dynamics of a problematic vulture roost in...
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DYNAMICS OF A PROBLEMATIC VULTURE ROOST IN SOUTHWEST FLORIDA AND
RESPONSES OF VULTURES TO ROOST-DISPERSAL MANAGEMENT EFFORTS
A thesis
Presented to
The Faculty of the College of Arts and Sciences
Florida Gulf Coast University
In Partial Fulfillment
Of the Requirement for the Degree of
Master of Science
By
Betsy A. Evans
2013
APPROVAL SHEET
The thesis is submitted in partial fulfillment of
the requirements for the degree of
Master of Science
__________________________________
Betsy A. Evans
Approved: December 2013
__________________________________
Jerome A. Jackson, Ph.D.
Committee Chair / Advisor
__________________________________
Edwin M. Everham III, Ph.D.
Committee Member
__________________________________
Charles W. Gunnels IV, Ph.D.
Committee Member
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable
presentation standards of scholarly work in the above mentioned discipline.
iii
ACKNOWLEDGMENTS
First and foremost I thank my thesis advisor, Dr. Jerry Jackson, whose support,
assistance, and interest throughout the whole process was vital in the preparation and completion
of this project.
I am grateful to my committee members, Dr. Billy Gunnels and Dr. Win Everham, who
provided statistical guidance and vital assistance in the interpretation and editing of the project.
I am appreciative to the Department of Marine and Ecological Sciences and the
Department of Biological Sciences at Florida Gulf Coast University for their support and
encouragement during the completion of my degree.
Additionally, I thank the Guadalupe Center of Immokalee for allowing me to conduct
research on their property and providing valuable information during the project. A special thank
you to the Ding Darling Wildlife Society for their generosity in providing supplemental funding
for the project.
I would like to thank my family and friends for their encouragement and positivity while
I completed my thesis work. And last, but not least I would like to thank my husband, Lucas
Halverson, whose assistance with data collection was invaluable and whose positivity gave me
the strength to finish my graduate degree.
iv
ABSTRACT
North American populations of Black Vultures (Coragyps atratus) and Turkey Vultures
(Cathartes aura) are steadily increasing and their ranges are expanding, and both species are
common in southwest Florida. Success of these species in the United States has led to a rise in
conflicts between humans and vultures and an increased need for effective vulture management.
The occurrence of a problematic vulture roost at the Guadalupe Center of Immokalee, an
early childhood education center in southwest Florida, was the focal point for a conflict between
humans and vultures. The Center had up to 250-300 vultures on their property on a daily basis
throughout most of the year. Vultures were observed perching on the Center building and
surrounding structures, as well as using an associated retention pond for bathing and drinking,
and adjacent mowed areas for social interactions.
The primary objective of this study was to evaluate and determine site-specific solutions
to a vulture roost location problem. In order to provide site-specific solutions to vulture roost
problems, an understanding of vulture behavior and roost habitats used by vultures was essential
to determine why the Guadalupe Center and areas surrounding it were attractive as a roost site. A
secondary objective included the evaluation of management techniques that were initiated by the
Center and were not site-specific. The overall goal was to evaluate vulture management
techniques available, to reduce and disperse the vultures on the property, and reduce vulture-
caused damage.
The Guadalupe Center roost was compared to 26 vulture roosts in the United States. Of
the 15 roosts observed in Florida, all were associated closely with water suggesting an
importance of water to vulture roost location. Air currents, such as obstruction currents were
v
present in over 70% of the roosts analyzed suggesting the importance of air current production to
the proximity of roost sites.
After the implementation of site-specific management techniques, the total population of
vultures on the property decreased significantly. The total population of vultures did not decrease
significantly after the initiation of non-site-specific management techniques.
The areas of the property the vultures occupied differed between pre- and post-
management. During roof-area management, vultures used the forested area north of the Center’s
property more often than expected and used the fences, pond, and building less often than
expected, suggesting that site-specific management efforts were effective at dispersing the birds
and reducing their impact on the property. Non-site-specific Center-initiated management did not
have a long-term effect on the population; birds readily habituated to the strategies (deployment
of vulture carcasses and pinwheels) and were observed more often than expected on all areas of
the Center’s property.
The behavior of the birds at the Guadalupe Center did not differ significantly when
comparing pre-management to any management strategy suggesting that management did not
prevent vultures from engaging in behaviors essential for survival.
Consistency and persistency are keys to effectively managing a problematic vulture roost.
When the Center initiated non-site-specific management techniques, the vultures did not alter
their use of the property. Since the management initiated by the Center was not consistent,
vultures were able to readily habituate to the deterrents. Management of vultures should be a
site-specific gradual process. Understanding behavior and roost habitat parameters is essential to
developing effective management strategies for vultures.
vi
TABLE OF CONTENTS
ACKNOWLEDGMENTS .............................................................................................. iii
ABSTRACT .................................................................................................................. iv
TABLE OF CONTENTS ............................................................................................... vi
LIST OF FIGURES ..................................................................................................... viii
LIST OF TABLES .......................................................................................................... x
INTRODUCTION .......................................................................................................... 1
MATERIALS AND METHODS
Study area ............................................................................................................ 5
Roost habitat characteristics............................................................................... 11
Pre-observational control data collection ........................................................... 12
Management techniques .................................................................................... 15
Post-management data collection ....................................................................... 23
Data analysis ..................................................................................................... 23
RESULTS
Roost habitat characteristics............................................................................... 25
Guadalupe Center roost composition ................................................................. 28
Management techniques .................................................................................... 30
Behavior ............................................................................................................ 43
vii
DISCUSSION
Guadalupe Center vulture roost composition...................................................... 45
Vulture response to management efforts ............................................................ 45
Use of noise, visual, and chemical repellants in vulture management ................. 47
Role of roost habitat characteristics in vulture management ............................... 51
Importance of vulture behavior in management plans ........................................ 53
Community activities contributing to vulture roost problems ............................. 55
What made the Guadalupe Center attractive as a roost site? ............................... 57
Need for effective vulture management ............................................................. 59
APPENDIX A
Black and Turkey vulture species information ................................................... 62
APPENDIX B
Problems associated with vulture use of the Guadalupe Center property ............ 69
APPENDIX C
Ineffectiveness and limitations in the implementation of the management
techniques of the Guadalupe Center roost .......................................................... 78
APPENDIX D
Future management suggestions for the Guadalupe Center roost site and other
problematic vulture roosts.................................................................................. 83
LITERATURE CITED ................................................................................................. 87
viii
LIST OF FIGURES
Figure 1: Map of study area in Collier County, Florida .................................................... 6
Figure 2: Aerial photograph of Immokalee, Florida ........................................................ 8
Figure 3: Aerial photograph of the Guadalupe Center .................................................... 10
Figure 4: Types of vulture behavior observed at the Guadalupe Center .......................... 14
Figure 5: Fence management ......................................................................................... 17
Figure 6: Pond management .......................................................................................... 18
Figure 7: Water pump management ............................................................................... 19
Figure 8: Mowed area management ............................................................................... 20
Figure 9: Roof and awning management ....................................................................... 21
Figure 10: Light pole management ................................................................................ 22
Figure 11: Vulture population at Guadalupe Center ...................................................... 28
Figure 12: Average number and age of Black Vultures during pre- and
post-management ......................................................................................... 29
Figure 13: Average number of Black Vultures during pre- and post-management.......... 32
Figure 14: Average number of adult Black Vultures during pre- and
post-management ......................................................................................... 33
Figure 15: Average number of immature Black Vultures during pre- and
post-management ......................................................................................... 34
Figure 16: Average number of Black Vultures during pre- and
pond-area management ................................................................................ 35
Figure 17: Average number of Black Vultures during pond-area and
roof-area management ................................................................................. 36
ix
Figure 18: Average number of Black Vultures during roof-area management and management
disruption..................................................................................................... 37
Figure 19: Average number of Black Vultures during management disruption and Center-
initiated management ................................................................................... 38
Figure 20: Dispersion of Black Vultures on the Guadalupe Center property .................. 39
Figure 21: Proportion of Black Vultures engaged in behaviors at Guadalupe Center ...... 44
Figure 22: Fence located at property across from Guadalupe Center .............................. 58
Figure 23: Black Vulture using obstruction current at Guadalupe Center ....................... 70
Figure 24: Vulture use of Guadalupe Center roofs ......................................................... 72
Figure 25: Vulture use of light poles and light shields ................................................... 73
Figure 26: Vulture damage to air-conditioning units at Guadalupe Center ..................... 74
Figure 27: Vulture use of mowed areas at Guadalupe Center ......................................... 75
Figure 28: Water pump at the Guadalupe Center ........................................................... 76
Figure 29: Water created by excessive irrigation at the Guadalupe Center ..................... 77
Figure 30: Fence damage at the Guadalupe Center ........................................................ 80
x
LIST OF TABLES
Table 1: Habitat characteristics and composition of vulture roosts ................................. 26
1
INTRODUCTION
Vultures are part of the most threatened avian guild in the world, the avian scavengers
(Sekercioglu 2006). Two of the seven species of New World vultures (i.e. California Condor
(Gymnogyps californianus) and Andean Condor (Vultur gryphus)) are currently listed as
threatened or endangered, and twelve out of the sixteen Old World vulture species are listed as
vulnerable, threatened, or endangered (IUCN 2012).
In contrast to the many species of vultures that are experiencing population declines,
Black Vulture (Coragyps atratus) and Turkey Vulture (Cathartes aura) populations in the
United States are increasing throughout most of their range (Avery 2004). Data from migration
counts, Christmas Bird Counts (CBC), and Breeding Bird Surveys (BBS) suggest that Black and
Turkey vulture populations have increased significantly in many areas across the eastern United
States (Farmer et al. 2008).
While urban development has been cited as a growing threat to birds across the world,
Black and Turkey vulture population increases appear to be partially explained by vultures’
ability to take advantage of urbanization. There are clearly some adaptations of vultures that
allow these birds to take advantage of human-caused habitat alterations (Avery 2004, Mandel
and Bildstein 2007). For example, an increased albedo of the Earth’s surface (e.g., global
warming, clearing of forests, construction of cities, roads, and parking lots) has created an
increasing frequency in both time and space of anthropogenic thermals, which vultures use for
energy-efficient flight (Mandel and Bildstein 2007), and contributed to the northward range
expansion of both species (Mossman 1991, Sauer et al. 1997). Vultures have also adjusted to
human-altered landscapes by taking advantage of the architecture of many anthropogenic
structures, such as tall buildings and communication towers (Audubon 1967 [1840], Buckley
2
1998, Avery et al. 2002, Evans and Sordahl 2008). Vultures are drawn to these structures not
only because of the perches provided, but because of the obstruction currents that are created
when a strong wind hits a vertical surface, like a building, creating an updraft (Cone 1962),
which vultures use for energy-efficient flight. Another manner in which vultures have taken
advantage of human-altered landscapes is through the exploitation of road-killed animals as a
food source. Vultures are able to take advantage of the food source, and spend less time
searching for food (Kelly et al. 2007).
As both human and vulture populations continue to grow, negative interactions between
humans and vultures have become more common. For example, vultures are susceptible to high
levels of heavy metals; such as lead (Carpenter et al. 2003), stationary and moving object
collisions (Sprunt 1937, Blackwell and Wright 2006, De Lucas et al. 2008), habitat loss (Jackson
1983), and other anthropogenic disturbances. Globally, there is an increasing frequency of
human-vulture conflicts that have risen with urban and rural land development.
The increase of Black and Turkey vultures throughout the United States has also
impacted humans and their property. Vultures have been estimated to cause over a million
dollars in property damage to residential, commercial, industrial, and agricultural areas in Florida
between 1993-2001 (USDA 2005).These problems include, but are not limited to the (1) tearing
of plastic, rubber, and leather coverings of personal property (USDA 2003); (2) destruction and
removal of insulation and linings on personal and commercial property such as vehicles and
buildings (USDA 2010); (3) scratching of paint on vehicles (USDA 2010); (4) tearing of roof
shingles on buildings (USDA 2003); (5) reported damage to landscaped plants due to excessive
roosting numbers and fecal matter (USDA 2003); (6) power outages associated with the
accumulation of fecal matter on electric transmission towers; and (7) loss or injury of livestock
3
in Florida and elsewhere (USDA 2005). Many communities in the United States encourage the
removal of vultures due to real or perceived property and agricultural damage.
To address adverse human-vulture interactions, several approaches have been taken in the
management of vultures. Among these are: deployment of vulture carcasses, taxidermic effigies,
or decoy effigies (Avery et al. 2002, Tillman et al. 2002); relocation of problem individuals
(Humphrey et al. 2000); broadcasting of noise deterrents (Schlierf et al. 2007); as well as lethal
measures (Runge et al. 2009). While such management techniques may be – or may seem to be
individually – effective in certain situations, they may not work at every site that has a vulture
problem. For example, the deployment of vulture carcasses is not always an effective long-term
solution (Sordahl 2013). Humans with a perceived vulture problem become frustrated when
these techniques do not work, which has led to use of lethal measures to remove vultures
(Virginia Department of Game and Inland Fisheries 2013).
As vulture and human populations increase, the need for more frequent and more
intensive control efforts could negatively impact North American vultures. There is an evident
need for evaluation of the efficacy and impacts of current efforts, development of sound
management guidelines, and education on the role of vultures in ecosystems, their ecological
needs, and the nature and context of the behavior that sometimes is found problematic for
humans (Appendix A). Many of the conflicts and management techniques that arise from human-
vulture interactions in the Americas may not only inform management efforts for problem
species elsewhere, but also provide insight useful in the conservation of threatened vulture
species.
The occurrence of a problematic vulture roost at the Guadalupe Center of Immokalee, an
early childhood education center in southwest Florida, was the focal point for a conflict between
4
humans and vultures. The Center had up to 250-300 Black and Turkey vultures on their property
on a daily basis throughout most of the year (pers. comm.). Vultures were observed perching on
the Center building and surrounding structures, as well as using an associated retention pond for
bathing and drinking; and adjacent mowed areas for social interactions.
My primary objective in this study was to evaluate and determine site-specific solutions
to a problematic vulture roost location problem. In order to provide site-specific solutions to
vulture roost problems, an understanding of vulture behavior and roost habitats used by vultures
was essential to determine why the Guadalupe Center and areas surrounding it were attractive as
a roost site. A secondary objective included the evaluation of management techniques that were
not site-specific. Of the techniques implemented, those techniques that were site-specific were
expected to be more effective in discouraging and altering the birds’ use of the property than
those techniques that were not site-specific.
5
MATERIALS AND METHODS
Study area
Fieldwork was conducted at the non-profit Guadalupe Center of Immokalee, an early
childhood education center that served as a year-round roost site for up to 250-300 Black and
Turkey vultures. The Guadalupe Center is located at the southern edge of Immokalee (ca. 24,000
population), Collier County, Florida (26°25′16″N, 81°25′22″W) (Figure 1). Agricultural fields,
as well as some natural landscapes, dominate the area surrounding Immokalee.
6
Fig
ure
1:
Map
of
study a
rea
in C
oll
ier
County
, F
lori
da.
Sta
r sh
ow
s lo
cati
on o
f G
uad
alupe
Cen
ter
roost
. (F
igure
adap
ted
from
the
Flo
rida
Cen
ter
for
Inst
ruct
ional
Tec
hnolo
gy,
Univ
ersi
ty o
f S
outh
Flo
rida,
Tam
pa,
2009)
7
The Center is surrounded by southern slash pine (Pinus elliottii var. densa) flatwoods
with a saw palmetto (Serenoa repens) understory and scattered cabbage palms (Sabal palmetto)
to the north, east, and west. This area also fringes an extensive bottomland hardwood, wetland
drainage composed primarily of baldcypress (Taxodium distichum), red maple (Acer rubrum),
and Brazilian pepper (Schinus terebinthifolius) (Figure 2). This natural area is privately owned
and part of the Big Cypress Basin managed by the South Florida Water Management District.
8
Fig
ure
2:
The
Guad
alupe
Cen
ter
of
Imm
okal
ee i
s lo
cate
d s
outh
of
Imm
okal
ee a
nd i
s par
t of
a sm
all
subdiv
isio
n,
Coll
ier
Vil
lage.
T
he
area
nort
h,
east
, an
d w
est
of
the
Cen
ter
consi
sts
of
a pin
e fl
atw
oods
eco
syst
em t
hat
fri
nges
an e
xte
nsi
ve
bott
om
land h
ardw
ood, w
etla
nd d
rain
age
(shad
ed i
n y
ello
w).
(G
oogle
TM
Ear
th i
mag
e ta
ken
on 2
2 D
ecem
ber
2010)
9
A drainage canal runs east-west along the north side of the Guadalupe Center property
(Figure 3), which carries runoff from the parking lot of an apartment building approximately 300
meters to the west of the Guadalupe Center. Prevailing west winds are funneled down the canal
and hit the west facing wall of the west-most building of the Guadalupe Center. This wall has no
roof overhang, resulting in an upward obstruction current that creates an updraft “elevator” for
the vultures to use. Directly to the west of the Center is a 0.6-hectare retention pond with low
sloping sides allowing vultures to access water for drinking and bathing. To the west and south
of the Center are an isolated subdivision, Collier Village, and another retention pond. The city
landfill is located approximately 1.5 kilometers southeast of the Guadalupe Center. Throughout
the city, many dumpsters, including some at the Center and others at businesses nearby, provide
vultures with access to reliable and predictable food sources.
10
Fig
ure
3:
Aer
ial
photo
gra
ph o
f th
e G
uad
alupe
Cen
ter
in I
mm
okal
ee, F
lori
da.
The
Guad
alupe
Cen
ter
was
use
d a
s a
roost
loca
tion f
or
250
-300+
Bla
ck a
nd T
urk
ey v
ult
ure
s. (
TM
Ear
th i
mag
e ta
ken
on 2
2 D
ecem
ber
2010)
11
The Guadalupe Center property has many features that make it attractive to the vultures.
These features include the architecture of the Center building, chain-link fences surrounding the
Center’s property, eleven light poles located in the parking lot, an air-conditioning unit and
enclosure, open mowed areas, a retention pond, a water pump, and irrigation sprinklers. All of
these features together resulted in the attractiveness of the Guadalupe Center property as a roost
site. Many of the features provided perch sites for the vultures to engage in social behaviors, or
provided the birds with water for drinking and bathing.
Roost habitat characteristics
Data on habitat characteristics of the Guadalupe Center roost were collected prior to the
initiation of management techniques in order to compare the Center’s roost to other vulture
roosts in the United States. Habitat characteristic data were collected in order to determine why
certain areas attract more vultures, and if management techniques could be developed to
discourage the vultures from occupying urban roost areas. The Guadalupe Center vulture roost
was compared to 26 vulture roosts in the United States. Comparative data were obtained by
personal observation, as well as personal observations and data published by others.
Habitat data that were collected included:
1. Presence of structures, both natural and anthropogenic, that allowed for the creation of
obstruction currents and thermals: Structures promoting the creation of obstruction
currents included large vertical surfaces, such as a cliffs or buildings. Structures
promoting the creation of thermals included areas with an increased albedo, often devoid
of vegetation such as agricultural fields or large areas of pavement.
12
2. Availability of food: Due to the presence of roadways and trash receptacles throughout the
United States, all vulture roosts analyzed were assumed to be near food due to the
proximity of roadways to all roosts observed.
3. Availability of water: Any mention of water near the roosts, whether natural or anthropogenic
were considered to be used by the vultures.
4. Area type: The type of area was categorized as urban, rural, or natural. Urban areas were
considered to be regions with anthropogenic buildings or other structures used by the
vultures for roosting. Rural areas were considered to be regions with agricultural fields,
equipment, and buildings. Natural areas were those areas where anthropogenic buildings
were not present or used by the vultures when roosting.
5. Roost tree type: If vultures roosted in trees, the type of trees was recorded. In some cases,
vultures only roosted on anthropogenic structures, thus no roost tree type was recorded.
6. Species composition of the roost: Data were collected on species of birds at each roost to act
as a comparison to the Guadalupe Center roost. Species composition was categorized as
only Turkey Vulture, only Black Vulture or Black and Turkey vulture.
Some roost descriptions did not mentioned any of the above characteristics; these were
categorized as “information not available or N/A” and were not included in the analysis of those
characteristics.
Pre-observational control data collection
Preliminary observational control data were collected from 7-11 March 2011 using 10 x
35 binoculars. Observations were made during all daylight hours, to understand the daily use of
the area by the vultures. The age (immature or adult), species (Black Vulture or Turkey Vulture),
13
behavior, and location of the vultures were recorded hourly to gain an understanding of what
aspects of the Guadalupe Center were attractive to the birds.
Vultures were classified as either immature or adult. Since transition to complete adult
plumage is gradual, hatching-year and second-year birds were categorized as immature and all
after-second-year birds as adults (Pyle et al. 2008). Immature Turkey Vultures were identified by
their gray head and/or dark bill tip (Kirk and Mossman 1998, Pyle et al. 2008). Adult Turkey
Vultures were identified by their red head and lighter bill tip (Kirk and Mossman 1998).
Immature Black Vultures were identified by their darker and less wrinkled head, in addition to
the conspicuous presence of feathers on the crown and nape of the head (Buckley 1999, Pyle et
al. 2008). Adult Black Vultures were identified by their dark, very rugose, featherless appearing
head (Buckley 1999).
Pre-observational behavioral sampling observations were made using instantaneous-scan
sampling (Altmann 1974). The behavior of each individual vulture was recorded. This resulted in
32 vulture behavior counts during which 9 specific behaviors were identified for observation
during post-management efforts (Davis 1983a, b).
The behaviors of the vultures were initially classified using nine categories (Figure 4).
Because of functional relatedness and sample sizes, for data analysis the following behaviors
were reorganized: (a) spread-wing and delta-wing postures were grouped together into one
category referred to as sunning behavior, and (b) drinking and bathing were grouped together
into one category as water-related behaviors that occurred at the pond and at pools created by the
sprinklers or rain.
14
A. Inactivity: Vultures engaged in standing,
lying, or general perch behavior. There was
no movement or interaction with other
individuals.
B. Spread-wing: Vultures with wings
fully extended and exposed to the sun.
C. Delta-wing: Vultures with wings partially
extended inward toward the body and facing
the sun.
D. Preening: Vultures preening feathers
anywhere on their own body.
E. Allopreening: Vultures preening the
feathers anywhere on the body of another
individual.
F. Agonistic: Vultures engaged in
aggressive behavior with other
individuals. This behavior included
activities such as chasing or pecking
another vulture. Agonistic interactions
require and were recorded as two or
more individuals. Per Hinde (1970)
agonistic behavior was described
collectively as attacking, fleeing, or
submissive behaviors.
15
G. Fidgeting: Vultures using bills to peck or
pick with no apparent “goal-directed” intent
at nearby items. Vultures were seen pecking
at roof shingles, insulation, and other
anthropogenic items at the roost site.
H. Drinking and bathing: Vultures
drinking water in pond, canal, or from
temporary pools created by rain or the
Center sprinkler system Vultures
bathing and dipping either their wings
and/or head into water.
Figure 4: Types of vulture behavior observed at the Center (Davis 1983a, b).
Management techniques
Management techniques were implemented in the areas on the property where the
vultures caused the most problems (Appendix B). Management techniques were implemented
gradually as permitted in order to see how each alteration of the property affected the vultures’
location and behavior.
Vulture management efforts were categorized for data analysis as follows due to the
simultaneous implementation of some techniques: (1) pre-management, management prior to my
involvement; (2) pond-area management, management at the pond, mowed areas, fences and
water pump; (3) roof-area management, management on the roof and light poles; (4)
management disruption, management efforts employed, but no longer effective or intact; and (5)
Center-initiated management, non-site-specific management implemented and suggested by
others (i.e. pinwheels and vulture carcasses).
Vulture management categories were a posteriori defined categories based on the
chronological implementation of management by the Center. Categories were organized because
16
of the sequential and simultaneous implementation of the management techniques at the Center
(Appendix C). For example, management at the pond, mowed areas, fences and water pump
were included in one category (pond-area management) because of the simultaneous
implementation of those techniques. The chronological implementation of management at the
Center is as follows: pre-management, pond-area management, roof-area management,
management disruption, and Center-initiated management.
Management techniques included:
Pond-area management
1. Fences. The first management technique implemented was the placement of a thin wire
above the fence stretched between every other fence pole, approximately 6 meters apart.
The wire was stretched 10-15 centimeters above the fence, but vultures were able to push
the wire down and still perch. The wire was moved 20 centimeters above the fence, and
vultures were still able to stretch out the wire and perch. Since the wire was not effective,
two monofilament lines (0.75 mm diameter, 50 lb. fish line) were stretched 20-25
centimeters above the fence (Figure 5). The line needed to be tight in order to prevent
vultures from pushing it down allowing them to still perch as noted with the lower quality
wire. The monofilament line was stretched between each fence pole at 3 meters rather
than 6 meters to prevent vultures from pushing the line down. Lines were placed less than
20-25 centimeters above the fence to prevent vultures from perching underneath.
A sprinkler system was installed on the north fence, but the system was defective
and created excess water for the vultures to use for drinking and bathing. At times the
sprinkler was functioning during both pre- and post-management of the property.
17
Figure 5: Two strands of monofilament line were installed 20-25 centimeters above chain-
link fences on the Guadalupe Center’s property to discourage vulture perching.
18
2. Pond. The installation of silt fencing around the pond perimeter was used to discourage
vultures from the property (Figure 6). The silt fencing was one meter tall and made of
heavy plastic. Wooden stakes were pre-installed on the fence and were set at 3 meter
intervals. The fencing was installed at the water’s edge, but was not moved as water
levels increased and decreased.
Figure 6: Silt fencing was installed around the Center pond to prevent vultures from using
the water for bathing and drinking.
19
3. Water pump. A PVC pipe extending to the pond was installed to eliminate water access
to the birds (Figure 7).
Figure 7: PVC pipe was extended from the water pump to prevent vultures from using the
water.
20
4. Mowed areas. When the grass in the area was not mowed, allowing the grass to grow
tall, vultures were deterred from occupying the region. When the area was mowed,
monofilament was stretched in a zigzag pattern across the area to prevent the vultures
from landing in the region (Figure 8). The poles were approximately 6-12 meters apart
and the line stretched approximately 9 meters from pole to pole.
Figure 8: Monofilament line stretched across the mowed area of the Center property.
21
Roof-area management
1. Building architecture. Bird spikes (Bird B Gone Stainless Steel 5.0” width and 4.75”
height) were placed along the peaks and edges of the roofs and awnings on the west-end
to prevent perching (Figure 9A and 9B). Two motion-activated sprinklers were installed
on the roof as well to further deter the vultures from perching (Figure 9C).
A.
B.
C.
Figure 9: Management of the Center roof and awning. A. Bird spikes placed on roof.
B. Bird spikes placed on awning. C. Motion-activated sprinkler installed on roof.
22
2. Light poles. Polycarbonate plastic bird spikes (Bird-B-Gone polycarbonate plastic 3”
width) were installed on the light shields to prevent vultures from perching on these
structures; vultures were still able to perch atop the supporting light poles (Figure 10).
Figure 10: Bird spikes placed on the light shields of the light posts at the Center.
23
Center-initiated management
1. Pinwheels. The Center installed pinwheels on the north fence, mowed areas, and roof.
The pinwheels were installed arbitrarily throughout the mentioned areas.
2. Vulture carcasses. The Center hung three Black Vulture carcasses in the trees separating
the pond from the northern mowed area. The three carcasses were hung upside down in
three adjacent trees.
Post-management data collection
During and after the implementation of management techniques, the age, species,
behavior, and location of the vultures on the Center property were recorded daily from April
2011 – December 2012 for a total of 85 days.
Without individually-marked birds, I could not determine the frequency with which
individuals were observed. However, the number of vultures present varied from day to day and
Turkey and Black vultures are known to use multiple roosts such that the roost composition was
likely fluid (Rabenold 1983). Consequently I viewed the outcomes of the management of the
roost each day as independent events.
Data analysis
Each daily sample point was treated as a separate data point. For those days when more
than one count was recorded, the first sampling of the day was used. The first sampling was used
because the behavior of the vultures was most likely not altered by my presence during the first
count. Data from the wet season (May-October) were not included in statistical analyses because
the vultures on the property dramatically decreased each wet season without regard to
management as communicated by the Center.
24
To examine the overall effect of management techniques on the population of vultures at
the Guadalupe Center roost, a one-way permutation test was used with a Monte Carlo Simulation
of the p-value (B = 9,999). Total Black Vulture, adult Black Vulture, and immature Black
Vulture average population numbers were analyzed and compared during pre- and post-
management efforts.
A one-way permutation was used with a Monte Carlo Simulation of the p-value (B =
9,999) to test the effectiveness of each categorical management technique (pre, pond-area, roof-
area, management disruption, and Center-initiated). Average Black Vulture population numbers
were analyzed during each of the management technique categories.
To examine the dispersion of the vultures on the property during the different categories
of management (e.g. pre, pond-area, roof-area, management disruption, and Center-initiated), a
Pearson’s chi-squared test was used. The five categories were compared to each proceeding
management technique category (i.e. pre vs. pond-area, pond-area vs. roof-area, roof-area vs.
management disruption, and management disruption vs. Center-initiated) to determine if there
was a change in the dispersion of the vultures across the property.
To examine the difference in vulture behavior during each management category, a one-
way permutation test was used with a Monte Carlo Simulation of the p-value (B = 9,999). The
proportion of vultures engaged in each of the seven behavioral categories during each
management effort were calculated for data analysis. A one-way permutation test was used with
a Monte Carol Simulation of the p-vaule (B = 9,999) to compare overall pre- and post-
management behavioral differences.
All statistical tests were conducted using R (R Core Team 2013) using the package coin
(Hothorn et al. 2006). Significance was accepted at an α-level of 0.05.
25
RESULTS
Roost habitat characteristics
Data were gathered from 27 vulture roosts in the United States (Table 1). Comparative
data were obtained by personal observation, as well as personal observations and data published
by others. Of the 27 roosts analyzed, over 70% had both Black and Turkey vultures present in
the roost. All other roosts had only one species present, either the Black Vulture or Turkey
Vulture.
Over 70% of the roosts analyzed were associated with the formation of obstruction
currents similarly to the Guadalupe Center roost. Of vulture roosts analyzed including the
Guadalupe Center roost, all were associated with thermals or structures located nearby that
would create thermals, except one where there was no mention of either.
Roosts located near a source of water accounted for 89% of the roosts analyzed. All 15
roosts located in Florida, were located near water or were associated with water. Food was found
to be available near all vulture roosts due in part to the amount of roadways located throughout
the country.
The roosts analyzed were composed of a variety of species of roost trees. Of the roosts
observed, only 11% did not describe or include either conifers or snags, suggesting a strong
preference for those roost tree types. In the morning and evening at the Guadalupe Center roost,
vultures perched within the forested area on exposed horizontal branches of slash pines and
snags and on the top of cabbage palms.
Anthropogenic structures were located near almost every roost; only 22% of the roosts
did not directly mention anthropogenic structures in their descriptions.
26
Tab
le 1
: H
abit
at c
har
acte
rist
ics
and c
om
posi
tion o
f 27 v
ult
ure
roost
s fo
und i
n t
he
Un
ited
Sta
tes
Guadalu
pe C
ente
r, C
olli
er
County
, F
lori
da
urb
an
both
yes
yes
yes
yes
Babcock R
anch, C
harl
ott
e C
ounty
, F
lori
da
rura
lboth
no
yes
yes
yes
Harn
s M
ars
h, L
ee C
ounty
, F
lori
da
natu
ral
both
no
yes
yes
yes
Dis
ney W
orl
d, O
sceola
County
, F
lori
da
urb
an
both
yes
yes
yes
yes
Bonnet
Cre
ek, O
sceola
County
, F
lori
da
natu
ral
both
no
yes
yes
yes
Orl
ando W
ildern
ess
Park
, O
range C
ounty
, F
lori
da
natu
ral
both
N/A
*yes
yes
yes
Ora
nge C
ounty
Sew
age T
reatm
ent
Pla
nt, O
range C
ounty
, F
lori
da
natu
ral
both
N/A
yes
yes
yes
Nort
h F
ort
Myers
, L
ee C
ounty
, F
lori
da
urb
an
both
yes
yes
yes
yes
Lakes
Park
, L
ee C
ounty
, F
lori
da
urb
an
both
yes
yes
yes
yes
Bull
Pond, P
utn
am
County
, F
lori
da
rura
lboth
yes
yes
yes
yes
Okeechobee C
ounty
, F
lori
da
rura
lboth
N/A
yes
yes
yes
Lake B
utler,
Baker
County
, F
lori
da
rura
lboth
yes
N/A
yes
yes
Myakka R
iver
Sta
te P
ark
, M
anate
e C
ounty
, F
lori
da
natu
ral
both
no
yes
yes
yes
Peace R
iver,
DeS
oto
County
, F
lori
da
natu
ral
Bla
ck V
ulture
N/A
yes
yes
yes
Everg
lades
National P
ark
, M
iam
i-D
ade C
ounty
, F
lori
da
natu
ral
Bla
ck V
ulture
yes
yes
yes
yes
Decora
h, W
innesh
iek C
ounty
, Io
wa
urb
an
Turk
ey V
ulture
yes
yes
yes
yes
Cora
lvill
e D
am
, Jo
hnso
n C
ounty
, Io
wa
urb
an
Turk
ey V
ulture
yes
yes
yes
yes
Chath
am
County
, N
ort
h C
aro
lina
natu
ral
Bla
ck V
ulture
N/A
yes
yes
yes
Malh
eur
National W
ildlif
e R
efu
ge, H
arn
ey C
ounty
, O
regon
natu
ral
Turk
ey V
ulture
yes
yes
yes
yes
Gett
syburg
National M
ilita
ry P
ark
-Big
Round T
op, A
dam
s C
ounty
, P
ennsy
lvania
natu
ral
both
yes
yes
yes
N/A
Gett
syburg
National M
ilita
ry P
ark
-Harp
ers
Hill
, A
dam
s C
ounty
, P
ennsy
lvania
natu
ral
both
yes
yes
yes
N/A
Gett
syburg
National M
ilita
ry P
ark
-Quarr
y, A
dam
s C
ounty
, P
ennsy
lvania
natu
ral
both
yes
yes
yes
N/A
Hols
ton R
iver
Am
munitio
n P
lant, H
aw
kin
s C
ounty
, T
enness
ee
natu
ral
both
yes
yes
yes
yes
Nacogdoches
County
, T
exas
rura
lT
urk
ey V
ulture
N/A
yes
yes
yes
Leesb
urg
, L
oudoun C
ounty
, V
irgin
iaurb
an
both
yes
yes
yes
yes
Dutc
h G
ap C
onse
rvation A
rea, C
hest
erf
ield
County
, V
irgin
ianatu
ral
both
yes
yes
yes
yes
Radfo
rd A
rmy A
mm
unitio
n P
lant, M
ontg
om
ery
County
, V
irgin
ianatu
ral
both
yes
yes
yes
yes
*N
/A d
ata
not
availa
ble
RO
OS
T L
OC
AT
ION
WA
TE
RA
RE
AS
PE
CIE
SO
BS
TR
UC
TIO
N
CU
RR
EN
TS
TH
ER
MA
LS
FO
OD
27
Tab
le 1
(co
nti
nued
): H
abit
at c
har
acte
rist
ics
and c
om
posi
tion o
f 27 v
ult
ure
roost
s fo
und i
n t
he
Unit
ed S
tate
s
Gua
dalu
pe C
ente
r, C
ollie
r C
ount
y, F
lori
dapi
ne, c
abba
ge p
alm
, and
sna
gsye
spe
rson
al o
bser
vatio
n
Bab
cock
Ran
ch, C
harl
otte
Cou
nty,
Flo
rida
pine
yes
pers
onal
obs
erva
tion
Har
ns M
arsh
, Lee
Cou
nty,
Flo
rida
pine
no
pers
onal
obs
erva
tion
Dis
ney
Wor
ld, O
sceo
la C
ount
y, F
lori
daba
ldcy
pres
s an
d sn
ag p
ines
yes
pers
onal
obs
erva
tion
Bon
net
Cre
ek,
Osc
eola
Cou
nty,
Flo
rida
bald
cypr
ess
and
snag
pin
esno
S
tole
n an
d T
aylo
r 20
03
Orl
ando
Wild
erne
ss P
ark,
Ora
nge
Cou
nty,
Flo
rida
cabb
age
palm
sna
gsno
Sto
len
1996
Ora
nge
Cou
nty
Sew
age
Tre
atm
ent
Pla
nt, O
rang
e C
ount
y, F
lori
daba
ldcy
pres
sno
Sto
len
1996
Nor
th F
ort
Mye
rs, L
ee C
ount
y, F
lori
dapi
neye
spe
rson
al o
bser
vatio
n
Lak
es P
ark,
Lee
Cou
nty,
Flo
rida
bald
cypr
ess
and
snag
sye
spe
rson
al o
bser
vatio
n
Bul
l Pon
d, P
utna
m C
ount
y, F
lori
daba
y, p
ine
and
bald
cypr
ess
yes
Till
man
et
al.
2002
Oke
echo
bee
Cou
nty,
Flo
rida
cabb
age
palm
sna
gs a
nd o
aks
lack
ing
folia
geye
sT
illm
an e
t al
. 20
02
Lak
e B
utle
r, B
aker
Cou
nty,
Flo
rida
pine
yes
Till
man
et
al.
2002
Mya
kka
Riv
er S
tate
Par
k, M
anat
ee C
ount
y, F
lori
da
cabb
age
palm
, liv
e oa
k sn
ag, a
nd b
aldc
ypre
ssye
spe
rson
al o
bser
vatio
n
Pea
ce R
iver
, DeS
oto
Cou
nty,
Flo
rida
cabb
age
palm
and
live
oak
sna
gsye
spe
rson
al c
omm
unic
atio
n
Eve
rgla
des
Nat
iona
l Par
k, M
iam
i-D
ade
Cou
nty,
Flo
rida
cabb
age
palm
and
sna
gsye
spe
rson
al o
bser
vatio
n
Dec
orah
, Win
nesh
iek
Cou
nty,
Iow
ade
cidu
ous
tree
s an
d sn
ags
yes
pers
onal
obs
erva
tion
Cor
alvi
lle D
am, J
ohns
on C
ount
y, I
owa
N/A
ye
spe
rson
al o
bser
vatio
n
Cha
tham
Cou
nty,
Nor
th C
arol
ina
deci
duou
s an
d m
ixed
con
ifer
sno
Rab
enol
d 19
87
Mal
heur
Nat
iona
l Wild
life
Ref
uge,
Har
ney
Cou
nty,
Ore
gon
cott
onw
ood
yes
Dav
is 1
979
Get
tsyb
urg
Nat
iona
l Mili
tary
Par
k-B
ig R
ound
Top
, Ada
ms
Cou
nty,
Pen
nsyl
vani
api
neye
sW
righ
t 19
84
Get
tsyb
urg
Nat
iona
l Mili
tary
Par
k-H
arpe
rs H
ill, A
dam
s C
ount
y, P
enns
ylva
nia
pine
and
har
dwoo
dsye
sW
righ
t 19
84
Get
tsyb
urg
Nat
iona
l Mili
tary
Par
k-Q
uarr
y, A
dam
s C
ount
y, P
enns
ylva
nia
pine
yes
Wri
ght
1984
Hol
ston
Riv
er A
mm
uniti
on P
lant
, Haw
kins
Cou
nty,
Ten
ness
eesy
cam
ore
and
beec
hye
sT
anne
r 19
47 a
nd S
tew
art
1983
Nac
ogdo
ches
Cou
nty,
Tex
aspi
neno
Has
kins
197
2
Lee
sbur
g, L
oudo
un C
ount
y, V
irgi
nia
pine
yes
Nic
kens
200
8
Dut
ch G
ap C
onse
rvat
ion
Are
a, C
hest
erfi
eld
Cou
nty,
Vir
gini
aN
/Aye
sD
uerr
200
9
Rad
ford
Arm
y A
mm
uniti
on P
lant
, Mon
tgom
ery
Cou
nty,
Vir
gini
asy
cam
ore
and
pine
yes
Pra
ther
et
al.
1976
RO
OS
T L
OC
AT
ION
RO
OS
T T
RE
EA
NT
HR
OP
OG
EN
IC
ST
RU
CT
UR
ES
SO
UR
CE
28
Guadalupe Center roost composition
The total number of Black and Turkey vultures at the Guadalupe Center changed
throughout the year and roughly matched a change in the wet and dry season (Figure 11).
Black Vultures far out-numbered Turkey Vultures at the site in 17 out of 22 months of
observation during both pre- and post-management efforts (Figure 11). Since Turkey Vultures
were not present in as high of numbers as Black Vultures, were often only located in the forested
area north of the Center, and were only present early in the morning before leaving the roost and
at dusk when coming to roost, their data were excluded from further analysis.
Figure 11: Total number of Black and Turkey vultures present at the Guadalupe Center
from March 2011 through December 2012.
29
The average numbers of immature and adult Black Vultures were recorded at the
Guadalupe Center roost. Prior to management, there were more immature Black Vultures on
property than during management (Figure 12). After management, there were comparatively
more adult Black Vulture at the roost than immature Black Vultures (Figure 12).
Figure 12: Average number of immature Black Vultures and adult Black Vultures during
pre- and post-management (during the dry season at the Guadalupe Center).
30
Management techniques
The average number of Black Vultures on the property significantly decreased when
comparing pre- to post-management efforts (Figure 13: Z = 3.63, p < 0.01). Similarly, the
average number of adult Black Vulture significantly decreased when comparing pre- and post-
management efforts (Figure 14: Z = 2.2192, p = 0.017) as well as the average number of
immature Black Vultures (Figure 15: Z = 4.1, p < 0.001).
The average number of Black Vultures on the property decreased significantly when
comparing pre-management efforts to pond-area management efforts (Figure 16: Z=2.4673,
p<0.01). The average number of Black Vultures did decrease when comparing pond-area
management to roof-area management, but it was only marginally significant (Figure 17:
Z=1.9047, p=0.05). When comparing the roof-area management to when management was
disrupted there was a marginally significant increase in the average number of Black Vultures on
property (Figure 18: Z=1.7166, p=0.05). When comparing Center-initiated management to
management disruption, there was not a significant decrease in the average number of Black
Vultures on the property (Figure 19: Z=1.4871, p=0.162).
The areas the vultures occupied during pre-management and during pond-area
management differed significantly (Figure 20 A, B: X2=65.89, p<0.001). During pre-
management the vultures were found primarily at the pond (Figure 20 A, B). Prior to pond-area
management, vultures were on the fences 30% of the time (Figure 20 A, B). After pond-area
management only 15% of vultures were found on the fences (Figure 20 A, B). After pond-area
management was implemented, the vultures were also observed more than expected on the
building and at the pond. In addition, there was a significant difference between the areas
occupied by the vultures when comparing pond-area management to roof-area management
31
(Figure 20 C, D: X2=251.69, p<0.001). After roof-area management, vultures occupied the
forested area north of the Center more often than expected and were observed on the building
and at the pond less than expected.
Vultures used different areas at the Center when roof-area management was compared to
days during management disruption. During management disruption, vultures returned to the
building, fences, and pond (Figure 20 E, F: X2=86.46, p<0.001). Vultures were not observed as
often in the forested area north of the Center during days when management was disrupted.
When comparing the days during management disruption to Center-initiated management
there was no significant difference in the areas the vultures used on the property (Figure 20 G, H:
X2=5.42, p=0.144), suggesting that management disruption and Center-initiated management did
not alter the dispersion of the birds on the property.
32
Figure 13: Average number of Black Vultures during pre- and post- management at the
Guadalupe Center. The top and bottom of each box indicate the 75th and the 25
th
percentiles. The line through the box is the median, and the error bars are the 5th and 95
th
percentiles.
33
Figure 14: Average number of adult Black Vultures during pre- and post-management at
the Guadalupe Center. The top and bottom of each box indicate the 75th and the 25
th
percentiles. The line through the box is the median, and the error bars are the 5th and 95
th
percentiles.
34
Figure 15: Average number of immature Black Vultures during pre- and post-management at
the Guadalupe Center. The top and bottom of each box indicate the 75th and the 25
th percentiles.
The line through the box is the median, and the error bars are the 5th
and 95th percentiles.
35
Figure 16: Average number of Black Vultures during pre- and pond- area management at
the Guadalupe Center. The top and bottom of each box indicate the 75th and the 25
th
percentiles. The line through the box is the median, and the error bars are the 5th and 95
th
percentiles.
36
Figure 17: Average number of Black Vultures during pond-area and roof-area
management at the Guadalupe Center. The top and bottom of each box indicate the 75th
and the 25th percentiles. The line through the box is the median, and the error bars are the
5th
and 95th percentiles.
37
Figure 18: Average number of Black Vultures during roof-area management and
management disruption at the Guadalupe Center. The top and bottom of each box
indicate the 75th and the 25
th percentiles. The line through the box is the median, and the
error bars are the 5th and 95
th percentiles.
38
Figure 19: Average number of Black Vultures during management disruption and Center-
initiated management at the Guadalupe Center. The top and bottom of each box indicate
the 75th and the 25
th percentiles. The line through the box is the median, and the error bars
are the 5th and 95
th percentiles.
39
A.
B.
Figure 20: Dispersion of Black Vultures on the Guadalupe Center property
during pre- and pond-area management. A. Total number of Black Vultures
on areas of property. B. Percentage of Black Vultures on areas of property.
40
C.
D.
Figure 20 (continued): Dispersion of Black Vultures on the Guadalupe Center
property during pond-area and roof-area management. C. Total number of
Black Vultures on areas of property. D. Percentage of Black Vultures on areas
of property.
41
E.
F.
Figure 20 (continued): Dispersion of Black Vultures on the Guadalupe Center
property during roof-area and management disruption. E. Total number of
Black Vultures on areas of property. F. Percentage of Black Vultures on areas
of property.
42
G.
H.
Figure 20 (continued): Dispersion of Black Vultures on the Guadalupe Center
property during management disruption and Center-initiated management. G.
Total number of Black Vultures on areas of property. H. Percentage of Black
Vultures on areas of property.
43
Behavior
Vultures were engaged in similarly proportioned behaviors during all management
categories (Figure 21). Vultures engaged in behaviors described as inactive, the majority of the
time on the property. The second most common behavior was preening in all management
efforts. Sunning and water-related behaviors were exhibited more often than agonistic,
allopreening, and fidgeting.
The differences between pre- and post-management proportions were tested for each of
the seven behavior categories. There was a significance difference between most pre- and post-
management behaviors when analyzing each behavior separately against management type.
When comparing water-related (Z=-5.001, p<0.001), sunning (Z=-4.918, p<0.001), inactive (Z=-
5.5549, p<0.001), allopreening (Z=-2.9609, p=0.007) and preening (Z=-6.7252, p<0.001) during
pre- and post-management, all behaviors significantly decreased between pre- and post-
management. Fidgeting (Z=-2.5276, p=0.014) and agonistic (Z=-2.1475, p=0.0305) behaviors
were significantly different between pre- and post-management as well.
44
A. B.
C. D.
Figure 21: Proportion of vultures engaged in each of seven behaviors observed at the Guadalupe
Center roost during each management category.
45
DISCUSSION
Guadalupe Center vulture roost composition
The composition of the population of vultures at the Center was important in
management efforts. There were two species present on the property; Black Vultures and Turkey
Vultures. Black Vultures far outnumbered the Turkey Vultures and were present throughout the
day and in all areas of the property. Turkey Vultures, on the other hand, were often present only
early in the morning before leaving the roost or late at night when coming to roost. Turkey
Vultures were observed most often in the forested area north of the Center and not on the Center
property, thus they were not part of the main roost problem at the Guadalupe Center. In addition
Black Vultures are most commonly associated with vulture roost damage of the two species of
vultures.
The age composition of the roost was important to management as well. Of the Black
Vultures present at the Guadalupe Center roost, the majority were immature birds. Rabenold
(1987) found that immature Black Vultures were site faithful, as opposed to adults that did not
stay at the same roost consistently. Similarly, Stewart (1978) found that Black Vulture roosts
consisted largely of young nonbreeding aged birds. The birds at the Guadalupe Center most
likely consisted of nonbreeding birds either too old or too young to breed suggesting that the
birds would stay in the area as long as the benefits of the site outweighed the costs.
Vulture response to management efforts
The total number of Black Vultures at the Guadalupe Center decreased significantly
between pre- and post-management efforts suggesting that the management techniques
implemented discouraged vulture use of the Center’s property. The implementation of
46
management techniques also caused the vultures to alter their use of the property after each
management effort category.
During pre-management, vultures were found across all areas of the property. Areas of
the property that the vultures used included the building, fences, pond (including the surrounding
mowed area), and the forested area north of the Center. After pond-area management, vultures
were observed more often than expected on the buildings and at the pond. The birds, however,
were observed less than expected on the fences and in the forested area. Overall, the number of
vultures decreased, but a higher proportion of birds did occur at the pond. The birds may have
occurred at the pond more often due to the scarcity of the resource and difficulty in accessing the
resource. The vultures’ persistence of staying in areas near the water suggests that the water and
surrounding areas were the major draw of the property. It is also important to note that the silt
fence surrounding the pond often fell down, thus was never fully implemented allowing vultures
access to water.
Roof-area management followed the management conducted at the pond-area. The birds
used the property differently when comparing pond-area and roof-area management. Vultures
were observed more than expected in the forested area north of the Center and less than expected
in all other areas of the property during roof-area management. The movement of the birds to the
forested area suggests that the roof-area management was successful in deterring the majority of
the birds from the Center’s property. The combined management efforts at the pond and roof cut
off the vultures’ access to water, perch sites, and areas for social and maintenance behaviors.
Many of the management techniques deteriorated after their initial implementation (i.e.
management of the pond, mowed areas, fences, and building all fell into disrepair). The silt
fencing was under water, monofilament lines on the fences and in the mowed area were broken,
47
and the sprinklers and bird spikes on the roof were not functioning. During times of management
disruption, the birds were observed throughout all areas of the property. The birds were also
observed less often than expected in the forested area north of the Center’s property. This
suggested that the intact management did encourage the birds to use areas other than the Center’s
property.
After management disruption, the Center implemented a few techniques suggested by
other individuals. The techniques suggested by others were not designed for the specific
population of vultures occupying the Guadalupe Center property and were not designed for the
problems associated with the Guadalupe Center property. These techniques included the hanging
of vulture carcasses near the pond and the placement of pinwheels in the mowed areas, roofs, and
fences. There was no significant difference in the birds’ use of the property during the non-site-
specific Center-initiated management and management disruption. This suggests that our
management which was site-specific and problem-specific was more successful than the Center-
initiated management, which was not site-specific.
Use of noise, visual, and chemical repellants in vulture management
The Center often initiated management techniques that were not site-specific and were
suggested by other individuals. These non-site-specific management techniques were not based
on the behavior of the vultures or the habitats used by the vultures at the Guadalupe Center.
These techniques were suggested by individuals and agencies that had implemented these
techniques successfully at other vulture roosts or with other bird species, but had not visited the
Guadalupe Center site specifically. The non-site-specific management techniques failed to take
into account the population of vultures or the physical features of the Center. It seemed clear
from my observations that as long as the benefits for the vultures at the roost site outweigh the
48
energy costs required to continue to use the site, the vultures would continue to occupy the
Center grounds. The use of noise repellants, such as shouting and creating loud sounds,
fireworks and other pyrotechnics; visual repellants, such as pinwheels, dead vultures, and owl
decoys; and chemical repellants only garnered short-term successes. Birds, including vultures,
readily habituate to stimuli that cause no harm. The attraction of the site remained with the use of
noise, visual, and chemical repellants and the vultures continued to return until the site was
altered to make it less attractive.
Use of noise repellants. Prior to my involvement at the Center, personnel attempted to deter the
vultures with a variety of noise repellants. Center personnel used firecrackers to discourage
vulture use of the Center. The vultures left the property only momentarily after the use of
pyrotechnics. The Center staff would also yell and clap their hands at the vultures, but the
vultures would only fly to another area of the property. The vultures habituated to the loud noises
produced by the Center staff, thus noise repellants did not deter the vultures from the property.
Use of visual repellants. Pinwheels, toys incorporating a folded plastic rosette attached by a pin
or nail to a stick so that the vanes of the rosette catch the wind and cause it to spin, were
suggested and installed by Center personnel in hopes of dispersing vultures. Center personnel
installed the pinwheels on 22 March 2012, and the vultures continued to use the site, but ignored
areas where the pinwheels were installed. A week after the installation of the pinwheels the
vultures began to perch immediately next to them.
Other visual deterrents, such as owl decoys were used prior to my visit to the site. Center
personnel noted that the owl decoys did not work. Similarly, I observed Black Vultures at the
Seminole Casino of Immokalee perched right next to an owl decoy intended to scare the vultures.
Clearly any deterrent value of owl decoy was short-lived.
49
A common vulture management technique employed across the United States is the use
of vulture effigies or carcasses in efforts to disperse roosts. Center personnel obtained a permit
from the United States Fish and Wildlife Service for the allowable take of Black Vultures.
Against recommendations, three Black Vultures were shot on Center property on 31 March
2012. The dead vultures were hung upside down by their feet in trees adjacent to the north side
of the pond, where most vultures were located. This was in accordance with instructions
provided by the U.S. Department of Agriculture (Seamans 2004, Humphrey et al. 2010, [and
others]). I visited the property later that evening and viewed vultures in the forested area north of
the Center and on light poles. Vultures continued to increase in numbers over the next few weeks
as the dead vultures decomposed. Often, USDA staff members recommend obtaining permits for
the allowable take of vultures after all other management resources have been exhausted (USDA
2003). The allowable take of vultures is becoming more common in vulture management across
the country as USDA officials promote the use of vulture effigies and carcasses as successful
roost dispersal mechanisms. The few studies that have examined the long-term effectiveness of
vulture effigies and carcasses as roost dispersers have been inconclusive and most studies have
been conducted at communication tower roosts (Avery et al. 2002).
The effectiveness of vulture effigies after the initial displacement of a vulture roost has
not been studied adequately. In June 2012, a Turkey Vulture carcass was hung from one of two
communication towers at a vulture roost in northeast Iowa. The carcass provided by the USDA,
initially displaced the vultures from both communication towers and even led them to abandon
their roost a ½ kilometer away (Sordahl 2013). The vultures had occupied the roost area for over
20 years. In September 2012, approximately 30 vultures returned to use the roost, but it is
assumed these vultures were northern migrants that used the roost as a stopover site while
50
migrating south (Sordahl 2013). The resident Turkey Vultures would typically have departed
from the roost in October to migrate south. The vultures did return in March 2013. Currently,
there are 50-100 vultures using both communication towers and the night roost. The vultures are
not occurring in as high of number as the past (as many as 300), but they have returned,
suggesting that effigies and carcasses are not a long-term permanent solution to dispersing
roosts.
Researchers do not fully understand why vultures will abandon a roost site permanently
or temporarily when a carcass or effigy is hung. The public and researchers often
anthropomorphize vultures and attribute roost dispersal to the fact that vultures do not want to be
near other dead vultures. At the Guadalupe Center, vultures at times were found dead and on
occasion other vultures were seen eating the vulture carcasses (pers. obs.). Vultures are known
scavengers and will scavenge their own species if the opportunity arises (Audubon 1967 [1840],
personal observations). My study shows similar ineffectiveness of this management technique;
the use of vulture effigies and carcasses has limited short-term effectiveness as a deterrent
strategy of vulture roosts.
Use of chemical repellants. The vulture roost at the Guadalupe Center was a human-interest
story that was told through radio, television, and newspapers. Responses and suggestions from
the public for dispersing the vultures were numerous. One individual offered to spray a
homemade concoction that was said to be effective in deterring birds from properties. The
individual would not disclose what ingredients were in the mixture, but said it would be effective
against the vultures. The product was sprayed in an area where the vultures were roosting on a
fence. While the vultures did leave the immediate area while the individual was spraying, they
returned within 30 minutes after the spraying was completed. The spray had no visible lasting
51
influence on the vultures, but did kill some of the vegetation in the area where it was sprayed.
Other studies support the ineffectiveness of chemical repellants in dispersing vulture roosts
(Schlierf et al. 2007).
Role of roost habitat characteristics in vulture management
After gaining an understanding of the problems and areas used by the birds, I looked at
other roosts in the United States to understand what habitat characteristics were shared between
the Guadalupe Center roost and other roosts.
A comparison of the vulture roost at the Guadalupe Center to other roosts in the United
States was important to determine what habitat characteristics were typically associated with
large roosts. Understanding what habitat characteristics are attractive for a communal roost will
allow researchers and wildlife management personnel to better assist individuals with roost
problems on their property.
Most of the vulture roosts I surveyed were associated with habitat parameters that
promote the production of air currents, such as obstruction currents and thermals, for energy
efficient flight in vultures. Before the arrival of Europeans to North America, the eastern United
States was predominately covered by forests, which were heat absorbing. This feature limited the
ability of vultures to use the pre-European forests of eastern North America. Today, much of the
United States is covered by agricultural fields and cities, which reflect large amounts of heat
(Jackson 1991). Roads, large concrete lots, water surfaces, and even buildings may enhance the
production of thermals (Geiger 1965). The reflection of heat creates thermals, which create eased
flight for soaring birds, such as vultures.
The unevenness of the land surface and borders between forests and clearings promote
the development of obstruction currents. Large vertical surfaces, like the Guadalupe Center
52
building contribute to the formation of obstruction currents as well. Studies show that areas that
promote obstruction currents and thermals, allowing for effortless flight, are preferred sites for
vulture roosts (Thompson et al. 1990).
Data on roost tree type were collected because some trees have been reported to be more
attractive to birds as they create more accessible perch sites than others (Wright et al. 1986,
Thompson 1987). Of the roosts analyzed in this study, conifers and snags were consistently used
as preferred roost trees. Vulture roost habitat studies found similar results. Thompson found in
his study that taller trees and conifers were preferred to smaller deciduous trees (1987). At the
Guadalupe Center, the roost trees most commonly occupied were conifers or snags. There are
several reasons why large birds, like vultures prefer to roost in conifers. Tall trees, like conifers
have branches that are well-spaced which allow for a vulture’s large wingspan and easy takeoff
(Thompson 1987). Conifers tend to have larger diameter branches that move less in the wind,
providing a steady perch for vultures (Koford 1953). Conifers also have a horizontal arrangement
of branches that allow for easy flight takeoff (Wright et al. 1986). Similar to conifers, snags,
trees without an increased amount of foliage were found to be preferred by vultures due to eased
flight (Ceballos and Donazar 1990).
The preference of vultures to conifers and snags at roosting sites has been well-
researched, but the preference to have a roost located by a water source has not been discussed as
thoroughly. Similar to the Guadalupe Center roost, Stolen (1996) found that vultures in Florida
prefer tall trees that are near water. In addition, Audubon in his accounts of the Black Vulture
and Turkey Vulture described both birds as preferring roost areas near water (1967 [1840]).
Other species of vultures, like the California Condor, are known to roost near water sources as
53
well (Snyder and Snyder 2000). There seems to be a preference in areas with characteristic dry
seasons, for vulture roosts to be located near a source of water.
At the Guadalupe Center, there was a large open mowed area that vultures frequented. In
one study, researchers found that areas of open ground near a roost were used between one to
three hours before leaving to roost (Rabenold 1987). In addition, Audubon described a roost of
over a thousand Black Vultures along a river with the ground beneath the roost devoid of
vegetation (1967 [1840]). Vultures at both the Guadalupe Center and in other studies used the
open ground for social interactions and maintenance behaviors. At the Guadalupe Center,
monofilament line was stretched across mowed areas to discourage vulture use in those regions.
Management techniques should be implemented in open areas of vulture roosts to prevent
vultures from engaging in daily social and maintenance behaviors.
An understanding of vulture roost habitat characteristics is important not only for
management, but for conservation. Many species of Old World vultures are either listed as
threatened or endangered (IUCN 2012) and researchers have found that Old World vultures
share similar roost site characteristics (Yamac 2006). The roost sites studied by Yamac showed a
strong preference to areas that promoted the creation of obstruction currents and a preference of
conifers as roost trees (2006). Old World vultures are at risk for extinction, thus any information
about the preferred habitat characteristics for roosting in New World vultures has the potential to
assist with the conservation efforts of the Old World species.
Importance of vulture behavior in management plans
The majority of birds observed at the site were engaged in maintenance behaviors
(preening) or behaviors described as inactive (lying or perching) during all management efforts.
Other individuals have found similar results at roosts throughout the United States. Stolen (1996)
54
found that Black Vultures at a roost in central Florida spent most of their time engaged in
maintenance behaviors such as, preening and scratching; and social behaviors, such as agonistic
interactions and allopreening (1996). In addition, Clark and Ohmart (1985) found that Turkey
Vultures spent the majority of their time engaged in preening and resting behaviors. Davis
(1983b) also found that over 50% of Turkey Vultures engaged in preening behavior pre- and
post-roosting. Similarly behavioral patterns have been observed in other New World vultures.
For example, Koford (1953) found that preening was the most common behavior exhibited by
California Condors at roosts.
An understanding of what the vultures are doing at a particular site and how the birds use
the site are important in effective vulture management. This information will help determine
what management techniques should be implemented to deter vulture use of the property.
Vultures at the roost were often observed drinking or bathing at the pond and preening in the
mowed area after using the water. Many of the birds at the Guadalupe Center engaged in
preening were wet suggesting the importance of the water to the roost location. While the
vultures did engage in behaviors the same proportionate amount of time during pre- and post-
management efforts, the total population was significantly reduced after management techniques
were initiated. The understanding of the birds’ behavior at the roost site allowed me to suggest
management techniques based on their location and overall behavior at the site.
Vulture social and maintenance behaviors at the Guadalupe Center did not change overall
between pre- and post-management strategies. When analyzing each behavior separately, the
behaviors most common were found to be the most significantly different between pre- and post-
management. This is mostly likely caused by the reduction in vulture numbers between pre- and
post-management. Vultures seemed to exhibit the same proportionate amount of behavior
55
regardless of management efforts. This suggests that management does not affect these behaviors
negatively. Vultures were still able to engage in maintenance behaviors, behaviors essential for
survival, regardless of management efforts.
While the social and maintenance behaviors of the birds did not change, how the birds
used the property changed dramatically depending on the management strategy. Vultures were
present throughout all areas of the property when management was not intact and not site-
specific. When the pond-area and roof-area management were intact vultures used the areas of
the property differently. In addition, management strategies at the pond and roof areas affected
the birds by dramatically reducing the number of vultures on the property.
Community activities contributing to vulture roost problems
While the Center may be able to alter and manage their property to make it less attractive
to the vultures, they will be unable to alter the community’s actions that contribute to the large
population of vultures in the area. The community of Immokalee is partially accountable for the
vulture roost problem at the Guadalupe Center. In Collier County, there are over 100,000 acres
of farmland and in 2007 over 175,000 acres of vegetables were produced (USDA 2009). Large
numbers of Black Vultures and a few Turkey Vultures have been observed feeding on rotting
vegetables in the agricultural fields that surround Immokalee (Jerry Jackson, pers. comm.). Black
Vultures are known to take advantage of vegetative matter as a food source. Black Vultures in
Louisiana were reported to have eaten sweet potatoes when traditional food sources were scarce
(McIlhenny 1945). In addition, Haverschmidt (1947) reported that Black Vultures would
occasionally eat the African oil palm fruits that fell to the ground and would eat coconut flesh
left over at factories in Coronie. In Immokalee, culled vegetables are left to rot in large piles,
which were a predictable food source for many birds in the area, including vultures.
56
There may be some form of seasonality to the presence of vegetable cull piles. I visited
two cull pile sites from April 2011-December 2012. Culled vegetables were not present or very
limited between June-October 2011 and June-October 2012. The vultures at the Guadalupe
Center were not present in high numbers during the characteristic wet season months, but these
months also coincide with the lack of culled vegetable piles in the area. The agricultural practices
of Immokalee may contribute to the vulture problem at the Guadalupe Center.
The availability of culled vegetables is not the only food source that vultures may depend
on in the area. Dumpsters and garbage litter the streets of Immokalee, often trash receptacles are
left open for vultures to take advantage of and have contributed to the large vulture population in
the area. The Seminole Casino of Immokalee is located less than one kilometer from the
Guadalupe Center and has a large dumpster that is regularly left uncovered. I visited the
dumpsters 56 times between April 2011 and December 2012 and viewed over 1,000 Black
Vultures, with a maximum of 72 Black Vultures observed on one occasion. There are countless
dumpsters located throughout Immokalee and very rarely are the dumpsters covered properly.
While vultures at the Guadalupe Center roost can be encouraged to leave the site,
vultures will continue to exist in high numbers in the community due to human activities.
Changing agricultural practices (the elimination of large rotting vegetable piles), encouraging an
anti-litter behavior (closing trash receptacles and trash clean-up), and the removal of road-killed
animals found within city limits would help reduce local vulture populations. A typical vulture
diet consists primarily of carrion which is unpredictable and unevenly distributed, thus the
predictability of food (i.e. trash receptacles or culled vegetable piles) in the city must be
addressed.
57
Vandalism of management methods at the Guadalupe Center was an issue as well.
Individuals would often break into the Center’s property to use the pond for fishing. The
individuals would knock down the silt fencing that surrounded the pond. The silt fencing was
installed to prevent vultures from gaining access to the water and vultures were able to access the
pond after vandalism. In addition to the vandalism, individuals that broke into the Center
property would also leave litter and fish remains around the pond, providing the vultures with
more foraging opportunities. Community awareness of the vulture problems within the city
limits should be addressed. The Guadalupe Center could have signs attached to their fences
explaining the management and importance of keeping the vultures off of the property.
What made the Guadalupe Center attractive as a roost site?
The vultures’ use of the Guadalupe Center as a roost site was not due to one singular
reason or feature of the property. The Guadalupe Center provided the vultures with all the
necessary needs for a relatively easy and comfortable existence, including food, water, perch
sites, areas for social interactions, and proximity to nesting and roosting areas. All of the features
together and their interconnectedness provided the ideal roost site for the vultures. If all of these
features had not been present together the vultures may not have occurred in such high numbers
on the property.
The property across from the Center had a few characteristics similar to that of the
Guadalupe Center property, but no vultures. One feature shared between the Guadalupe Center
and the nearby property was the presence of chain-link fences. The chain-link fences differed
from the Center’s in that the chain-links projected above the fences’ top support (Figure 22).
Vultures could not easily perch on these fences as compared to the Center’s fence.
58
Another feature shared between the Center and the other property was the presence of a
retention pond surrounded by trees. The retention pond at the other property had much steeper
slopes than the pond at the Guadalupe Center. The Guadalupe Center pond had shallow areas
that provided easier access for bathing and drinking for the vultures.
While the two properties shared the presence of fences for perching and a retention pond
that provided the birds with water, it did not have the combination of features that the Guadalupe
Center property had. The Center property provided the birds with not only perch sites and water,
but also food (dumpsters), areas for socialization (large mowed areas and roost tree habitat),
proximity to ideal nest habitat and roost trees (forested area north of the Center), and areas that
promoted the creation of thermals and obstructions currents (building architecture).
Figure 22: Fence located across from the Guadalupe Center property that possibly
discouraged vultures from using it as a perch site.
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Need for effective vulture management
Vultures have long been living in close association with humans, taking advantage of
their middens and waste. Human-vulture conflicts have been documented in as early as the 1800s
(Audubon 1967 [1840]). Audubon (1967 [1840]) described both Turkey and Black vultures as
being “half domesticated” in cities, taking advantage of human food scraps. In his account of the
Black Vulture, he described early vulture management of roofs and chimney tops. Individuals
would place spikes and pieces of glass on roofs and chimneys tops to prevent the roosting of
vultures in the southern United States (Audubon 1967 [1840]).
The need for information and research regarding Black and Turkey vulture roosts in
association with humans is recognized (Bruggers et al. 2002), but there have been few long-term
studies on vulture management. The importance of developing an understanding of vulture roost
management is crucial to continue on a path of positive coexistence between humans and
vultures.
If vulture management methods that result in positive outcomes for both humans and
vultures are not reached, lethal forms of management may be employed. Because of the
ecosystem services that Black and Turkey vultures provide and because they are long-lived
species (Clapp et al. 1982) with low reproductive rates, laying only two eggs per nesting season
(Jackson 1983, Stewart 1983), the use of lethal means to control them may have significant
negative consequences for the species.
Lethal management techniques have not been proven successful in dispersing vulture
roosts. In the Dutch Gap area of Chesterfield, Virginia, Black Vultures were routinely killed
from 2002-2006. Over 1,200 Black Vultures were killed, but vultures still continued to cause
significant property damage to the area (Virginia Department of Game and Inland Fisheries
60
2013). As of 2007, there were over 400 Black Vultures and approximately 30-40 Turkey
Vultures occupying the area. The local culling of vultures was ineffective at reducing the amount
of property damage caused by vultures due to the immigration of new vultures into the area.
Long-term studies on vulture demographics are needed to understand the effect of lethal methods
in vulture management. The lack of data on vulture demographics and the compliance with state
and federal regulations protecting these native birds have created difficulties (Runge et al. 2009)
and should discourage individuals from including lethal measures in their management plans.
The ability to effectively manage a large vulture roost will unquestionably lead to fewer
human-vulture conflicts. Less human-vulture conflicts will ensure not only the safety of vulture
populations, but result in less property and agricultural damage. Any knowledge as to why
vultures are attracted to certain areas will benefit not only the Guadalupe Center site, but other
areas that are experiencing similar vulture-related problems. Finding an economical and effective
way to deter the vultures from agricultural and personal property is essential for vulture
populations to continue to thrive.
The various management techniques employed at the Guadalupe Center roost site
dramatically reduced the number of vultures present on the property and affected their overall
use of the site. Persistence and consistency are keys to successful management of vulture roost
problems. Management should be site-specific and gradual. The roost at the Guadalupe Center
had several areas that were attractive to the birds, and each area needed to be addressed
individually to discourage vulture use of the property. Every area that was attractive to the
vultures required a different management method. While there are instances where each
technique of managing vultures has proven successful, none provide a conclusive solution for all
vulture management problems. Understanding site-specific and problem-specific parameters is
61
essential to developing effective management for vultures. In most cases, vulture roost problems
will need to be solved using a variety of techniques (Appendix D).
62
APPENDIX A
Black and Turkey vulture species information
Current and historical range. The family Cathartidae includes all extant species of New World
vultures. There are three species of New World vultures that occur within the United States;
Turkey Vulture (Cathartes aura), Black Vulture (Coragyps atratus), and the California Condor
(Gymnogyps californianus). Two of the species, the Black and Turkey vulture occur throughout
the state of Florida. Both Black and Turkey vultures act as frequent avian scavengers in the
southeastern United States, where they share similar habitats and food resources (Coleman and
Fraser 1987).
While both Black and Turkey vultures only occur in the New World today, fossil records
show that cathartids did exist in Europe from the early Eocene (Feduccia 1996) to the early
Miocene (Rich 1983). The earliest fossil records show that cathartids occurred during the early
Oligocene in the New World (Feduccia 1996), and fossils of both Black and Turkey vultures
have been found in lower Pleistocene deposits in Florida (Wetmore 1956). Based on fossil
records, both species of vultures have had extensive opportunities for interaction and coexistence
in the state of Florida.
Black Vultures are a resident species throughout much of their range, including Florida
(Buckley 1999). The Black Vulture range extends north to Pennsylvania, encompasses the
southeastern United States and extends south through South America (Buckley 1999). Within the
last seventy years, the Black Vulture range has shifted, moving farther north through the United
States (Greider and Wagner 1960). While Black Vulture populations have been increasing in the
United States since 1966 (Sauer et al. 1997), their populations are still at risk due to human
63
activities including trapping, shooting, poisoning, pesticide contamination, loss of habitat, and
frequent disturbance of nest and roost sites (Buckley 1999).
Turkey Vultures differ from Black Vultures as they are a highly migratory species.
Populations in south Florida are augmented by migrating birds as early as October through the
end of winter (Stevenson 1970). The Turkey Vulture range extends north into southern Canada
and extends south through southern South America (Kirk and Mossman 1998). During the winter
months, Turkey Vultures migrate to the southern parts of their range which include the
southeastern United States, western California, the eastern Caribbean islands, Mexico, Central
America, and South America (Kirk and Mossman 1998). Turkey Vulture populations are
increasing, along with their breeding range extending farther north (Mossman 1991), but they
face similar population risks as Black Vultures.
Ecosystem services. Black and Turkey vultures play an essential ecological role in the
environment. They are the only living obligate terrestrial vertebrate scavengers (Houston 1979,
Ruxton and Houston 2004). Vultures are key scavengers, signaling (by their presence) to other
terrestrial vertebrate scavengers, where animal carcasses are located (Houston 1979).
Because vultures act as the “garbage disposals” of our ecosystems, they and other
scavengers are fundamental to terrestrial ecological communities. While performing ecosystem
service, vultures, other vertebrate scavengers, insects, other invertebrates and fungi assist in
nutrient cycling by quickly disposing of decaying carcasses and thus reduce the potential spread
of disease (Markandya et al. 2008).
A vulture’s ability to dispose of diseased carcasses is an ecosystem service that is
essential for the health of humans and other animals worldwide. Bacteria associated with
diseases such as anthrax (Darling and Bates 1912) and botulism (Kalmbach 1939) are destroyed
64
in the digestive tracts of vultures. If vultures were not present, these carcasses would decompose
slowly, thus facilitating the spread of these pathogens. Most wild animals do not die because of
predator-prey interactions, but from such things as disease, parasites, starvation, and a variety of
other non-predatory related causes (Houston 1979).
The measurement of metals in feathers may allow birds to act as bioindicators for the
health of many different ecosystems (Burger 1993). The position of vultures at the top of the
food chain and their ability to bioaccumulate heavy metals allow vultures to act as biological
indicators of pollutants in the environment (van Wyk et al. 2001).
Foraging and diet. Vultures feed on a wide variety of both domestic and wild carrion (Coleman
and Fraser 1987), and exploit the high numbers of road-killed animals found throughout the
United States (Sprunt 1937, Thiel 1976, Stewart 1978). Vultures typically prefer to feed on fresh
carrion (Rapp 1943, Houston 1986); however they cannot always do so because of the toughness
of the skin; therefore they either wait for a carcass to decompose further, or wait for the
assistance of other carrion feeders (Wallace and Temple 1987).
Black and Turkey vultures share similar food resources, but their food-finding abilities
differ greatly. Turkey Vultures differ from Black Vultures in that they use their highly developed
olfactory organs to find their food primarily by smell (Bang 1964, Stager 1964). Turkey Vultures
prefer to feed on smaller food items, are less likely to feed near humans, and are more often to
feed in smaller groups or alone (Kirk and Mossman 1998). They are less aggressive than Black
Vultures and are often outcompeted at food sources (Kirk and Mossman 1998).
The Black Vulture diet is highly variable when compared to the Turkey Vulture; it
includes carrion, vegetative matter (McIlhenny 1945, Buckley 1996), and even in extreme cases
live prey (Sprunt 1946, Lovell 1947, Jackson et al. 1978). Black Vultures rely heavily on sight as
65
well as on Turkey Vultures to find carrion (Buckley 1996, 1997). The ability of the Turkey
Vulture to use its heightened sense of smell often involves Black Vultures following them to
carcasses (Buckley 1996). Turkey Vultures typically fly at lower heights where their olfactory
senses are the strongest and Black Vultures soar much higher looking for Turkey Vultures on or
near a carcass (Buckley 1996).
A dominance hierarchy exists between and within species of vultures at carcasses
(Wallace and Temple 1987). Turkey Vultures almost always arrive at a carcass first because of
their superb sense of smell (Kirk and Houston 1995). Black Vultures are far more aggressive
than Turkey Vultures and will often displace Turkey Vultures from a carcass (Stewart 1978,
Wallace and Temple 1987, Buckley 1997). While a large group of Black Vultures may displace a
single Turkey Vulture, a solitary Black Vulture will have a more difficult time getting rid of a
Turkey Vulture (Wallace and Temple 1987, Houston 1988, Buckley 1996). Because large groups
of Black Vultures can easily displace Turkey Vultures (Stolen 1996) Turkey Vultures will
typically feed on smaller carcasses (Stewart 1978). Feeding vultures will seldom tolerate more
than one vulture at a carcass (Prior and Weatherhead 1991). Typically one vulture will feed
while other vultures wait close by (Kirk 1988).
For vultures that scavenge in groups, the potential advantages presumably outweigh the
costs. There are definite disadvantages to feeding in a large group such as the obtainment of less
food for individuals (Lindström and Ranta 1993). There are also advantages of foraging in large
groups such as increased alertness for predators (Clark and Mangel 1986) and the possible
assistance in opening a carcass (Wallace and Temple 1987).
Communal roosting. Black and Turkey vultures both roost communally and will often do so
together; communal roosts are important to many species of birds, not only vultures during at
66
least part of their annual cycle (Beauchamp 1999). The communal roost is a significant aspect in
the social behaviors of vultures. Communal roosts may be permanent (roosts used yearly),
seasonal (roosts formed during breeding season), or temporary based on location to food sources
or migration (Sweeney and Fraser 1986). Black Vultures may use the same communal roost site
for decades (Prather et al. 1976). The number of individuals in Black Vulture roosts varies and
may even contain up to thousands (Prather et al. 1976). Turkey Vultures often will roost with
Black Vultures at a communal roost site increasing the number of individuals. Turkey Vultures
will form single species roosts of up to 300 individuals (Kirk and Mossman 1998).
While it is known that vultures and other birds communally roost, there are several
hypotheses as to why birds choose to roost communally. Potential advantages of communal
roosting include, but are not limited to: thermoregulatory benefits (Brenner 1965), reduced
predation risks (Lack 1968), transfer of information regarding foraging (Ward and Zahavi 1973),
reduced travel distances between daily activity centers (Caccamise and Morrison 1986), and the
dispersion of foragers to reduce intraspecific competition (Chapman et al. 1989). Communal
roosts act as a gathering place for both adult and juvenile vultures (Buckley 1999). There are
disadvantages of communal roosting as well. There may be increased competition for food
(Alexander 1974), mates (Hoodgland and Sherman 1976) and preferred roost sites (Swingland
1977). Vultures use roosts to communicate and socialize. Vultures do not have a syrinx or voice
box so gathering in large groups is essential for communication.
Breeding and nesting. Black and Turkey vultures do not build nests. They lay their eggs,
usually two, on the ground, in abandoned buildings, caves, or hollow trees (Jackson 1983). Black
and Turkey vultures differ somewhat in nesting locations because Turkey Vultures are solitary
nesters and Black Vultures are known to nest in close proximity to others (Jackson 1983).
67
Black Vultures maintain long-term pair bonds and are monogamous (Decker et al. 1993).
They will reproduce annually (Rabenold 1983) once sexual maturity has been reached. Prior to
nesting, Black Vultures will perch together near the nest site before the nesting begins (Jackson
1983). Black Vultures may begin nesting as early as February and as late as May. The peak of
egg-laying occurs from late February to the end of April (Buckley 1999). Black Vulture pairs
may continue to use the same site for nesting for years if breeding is successful (Rabenold 1983).
In Florida, both parents will incubate the eggs for a period of 28-30 days and young will fledge
the nest at approximately 98 days (Jackson 1983). The birds may have young on the nest as early
as late March and as late as early August (Buckley 1999). The peak months for young on the nest
are from early April until early July (Buckley 1999). Young are long-term dependents on their
parents (Jackson 1975) and also may associate at communal roosts and while foraging after
fledging (Rabenold 1986).
Turkey Vultures are most likely monogamous and will mate for life (Buckley 1999).
Turkey Vultures differ from Black Vultures in breeding season. Turkey Vultures will lay eggs
from mid-April until early July with the peak season occurring from the end of April to the end
of May (Kirk and Mossman 1998). Turkey Vultures may have young on the nest from late May
to mid-September with the peak season occurring from late May to mid-August (Kirk and
Mossman 1998). Turkey Vulture incubation periods are long and similar to those of the Black
Vulture. Turkey Vultures most likely breed annually, similar to Black Vultures, once sexual
maturity is reached (Jackson 1983). There is probably a long-term dependency of young to the
parents, comparable to the Black Vulture (Jackson 1983).
Habitat use. The habitat of the Black and Turkey vulture differs based on use. The nesting
habitat of both vultures should include forested areas or partially forested areas with nest sites,
68
such as hollow trees, abandoned buildings, or other dark recesses (Kirk and Mossman 1998).
Undisturbed sites are preferable since human disturbance can reduce breeding success (Jackson
1983).
The preferred roosting habitat of vultures should include areas with large trees (Davis
1983b) and areas that promote the creation of thermals and updrafts for flight (Thompson et al.
1990). Typically, roosts will be found near some source of water (Prather et al. 1976).
Throughout the United States vultures have been known to roost on anthropogenic structures,
such as communication towers (Evans and Sordahl 2009) and buildings (Audubon 1967 [1840]).
Foraging habitat should include open areas, such as fields or pastures where food can be
easily found visually for Black Vultures or by scent for Turkey Vultures (Kirk and Mossman
1998, Buckley 1999). The preferred habitat of both vultures should be a mixture of open areas
for foraging; areas that include remote nest sites, such as dense thick woodlands, hollow trees, or
abandoned buildings; and large stands of tall trees for roosting (Kirk and Mossman 1998,
Buckley 1999).
69
APPENDIX B
Problems associated with vulture use of the Guadalupe Center property
The main difficulty associated with the Guadalupe Center roost was finding a way to stop
the vultures from creating the problems they were causing on the property. Ultimately and
optimally this required encouraging the birds to abandon the roost.
To effectively manage the problems, all areas that the vultures occupied on a daily basis
needed to be addressed.
The problems associated with the vultures on the property included:
1. Fences. The chain-link fences that surrounded the Center’s property provided the vultures
with access to abundant perch sites.
2. Drainage canal. A drainage canal ran parallel to the fence immediately adjacent to the north
side of the Guadalupe Center’s property and the forested wetland (Figure 3). The canal
ran east-west, creating an almost effortless flight path for the vultures due to the
channeling of the prevailing winds. The vultures on occasion used the canal for bathing
and drinking. The canal also provided a food source for the vultures when it dried
seasonally; invertebrates and fish that lived in the canal were isolated in smaller pools
and were easily caught or died as the pools dried.
3. Pond. A pond located immediately to the west of the Center building was used by the vultures
for drinking and bathing. Vultures were present in lower numbers during the wet season
suggesting that the pond may be the reason the vultures were on the property.
4. Building architecture. The architecture of the Center building encouraged the perching of
vultures on the roof and surrounding structures. The Center roof generated thermals that
facilitated nearly effortless flying for the vultures. Vultures damaged the roof by tearing
70
shingles with their bills and through the high levels of excrement deposits on the
building. There were five skylights located at the peak of the Center’s main roof. The
vultures perched on the skylights resulting in excrement accumulation and scratching of
the plastic.
At the west end of the building, where most the birds congregated, the vultures
took advantage of an obstruction current (Figure 23). The eaves of the building did not
overhang which encouraged the formation of obstruction currents; wind hitting a vertical
object and shunting upwards. Without the overhanging eave the wind was shunted
vertically, creating an “elevator” for the birds, allowing for flight with minimal energy
expenditure.
Figure 23: Black Vulture using obstruction current created by the architecture of the
building.
71
There were definite patterns to where the vultures perched on the buildings based
on the location of whitewash from fecal matter (Figure 24A). Vultures typically perched
along the edges or peaks of the buildings and awnings (Figure 24B). The peak of the
buildings facilitated easy takeoff for the birds. Birds perched there could take advantage
of wind from any direction. Some of the roofs at the Center had lightning rods at the
peak (Figure 24). Vultures were unable to perch on the apex of roofs with lightning rods
suggesting that bird spikes could prevent vultures from perching on other Center roofs.
The nature of the Center’s roofs contributed to the vulture problem as well. The
rough asphalt shingles that covered all of the rooftops provided the vultures with a
surface that promoted traction and allowed for ease of movement on the roof as well as a
firmer footing for takeoff and landing. The steepness of some of the roofs (8/12 pitch;
34o) (Figure 24A) also discouraged perching. The Center’s main roof was low sloping
(3/12 pitch; 14o) allowing the vultures access to abundant perch sites (Figure 7).
72
A. B.
Figure 24: A. Vultures preferred to roost at the peaks and edges of roofs as can be seen
by the pattern of excrement. B. Vultures were able to take off more easily from the edges
and peaks of the roof. The lightning rods prevented the vultures from perching on top of
the roofs.
73
5. Light poles. Eleven light poles located to the south of the Center building in the parking lot
were used by the vultures for perching. The light poles and light shields were covered in
fecal matter due to the consistent perching of the vultures (Figure 25). Of the 11 light
poles in the Guadalupe Center parking lot, only the four most westerly poles were used
by the vultures. These poles were the nearest to the obstruction current created by the
building architecture and closest to the retention pond.
Figure 25: Vultures used the Center’s light poles and light shields for perching.
74
6. Air-conditioning units. An air-conditioning unit and enclosure located immediately to the
west of the Center building was used as a perch site for the vultures (Figure 26A). A low
concrete wall that surrounded the air-conditioning unit attracted the vultures for
additional perch sites and was covered in fecal matter. Vultures destroyed pipe insulation
and feathers were drawn into the air-conditioning unit filters (Figure 26B).
A. B.
Figure 26: A. Vulture perching on air-conditioning unit and enclosure. B. Damage caused
by vultures ripping apart the installation around the air-conditioning unit pipes.
75
7. Mowed areas. Vultures were consistently present and engaged in maintenance and social
behaviors in a mowed area located directly north of the pond (Figure 27).
Figure 27: Vultures used a mowed area north of the pond for social and maintenance
behaviors.
8. Forested area. A forested area located north of the Center was an attractive roost site for the
vultures. The vultures used the snags (standing dead trees), pines, and cabbage palms for
perch sites. The cabbage palms that the vultures used for roosting had tops that had been
flattened by the roosting of the vultures, thus further increasing their suitability as vulture
perch sites. The area was immediately adjacent to, but not located on the Guadalupe
Center property.
76
9. Water pump. A water pump located east of the pond and west of the Center building was
used to maintain the water levels of the retention pond. Water levels were maintained to
avoid thermal stratification and minimize algal blooms and the resuspension of
previously deposited materials. Water spewing from the pump provided an additional
water source for the vultures (Figure 28).
Figure 28: Water from the water pump allowed vultures to have additional access to a
water source.
77
10. Irrigation sprinklers. The presence of irrigation sprinklers on the Center property provided
the vultures with additional water sources. Sprinklers would often run all day and create
streams and puddles for the vultures to bathe in and drink from (Figure 29).
Figure 29: Vultures using a puddle created by excessive irrigation of the lawns at the
Guadalupe Center.
78
APPENDIX C
Ineffectiveness and limitations in the implementation of management techniques at the
Guadalupe Center roost site
The immediacy of the vulture roost problem at the Guadalupe Center resulted in part to
the ineffectiveness of some of the management techniques. To generate true experimental data,
the roost should have been observed for longer than one week prior to the start of management in
order to collect more baseline data. Data on behavior and precipitation (and other abiotic factors)
was limited due to the immediacy of the vulture roost problem. Abiotic data was not included in
the study due to the lack of preliminary data and due to simultaneous management efforts.
The implementation of some of the techniques by Center personnel did not allow for the
collection of precise experimental data during management. Several suggestions were provided
to the Center personnel on how to adequately manage for vultures in each area of the Center’s
property. Occasionally, management techniques were not conducted as instructed or conducted
simultaneously with other management techniques, at times resulting in the prevention of true
experimental data.
Center employees would also sometimes use management techniques that were not
suggested in hopes of deterring the vultures more quickly, in those cases I incorporated
responses of the vultures to their management efforts into data collection efforts to document the
possible impact of those management techniques.
The limitations and effectiveness of the vulture management techniques as employed are
discussed below.
1. Fence management. Vulture fecal matter can be highly corrosive (Figure 26), therefore the
prevention of vultures from perching on the fences was critical. The vultures were
79
attracted to the fences because there were abundant perches that provided them with an
area to rest, socialize, preen, sun and easily takeoff.
Prior to my involvement at the Center, motion-activated sprinklers were installed
on the fences. The sprinklers did work to deter vultures from perching, but often broke
down. If sprinklers were not maintained, vultures would readily habituate when the
sprinklers no longer emitted water and the birds would continue to use fences as perch
sites.
Prior to the installation of the monofilament, a thin wire was stretched above the
fences (Figure 30A). Vultures were able to stretch the wire and enable perching. Due to
the ineffectiveness of the wire, the Center installed a monofilament line above the fence
to prevent birds from perching on the fences (Figure 30B). The monofilament line broke
easily and was not resistant to the excessive heat associated with Florida’s subtropical
climate.
The placement of the monofilament above the fences was somewhat ineffective in
preventing vultures from perching as well. Vultures were sometimes able to push the
monofilament up and perch beneath it.
80
A. B.
Figure 30: A. Damage caused by vultures excessively perching and defecating on the
Center’s fences. B. Vultures were able to push down and stretch a thin wire.
2. Pond management. The management of the pond was important in making the site less
attractive to the vultures. After the initial installation of the fencing around the north side
of the pond, most birds congregated away from the pond or on the south side that had
open water suggesting that silt fencing would adequately prevent vultures from using the
pond.
After the initial installation, silt-fencing was installed around the entire perimeter
of the pond to prevent vultures from using the water. The fence was not placed directly at
the waterline to prevent vultures from using the water. Pond levels would increase and
decrease during the wet and dry season, thus the fence needed to be moved. Silt-fencing
was often underwater or would have dry areas exposed allowing vultures access to the
water.
3. Roof management. The architecture of the Guadalupe Center encouraged the high number of
vultures roosting on the property. The birds were concentrated on the west end of the
Center’s main building where the presence of a vertical updraft occurred. The eaves on
81
the Center building did not overhang promoting the creation of the obstruction current. If
the eaves did overhang, the upward flowing air would hit the eave and be pushed outward
horizontally, resulting in little vertical airflow.
The prevention of the vultures from easily using the obstruction current created on
property was important. Bird spikes were installed along the edges and ridges of the main
roof. The bird spikes were not adhered properly to the roof and often detached from the
shingles and were not replaced.
In addition to the bird spikes, two motion-activated sprinklers were installed on
the Center roof due to the ineffectiveness of the bird spikes. The sprinklers on the
Center’s roof were battery operated and would often break down allowing the vultures to
habituate to non-working sprinklers.
4. Light pole management. Vultures used light shields and light poles for perching on the
Center’s property. The bird spikes attached to the top of light shields prevented vultures
from perching on the shields, although vultures were still able to perch on top of the pole.
Overall, bird spikes were effective in deterring the vultures from perching at the specific
site where the spikes were affixed.
5. Air-conditioning unit management. It was important to discourage vultures from using the
concrete wall that surrounded the air-conditioning units as a perch site. Black Vultures
are known to peck and rip apart rubbers seals, leather boat cushions, and other similar
structures. At the Guadalupe Center, Black Vultures ripped apart and destroyed foil
wrapped insulation around pipes leading to the air-conditioning unit. The vultures were
likely attracted to the foil insulation because they often will rip apart foil wrappers found
in dumpsters and along roads and find food inside. They were attracted to the vibrations
82
of the pipes as well as they are similar to the vibrations produced by dying animals. After
the installation of the monofilament line on the fences, most vultures stopped perching on
the enclosure around the air-conditioning units, but no direct management was
implemented to the enclosure or unit.
6. Management of mowed area. The monofilament line that was stretched across the mowed
area north of the Center did prevent the high numbers of vultures from using the area.
Often lines were found broken and were never replaced, allowing the vultures to continue
to use the area.
7. Management of forested area north of Center property. The Center did not own the
forested area north of their buildings, thus no direct management could be implemented.
8. Water pump management. The Center personnel extended the pipe from the water pump to
the pond to prevent the vultures from using the pump as an additional water source. The
pipe was never fully extended to the water line, thus vultures were able to still access the
water to some extent. It was critical to prevent vulture access to water on the Center
property.
9. Irrigation sprinkler management. The regular irrigation system at the Center sometimes
provided a water source for the vultures. The Center was advised to stop the use of the
sprinklers during the day or to run the sprinklers less often to discourage vulture use.
When irrigation sprinklers were allowed to run continuously for long periods of time,
vultures were present in high numbers. They bathed in the “shower” provided by
sprinklers and in puddles created by the sprinklers.
83
APPENDIX D
Future management suggestions for the Guadalupe Center roost site and other problematic
vulture roosts
A variety of management techniques to discourage vulture use of the property were
suggested to the Guadalupe Center. In some cases the Center was unable or did not follow
through with management suggestions. These suggestions are listed below and may be
considered in future management efforts.
1. Fence management. The thin wire and monofilament line installed at the Center were not
adequate in preventing vulture use of the fences. Future management should include a
wire that can withstand a variety of environmental conditions and is not susceptible to
stretching in lieu of monofilament line. Additional lines of wire may be needed to further
prevent vulture perching. Two wires should be installed above the fence, one on either
side, in order to prevent the birds from pushing the line to one side and perching.
Staggering the two lines may be beneficial to prevent even further perching underneath
the line.
If the installation of wire is not possible, the planting of vegetation along fences
may prevent the vultures from using the fences. A fence located north of the Center
building had a hedge that grew above the fence line; vultures did not use this fence for
perching. The Center and other individuals that own fences that are susceptible to
perching vultures, could plant hedges adjacent to fences to prevent vulture perching.
The location of fences is important in vulture management as well. The fence
located north of the Center’s playground was not used by the vultures. The vultures most
likely did not make use of this fence due to the shade from nearby trees, distance to water
84
source, and the bend in the canal that prevented wind-tunneling from creating an
effortless flight path for the vultures.
2. Pond management. If the installation of fencing around a water source is not an option
individuals could encourage the growth of tall vegetation along the water’s edge. On the
northwest side of the Center’s pond there was a heavy concentration of cattails. Vultures
were never observed inside or in front of the cattails suggesting that the vegetation acted
as a natural barrier between the vultures and the water source.
Vultures also frequented the areas around the pond that were shaded by trees. To
discourage further use, trees could be pruned to allow more sunlight and decrease the
amount of shade available to the vultures.
Future property owners with retention ponds used by vultures may be able to
discourage vulture bathing by eliminating shallow areas for bathing when other
management options prove ineffective. In doing so they might thus reduce, but not
eliminate attraction of the site to the vultures. Even with deep water, if the vultures have
access, the water might still be an attractant for drinking and possibly for foraging
(Jackson et al. 1978). It is important to consider multi-species management when altering
habitats. The shallow areas attract vultures, but they also provide refugia for fish and
foraging opportunities for wading birds, such as herons and egrets.
3. Roof management. The Center installed bird spikes and motion-activated sprinklers to deter
the vultures from occupying the roof. Both the bird spikes and sprinklers required a high
amount of maintenance. Future management efforts could include the placement of a
vegetative barrier to the west of the building to prevent the creation of the obstruction
85
current. A vegetative barrier, like a hedge would absorb the wind and discourage
additional vulture use of the roof.
Other property owners, who want to discourage vulture use, should construct
buildings with overhanging eaves or plant hedges to absorb wind to prevent the creation
of obstruction currents on their property.
The type of materials used on the roofs may contribute to vulture use of rooftops
as well. The Center roof was covered in asphalt shingles. A metal roof would not provide
the traction needed for the vultures to take off and land. It seems likely that the damage
caused by the vultures would have been less if the roof had been metal. Vulture
excrement could have been removed easily and without further damage caused by the
removal, and the vultures would not have been able to pick apart the roof as they did with
the asphalt shingles.
4. Light pole management. The placement of bird spikes on the light shields did prevent from
vultures perching, however no management occurred on the light poles. Future
management could include the placement of a plastic funnel or small section of bird
spikes on the top of the poles to prevent perching.
5. Air-conditioning unit management. Center personnel were encouraged to enclose the air-
conditioning units to prevent vulture damage, but this was never implemented. Future
management should include the placement of bird spikes along the enclosure or the
complete enclosure of the air-conditioning unit to prevent damage.
6. Management of mowed area. Management of the mowed area included the placement of
monofilament line near the ground to prevent vulture use of the area. Monofilament was
often broke due to the mowing of the lawn. Future management could include stretching
86
monofilament or wire from the fence top to the trees, allowing mowers to access the area
without breaking lines.
An inexpensive solution to preventing vulture use in these areas would be to stop
mowing the grass. There were often times when the Center would not mow the grass for
weeks. When the grass was allowed to grow long, vultures did not occupy the area
because it was difficult to see and maneuver through the longer vegetation.
7. Management of forested area north of Center property. No management efforts could be
initiated in an effort to deter the birds from these roosts since the property was not owned
by the Center. In extreme cases, future management could include habitat modification.
The trees used most often for roosting could be cut down to prevent vulture roosting.
Since the trees were not on Center property, management was not specifically conducted.
The use of the cabbage palms by the vultures suggests that the vultures are capable of
creating their own roost tree sites to some extent.
8. Water pump management. Future management at the Center should include the full
extension of the pipe to the pond, preventing vulture use of the water on the Center
property.
9. Irrigation sprinkler management. The Center and other property owners should use the
irrigation sprinklers for shorter periods of time or after dark; vultures would not be able
to take advantage of this as an additional water source.
87
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