2009 san diego river brown-headed cowbird trapping program

2009 San Diego River Brown-Headed Cowbird Trapping Program

Prepared for: San Diego River Conservancy

1350 Front Street, 3024 San Diego, CA 92101-3604

Prepared by: Jennifer Sexton

TW Biological Services 1119 Oriole Circle

Fillmore, California 93015

February 2, 2010

2009 San Diego River Brown-headed Cowbird Trapping Program, Sexton

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INTRODUCTION A Brown-headed Cowbird trapping program was operated by TW Biological Services along the San Diego River in Mission Trails Regional Park in 2009. Trapping was conducted in an effort to reduce the threat and impact of Brown-headed Cowbird (Molothrus ater) brood parasitism on the Least Bell’s Vireo (Vireo bellii pusillus) and to provide a first year of data in support of a study being conducted through the USGS Western Ecological Center by Dr. Barbara Kus: “Distribution, Abundance, And Breeding Activities Of The Least Bell’s Vireo Along The San Diego River, California”. This work was funded by the San Diego River Conservancy

The Least Bell’s Vireo is a federally endangered migratory songbird species that nests in riparian habitat along the San Diego River, San Diego County, California. The brown-headed cowbird (Molothrus ater) is an obligate brood parasite which lays its eggs in the nests of other bird species including the vireo and is dependent upon the host to incubate their eggs and rear their young. Averaging 6”-7” in length, the brown-headed cowbird is a medium sized songbird with sexually dimorphic plumage. Adult males are gloss black with a brown head and neck. Females are slightly smaller than males and dull tan to light brown with indistinct streaking on the breast. Originally restricted to the midwest region of North America, the brown-headed cowbird expanded in both range and abundance following the settlement and alteration of natural habitats, particularly with the increase in agriculture and livestock production. Reaching California in the late 1800’s, this species was first documented breeding in San Diego County in 1915 (Unitt 1984), and had become well established within southern California by the 1930’s (Willett 1933, Rothstein 1994). Songbird species or populations that did not evolve with the cowbird and have no experience with parasitism may be subject to significantly reduced reproductive success. Brood parasitism combined with other impacts, such as habitat loss and fragmentation can lead to declines in host species, particularly those with an already limited population and distribution.

Cowbird control through breeding season trapping programs has proven to be an effective

method in the conservation efforts of sensitive songbird populations throughout the United States, and was initially utilized in the recovery efforts of the Kirtland’s warbler (Dendroica kirtlandii) in Michigan (Mayfield 1977). Subsequently, cowbird trapping has become an important tool in the management of several other sensitive songbird species, including the black-capped vireo (Vireo atricapillus) (Eckrich et al. 1999), least Bell’s vireo (Kus and Whitfield 2005), and southwestern willow flycatcher (Empidonax trailli extimus) (Kus and Whitfield 2005). PROJECT AREA The project area is located along the San Diego River between Old Mission Dam and Mast Boulevard within Mission Trails Regional Park, San Diego County, California (Figure 1). Vegetation communities within the project area include mixed willow riparian forest, cottonwood-willow riparian forest, southern willow scrub, mulefat scrub, and annual grassland. Surrounding habitat includes areas of coastal sage chaparral.


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Figure 1. Project location, Mission Trails Regional Park, San Diego County, California.

METHODS

On April 25, 2009 three cowbird traps supplied by Mission Trails Regional Park were set-up and activated within or near riparian habitat along the San Diego River within the park, between Old Mission Dam and Mast Blvd., to facilitate the best possible protection for breeding vireos (FigureA-1


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and A-2, Appendix A). Trapping operations were performed by Jennifer Sexton, Tommie Ebanez, Ian Maunsell, and Anna Schochenmaier.

Traps were furnished with fresh water, seed, perches, shade, and decoy cowbirds. All traps were

supplied with two male and three female cowbirds on April 25. The right primary wing feathers of both male and female decoy cowbirds were clipped for identification and prevention of accidental escape or release back into the wild. This practice also greatly diminishes their likelihood of survival in the wild. A sign was placed on each trap providing trap information and contact phone numbers. Global Positioning System (GPS) coordinates for all trap locations were recorded with a Garmin 276C hand held GPS unit. Coordinates were recorded in World Geographic System 1984 (WGS 84) (Table 1).

Traps were checked on a daily basis from April 25 through July 30. This was done to record

trap capture events, release non-target species incidentally captured, add or remove cowbirds to maintain the 2:3 (male:female) decoy ratio, provide fresh seed and water, and repair trap damage if needed. Information recorded for all newly captured cowbirds included capture location, date, sex, and age. Newly captured cowbirds not utilized as decoys, were removed on a daily basis and humanely euthanized. European Starlings (Sturnus vulgaris) and House Sparrows (Passer domesticus) caught in the traps were also euthanized in accordance with U.S. Fish and Wildlife Service (USFWS) cowbird trapping protocol. All other non-target birds captured were released unharmed at the trap sites. On July 30 all traps were de-activated and returned to the MTRP storage facility.

Table 1. 2009 Trap locations, Mission Trails Regional Park, San Diego County, CA.

Trap ElevationNo. GPS Y GPS X (m)

1 32.84029 117.04273 882 32.84249 117.03761 913 32.84051 117.02784 90

WGS 84 Coordinates

RESULTS Cowbird Captures A total of 50 cowbirds were captured between April 25 and July 30, 2009, including 38 males and 12 females (Table 2). The first male and female cowbird capture occurred on April 26. No juvenile cowbirds were captured during the 2009 trapping period. There were 291 actual trap days out of a potential of 291. Total trap days are calculated by multiplying the number of traps by the number of days they are in operation, then subtracting the number of days individual traps are inactive for various reasons such as vandalism. A few incidences of vandalism to the traps occurred during the project period. One male and one female bait bird escaped as a result but the female was subsequently recaptured. No trap days were lost as a result of the vandalism. With 50 cowbird captures, there were 0.17 cowbirds captured per trap day during the 2009 project period. The ratio of male to female captures during 2009 was 3.2:1. Individual trap performance ranged from nine cowbirds captured in Trap 2 to 27 cowbirds captured in Trap 3 (Table 2). There were no banded cowbirds captured during the 2009 trapping period.


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Table 2. Weekly cowbird captures by trap from April 25 through July 30, 2009.

TRAP-1 TRAP-2 TRAP-3

Mal

eFe

mal

eJu

veni

leM

ale

Fem

ale

Juve

nile

Mal

eFe

mal

eJu

veni

le

1 25 Apr. -1 May 4 3 0 1 0 0 13 2 0 232 2-8 May 3 0 0 3 1 0 3 2 0 123 9-15 May 0 0 0 0 1 0 2 0 0 34 16-22 May 1 0 0 1 0 0 0 0 0 25 23-29 May 0 0 0 0 0 0 0 0 0 06 30 May -Jun.5 0 0 0 1 0 0 0 0 0 17 6-12 Jun. 1 0 0 0 0 0 0 0 0 18 13-19 Jun. 0 0 0 0 0 0 0 0 0 09 20-26 Jun. 0 0 0 0 0 0 0 0 0 0

10 27 Jun. - 3 Jul. 1 0 0 1 0 0 0 0 0 211 4-10 Jul. 0 1 0 0 0 0 0 0 0 112 11-17 Jul. 0 0 0 0 0 0 3 2 0 513 18-24 Jul. 0 0 0 0 0 0 0 0 0 014 25-30 Jul. 0 0 0 0 0 0 0 0 0 0

10 4 0 7 2 0 21 6 05027

Capture TotalTrap Total

Wee

kly

Tot

al

Wee

k N

o.

Date

14 9

0

2

4

6

8

10

12

14

16

18

20

25 A

pr. -

1 M

ay

2-8

May

9-15

May

16-2

2 M

ay

23-2

9 M

ay

30 M

ay -J

un.5

6-12

Jun

.

13-1

9 Ju

n.

20-2

6 Ju

n.

27 J

un. -

3 J

ul.

4-10

Jul

.

11-1

7 Ju

l.

18-2

4 Ju

l.

25-3

0 Ju

l.

Week

Cow

bird

s C

aptu

red

MalesFemalesJuveniles

Males 18 9 2 2 0 1 1 0 0 2 0 3 0 0 Females 5 3 1 0 0 0 0 0 0 0 1 2 0 0 Juveniles 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Figure 2. Brown-headed Cowbirds captured per week in 2009.


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Non-Target Species

There were five capture events of three non-target species (Table 3). There were no non-target bird deaths during the 2009 trapping period. All non-targets captured were released unharmed at the trap sites.

Table 3. Non-Target Species Capture Events from April 25 through July 30, 2009.

Species Capture Events California towhee Pipilo crissalis 3 house finch Carpodacus mexicanus 1 song sparrow Melospiza melodia 1

Total 5 Trap Vandalism

There were two incidences of vandalism during the 2009 trapping period. On May 19 Trap 3 was found with three bolts and nuts missing from one of the top panels of the trap. The bolts had been unscrewed and removed from the trap. One male and one female bait bird had escaped but two females were still present in the trap. The bolts and nuts were replaced the same day and an extra nut was added to the bolts to prevent this from reoccurring. The escaped female from this trap was subsequently recaptured. On May 28 Trap 2 was found with the door lock hasp bent and partially broken off the door and a large dent kicked in the side panel wire. All of the bait birds were still present within the cage. The door was fixed the same day. Parasitism Least Bell’s vireo nest monitoring was conducted within the project area by the U.S Geological Survey, Western Ecological Research Center in 2009. There were five confirmed instances of cowbird parasitism of least Bell’s vireo nests within the project area during the 2009 nesting season (Lynn et al 2010). Parasitism occurred in 16 percent (5/32) of completed vireo nests in the project area. Only two of these failed as a result of the parasitism. Three of the five parasitism events occurred before traps were in operation. The dates of these parasitism events were between April 13 and 23. Two parasitism events occurred during the trapping period (May 4 and June 22). Both of these nests failed to fledge young, however only one failed as a result of parasitism. DISCUSSION & RECOMMENDATIONS A total of 50 cowbirds were removed from the San Diego River trapping area during the 2009 trapping program. The removal of cowbirds from the target area during the breeding season reduces the incidence of parasitism and consequently increases productivity of the vireos in that area. There were five confirmed instances of cowbird parasitism of least Bell’s vireo nests within the project area during the 2009 nesting season.. Only two of these instances occurred during the Cowbird Trapping period.

In San Diego County, cowbird populations are at their highest levels during spring and fall

migration, with peak numbers occurring between April 1 and April 15, and again from early August


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through September (TW Biological Services unpub. data). During these periods there is considerable overlap of both migrant and breeding residents, as well as subspecies. Vireos initiate nesting in San Diego County and are vulnerable to parasitism in early April. Though a large percentage of cowbirds captured during the beginning of April are likely migrant or transient individuals which may not be breeding residents within the targeted trapping project area, the initiation of cowbird trapping on April 1 is important for the protection of the vireos. Activating traps on April 1 maximizes cowbird captures and provides optimum protection of the vireo from breeding resident cowbirds. Consideration should be given to beginning trapping on April 1 in the future.

All three traps performed well during the 2009 trapping period. Trap coverage appears to be appropriate; individual trap performance ranged from nine cowbirds captured in Trap 2 to 27 cowbirds captured in Trap 3. It would be beneficial for trapping to continue in the 2010 season with the 3 traps operated between Mission Dam and West Hills Pkwy, beginning April 1. Trap placement in the 2010 trapping season should remain in the same general locations as in 2009. References Cited Collier, G. 1989 Western San Luis Rey River Brown-headed Cowbird Trapping Project. Unpublished

report prepared for U.S. Army Corps of Engineers, Los Angeles District. Department of Biology, San Diego State University, San Diego, CA. 1989.

Eckrich, G. H., T.E. Kolozar and M.D. Goering. 1999. Effective landscape management of Brown-

headed Cowbirds at Fort Hood Texas. pp. 267-274. In: Research and Management of The Brown-headed Cowbird in Western Landscapes (M.L. Morrison, L.S. Hall, S.K. Robinson, S.I. Rothstein, D.C. Hahn andT. Rich eds.), Studies in Avian Biology No. 18.

Jones, B.L. A Report on the Status of the Least Bell’s Vireo on the San Diego, Sweetwater, and San

Luis Rey Rivers, San Diego County, CA. 1985. Kus, B.E. and M. J. Whitfield. 2005. Parasitism, productivity, and population growth: response of

Least Bell’s Vireos and Southwestern Willow Flycatchers to cowbird control. Ornithological Monographs 57:16-27.

Lynn, S., M. J. Wellik, and B. E. Kus. 2010. Distribution, abundance, and breeding activities of the Least Bell’s Vireo along the San Diego River, California. 2009 Annual Data Summary. Prepared for the San Diego River Conservancy, San Diego, California.

Mayfield, H.F. 1977. Brown-headed Cowbird: agent of extermination? Am. Birds 31:107-113. Rothstein, R.I. 1994. The Cowbirds Invasion of the Far West: History, Causes and Consequences

Experienced by Host Species. Studies in Avian Biology : 301-314.


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Unitt, P. 1984. The birds of San Diego County. Memoir 13. San Diego Society of Natural History, San Diego, California. 231 pp.

Whitfield, M.J., K.M. Enos and S.P. Rowe. 1999. Is Brown-headed Cowbird trapping effective for

managing populations of the endangered Southwestern Willow Flycatcher? pp. 260-266. In: Research and Management of The Brown-headed Cowbird in Western Landscapes (M.L. Morrison, L.S. Hall, S.K. Robinson, S.I. Rothstein, D.C. Hahn and T. Rich eds.), Studies in Avian Biology No. 18.

Willett, G. 1933. A revised list of the birds of southwestern California. Pacific Coast Avifauna No. 2.

APPENDIX A

COWBIRD TRAP LOCATIONS

Legend

Trap location and number

0 1000 2000Feet

Source: National Geographic TOPO! USGS La Mesa Quadrangle

Figure A-1. 2009 San Diego River Brown-headed Cowbird Trap Locations.

T1

T2

T3


A-1

A-1

Mission Dam

Father Junipero Serra Trail

Mission Gorge Road

SR -

52

350 feet

Project Area

Legend

Trap Location and Number

T1 T3

T2

Figure A-2. 2009 San Diego River Brown-headed Cowbird Project Area and Trap Locations.

2009 San Diego River Brown-headed Cowbird Trapping Program, Sexton A-2

2009 san diego river brown-headed cowbird trapping program

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