Nest Site Selection and Nest Predation Patterns

at Forest-field Edges

by

Timothy Douglas Demmons

A thesis submitted to the Department of Biology

in conformity with the requirements for

the degree of Master of Science

Queen's University

Kingston, Ontario, Canada

September, 2000

copyright O Timothy Douglas Demmons, 2000

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Abstract

The effects of forest-field edge structure on nest site selection and nest predation

at forest-field edges were tested using natural and artificial nests. In the first part of this

study, nest site selection patterns of a declining species of edge-nesting neotropical

migratory bird, the Golden-winged Warbler (Vermivora chrysopteru) were studied in

southeastem Ontario. Habitat features important in nest site selection, and those

distinguishing successful and depredated nests were identified. Edges used as nest sites

had a more gradual edge slope, and a greater stem density surrounding nests than unused

sites along the same edges. Successful nests were associated with greater Goldenrod

(Solidago sp.) density and greater nest visibility than depredated nests, but edge shape

and woody vegetation density had no effect on nesting success. These results suggest

that Golden-winged Warbler breeding habitat could be created by the conversion of

abrupt agriculturai edges to more gradual edges by mowing areas adjacent to the edge on

a rotating schedule. In the second part of this study, artificial ground nests containing

Chinese Painted Quail and plasticine eggs were placed on different forest-field edge

structural types to quanti@ the effects of edge Iinearity, edge shape and nest visibility on

predation pattems. Edge shape and nest visibility did not affect nest predation intensity,

however, nests located on linear edges tended to be depredated more fiequently than

those on curvilinear edges, which was attributed to reduced travel along curvilinear

boundanes by nest predators. Predator identificati~n revealed individual responses of

nest predator groups to variation in edge stmchire and nest Msibility, suggesting that nest

predator comrnunity differences between sites may explain the inconsistent results of

present studies examining the effects of edge structure on nest predation pattems.

Acknowledgements 1 would first like to thank Raleigh Robertson for giving me the oppomuiity to

conduct this project. He always had confidence in my abilities, supported my decisions,

and he never criticized me when things (fiequently) went wrong. Above all, his sense of

humour and fnendship have helped me through many stresshl situations and made this

expenence much more enjoyable.

1 would like to express my gratitude to Stephen Lougheed for his guidance that

shaped much of this project and his willingness to drop everything vrhen 1 needed his

help. 1 especially thank Steve for being disappointed in my decision ta take a hiatus fiom

academic biology.. .coming fiom a scientist like yourself, that's about the best

compliment that 1 could receive. 1 would also like to thank Demis Jelinski and Vicki

Fnesen for inspiring my interest in this topic, and for al1 of their help in focusing this

study.

Thanks to al1 the folks in the Robertson lab over the past few years for al1 the

good times and especially to Barg, Jason Jones and Javier Salgado-Ortiz for ail of

their help. For assistance in the field, 1 am indebted to Katharina Manno, Jamie and Erin

Beauchamp, and especially William McLeish who, in addition to being a great Wend, is

one of the best field ornithologists that I've met. I'd also like to thank Iason Pither and

Daniel Memil1 who have both improved this thesis with their many helpful comments.

Thanks to the staff (especially Frank Phelan and Floyd C o ~ o r s ) and al1 of my

good fnends at QUBS for making my sumrners of research so enriching, enlightening and

mernorable. Special thanks go to Ryan DeBniyn, Heather McCracken, and Chris Yourth

for their help and friendship, and for always being ready to do anything.

Above all, 1 would like to thank the love of my life, Kelly Pageau. 1 wish

somehow that 1 could express how much her love, friendship, support, and understanding

have helped me through this experience. Despite al1 of the time spent apart, and time

spent together where 1 was locked up in my office writing, she endured as a source of

constant happiness for me. Thanks KeI.

This work was made possible by fùnding from NSERC, Queen's Graduate

Fellowship, and Wildlife Habitat Canada.

Abstract

Table of Contents

Page

. . 11

Acknowledgements

Table of Contents

List of Tables

Chapter 1: General Introduction

Edge-related nest predation pattems Habitat requirements of individual species Study Objectives

Chapter 2: Nest Site Selection and Patterns Affecting Nest Predation in an Edge-nesting passerine, the Golden-wuiged Warbler

Abstract introduction Methods

Vegetation measurements Statistical analyses

Results General nest site attributes General nesting success Nest site selection Factors affecting nesting success

Discussion Conservation implications

Chapter 3: The Effects of Edge Linearity and Edge Shape on Nest Predation at Forest-field Edges

Abstract Introduction Methods Resul ts Discussion

General patterns of predation intensity Nest predator identification

iii

Chapter 4: General Discussion

s-ary

Literature Cited

Appendiv

Vita

Page

57

63

64

77

78

List of Tables

vi

Page

Chapter 2

Table 2.1 Golden-winged Warbler nesting success 32

Table 2.2 Vegetation charactenstics at nest sites and random sites 33

Table 2.3 Correlations of nest and edge structural variables to 34 discriminant functions separating nest sites from random sites and successfiil fiom depredated nests

Table 2.4 Vegetation charactenstics at successful and depredated 35 nests

Chapter 3

Table 3.1 Logistic regression examining the effects of edge shape, 54 edge linearity, and nest visibility on nest predation

Table 3.2 Nest predator identification 55

Table 3.3 Painvise logistic regressions comparing nest fate with 56 edge structure and nest visibility

Chapter 1

General Introduction

Habitat structure plays a critical role in detemining avian population patterns (eg.

MacArthur and MacArthur 196 1, Willson 1974, Rotenberry and Wiens 1980, MacKenzie

et al. 1982) and reproductive success (eg. Wray and Whitmore 1979, Martin and Roper

1 988, Kelly 1993, Clark and Shutler 1999). Currlont widespread population declines of

neotropical migratory bird species have been attnbuted to anthropogenic modification of

habitats in the wintenng and breeding areas (eg. Whitcomb et al. 198 1, Robbins et al.

1989, Askins et ai. 1990, Rappole and McDonald 1994). These population trends, along

with continued (and growing) exploitation of remaining habitat necessitate an

understanding of the specific effects of habitat structural changes on bird comrnunities.

In addition, management efforts intended to slow population declines require detailed

habitat use information, which is presently unavailable or inadequate for many

neotropical migrants (Martin 1992).

Edge-related nest predation patterns

In the temperate breeding grounds, timber harvesting and clearing of forests for

agriculture have dramatically reduced the total area of forest cover, with remaining

forests frequently existing as isolated kagrnents in many areas (eg. Robinson 1992,

Donovan et al. 1997). Neotropical migrants breeding in forest-intenor habitats were the

first group of birds identified as being adversely affected by these changes in landscape

configuration due to documented losses of these species fkom smaller forest fkagments

(Whitcomb et al. 198 1, Wilcove 1985). Although many variables have been s h o w to

affect this trend including reduced food abundance in smaller fragments (Blake 1983,

Burke and Nol 1 W S ) , differential extinction and colonization rates depending on

fraagment size (MacArthur and Wilson 1967, Whitcomb et al. 198 l), and lower pairing

success in smaller fragments (Villard et al. 1993), reduced reproductive success resulting

from an increased proportion of "edge*' in smaller habitat fragments appears to be

prirnarily responsible for species losses fiom smaIler fiagments.

An edge is the outer portion of a landscape element, where conditions differ from

those found in the intenor (Forman 1995). Due to increased density and diversity of

organisms near habitat edges ("edge effect"; Odum 197 l), the creation of edges was

generally thought to be beneficiaf for wildlife (eg. Lay 1938, Good and Dambacfi 1943,

Kroodsma 1982), and continues (though not as frequently) to be used as a management

tool (Alverson et al. 1988, Keyser et al. 1998). The populations of most game species

respond positively to introduced edges (Noss 1983) however, neotropical migratory birds

breeding near edges tend to suffer reduced reproductive success due to increased levels of

cowbird parasitism (Brittingham and Temple 1983, Johnson and Temple 1990) and

greater nest predation compared to those breeding in habitat intenor (reviewed by Paton

1993). Of these two factors, nest predation has a much greater influence on reproductive

success in birds, and is known to be responsible for approximately 80% of al1 nesting

faiIure (Ricklefs 1969, Martin 1992).

Edge proximity effects on nest predation were first described by Gates and Gysel

(1 978) in their study of the nesting success of a variety of bird species near forest-field

edges. They discovered a highly significant positive relationship between nest success

and distance fiom an edge, and concluded that edges function as "ecological traps" by

concentrating nests, thereby increasing density-dependent nest predation (Gates and

Gysel 1978). Their study (and most others testing sirnilar hypotheses) also noted that

increased nest predation at edges may be related to nest predator activity patterns,

referring to a paper by Bider (1968) demonstrating the tendency of marnrnals

encountering a linear habitat featwe to travel parallel to this feature, thus increasing the

use of edges relative to interior habitats. Generally, increases in nest predation intensity

are most evident within 50m of habitat edges (Paton 1994) and the strength of this trend

can be affected by factors such as nest height Wayne and Hobson 1997, S6derstr6m et al.

1998), adjoining habitat types (Bayne and Hobson 1997, Saracco and Collazo 1999) and

the productivity gradient between them (Angelstam 1986). Landscape-levd

fragmentation patterns c m also affect edge-related nest predation, and increased

landscape fragmentation generally results in elevated nest predation intensity near edges

(Andrén et al. 1985, Donovan et al. 1997, Bayne and Hobson 1997, Tewksbury et al.

1998).

Studies of edge-related nest predation almost exclusively group al1 edges of a

pariicular type (eg. forest-field edges) together for analysis (eg. Keyser et al. 1998, King

et al. 1998, DeGraaf et al. 1999) which necessitates the assumption that differences in

edge structure have ao effect on nest predation patterns. In fact, there are several

quantifiable dimensions of edges which are likely to affect nest predation intensity. At

forest-field edges (which are of primary interest in this paper), edge structure is mainly

dependent on the length of time since the last disturbance (mowing, cultivation) of the

field. If a forest-field edge remains undisturbed for several years, a dense shmb layer

foms at the base of the edge ('mantel"; Forman 1995), which is typically bordered on

the field side by a herbaceous layer ("saum"; Forman 1995). Comecting the mante1 to

the forest canopy is a vertical layer of foliage termed the "veil" (Forman 1995) or "side

canopy" (Matlack 1993). The term "edge shape" will be used to descnbe differences in

the development of the mante1 at the border of two habitat types, although it might be

argued that "border shape" would be a more appropriate tenn (a border is the line

behveen adjacent habitat edges; Forman 1995).

The presence of mantel vegetation at the base of an edge may affect nest

predation by reducing predator search efficiency (Bowman and Harris 1980, Yahner

1988), and by camouflaging the nest (Martin 1992, 1993a) however, vegetative cover

provided by the mantel may increase use of the edge by some forest-ciwelling mamrnalian

nest predators (S6derstr6m et al. 1998). Several studies using n a d or artificial nests

(generally quai1 anaor plasticine eggs placed in a wicker or grass nest, used to document

predation intensity in different habitats) have investigated the effects of edge shape on

nest predation (Ratti and Reese 1988, Yahner et al. 1989, Fenski-Crawford and Niemi

1997, Suarez et al. 1997, Huhta et al. 1 9%), but their results are inconsistent. Other

measurable edge attributes such as edge cunllinearity and edge orientation have never

been investigated with respect to nest predation patterns. Environmental variables in the

edge zone are strongly affected by veil closure (Matlack 1993, 1994) which would be

expected to affect nest predator use of edges, however this also has never been tested.

The determination and separation of the effects of edge structure on nest predation

patterns around edges will provide a much better understanding of the influence of

habitat edge on nest predation patterns than would be possible by adhering to the

common practice of grouping together al1 edges of a particular type.

Habitat requirernents of individual species

The breeding habitat niches of neotropical migratory birds are very complex (see

James et al. 1984), and differ between species (Rotenberry and Wiens 1980, MacKenzie

et al. 1982). Individuals must establish temtories that include appropriate foraging

substrate, nest sites, Song perches, cover, access to bathing sites, and other factors that

allow them to successfully reproduce. For those declining species for which basic habitat

use information is available, what factors should be promoted in order to increase

populations? And for the many declining species where habitat use information is

lacking, where should research efforts be focused?

Martin (1992) proposed that management efforts will be most effective when they

promote feanires of the habitat that are limiting and have the greatest influence on fitness.

It was long believed that breeding habitat selection by neotropical migratory birds was

based pnmarily on specialized foraging habitat availability (eg. MacArthur 1958,

MacArthur and MacArthur 196 1, Willson 1974, Wiens and Rotenberry 198 1) and that

food abundance limited reproductive success (see Martin 1987). There is however

evidence that food rnay not be limiting in the temperate breeding grounds (Anderson et

al. 1982, Rosenberg et al. l982), and a re-analysis of Willson's (1974) data suggests that

avian comrnunity structural organization is better explained by specialization on nesting

substrates than by specialization in foraging habitat (Martin 1988). Studies comparing

the importance of nesting and foraging habitat in affecting breeding habitat selection have

found that the availability of suitable nesting habitat best explains habitat use patterns

(S teele 1 993, Matsuoka et al. l Y 97a). Furthermore, reproductive success (and thus

fitness) in birds is known to be strongly affected by nest site selection (eg. Martin and

Roper 1988, Martin 1992, Martin 1993b). In short, although many features of the

environment are necessary in order for a bird species to be successful, identification and

management of their nesting habitat requirements should be an integral part of efforts to

increase the populations of declining bird species. Studies of nesting habitat

requirements must also document nesting success to ensure that management decisions

reflect characteristics of productive populations.

Study Objectives

The first part of this paper wilI examie nest site selection and nest predation

pattems of a declining neotropical migratory bird species, the Golden-winged Warbler

(Vemivora chrysoptera). My interest in the ecology of the Golden-winged Warbler is

two-foId. First, it is a declining species for which very little is known of its nesting

ecology, po tentiall y hindering fbture management efforts. Second, nests of this species

appear to be constructed predominantly on forest-field edges (Will 1986, Confer l992),

allowing me to investigate the effects of edge structure on nest site selection, and on nest

predation patterns using natural nests. Ln the next component of this study, 1 will again

examine the influence of edge structure on nest predation at forest-field edges, but

artificial nests will be employed to quanti@ predation intensity. The use of artificial

nests allows me to separate and specifically test the effects of two edge structural

variables (edge shape and curvilinearity) on nest predation pattems. Artificial nests aIso

permit the use of a much larger sample size, greater standardization between samples,

and a more balanced experimental design than would be possible using natural nests, as

well as the ability to identify predators using marks lefi on plasticine eggs in depredated

nests.

Chapter 2

Nest Site Selection and Factors Affecting Nest Predation in an Edge-nesting Passerine, the Golden-winged Warbler

Abstract

Nest site selection patterns of Golden-winged Warblers (Vermivora chrysoptera),

a declining neotropical migrant, were studied in southeastem Ontario in 1998 and 1999.

A total of 49 nests were found, and habitat features important in nest site selection and

those distinguishing successful and depredated nests were identified. Golden-winged

Warbler nests were located on the ground, pnmarily within the shrubby vegetation of

overgrown field and clearing edges (43/49) and remaining nests were conspricted away

from an edge within sirnilar vegetation. Nest sites were distinguished from random sites

primarily by a more gradual edge slope, and secondarily by greater woody vegetation

density surrounding nests. Overall nest success was 43%, and daily nest suwival was

greater dunng incubation than dunng the nestling penod. Nest predation was the

principal cause of nest failure in this study, resulting in 96% of unsuccessful nesting

attempts. Successfûl nests were associated with greater Goldenrod (Solidago sp.) density

and greater nest visibility than depredated nests, but edge shape and woody vegetation

density had no effect on nesting success. Nest predation by small marnmals that favour

areas of high cover may be responsible for the positive relationship between nest success

and nest visibility. Golden-winged Warbler breeding habitat could be enhanced by the

conversion of abrupt agricultural edges to more gradual edges by mowing areas adjacent

to the edge on a rotating schedule.

Introduction

Widespread population declines have recently been documented in many

neotropical migratory bird species breeding in scrub and early successional habitat (Sauer

and Droege 1992, Sauer et al. 1999). Breeding habitat loss resulting fiom changing

forestry and agricuiturai practices are likely pnncipally responsible for these declines

(Hagan 1993, Rodenhouse et al. 1993, Warner 19941, although winter habitat loss may

also be a major contributing factor (Rappole and McDonaid 1994). For one such

declining early successional species, the Golden-winged Warbler (Vermivora

chtysoprera), reductions in f m l a n d abandonment in the latter half of the twentieth

century have been implicated in reducing the amount of suitable breeding habitat over

much of its range (Confer and Knapp 198 1, Confer and Larkin 1998).

Exacerbating this problem, and perhaps primarily responsible for population

declines in the Golden-winged Warbler, are interactions and introgressive hybridization

with another closely related species, the Blue-winged Warbler (Vermivora pinrcs). The

two species are ecologically and genetically similar (though phenotypically dissimilar),

and interbreed to produce fertile hybrids when ranges overlap (Ficken and Ficken 1968,

Will 1 986, Gill 1 987). Likely formerly allopatric, and with a more southerly histoncaf

range, the Blue-winged Warbler has been expanding its range northwards into areas

formerly occupied exclusively by Golden-winged Warblers (Ficken and Ficken 1968,

Gill 1980). This process may have been hastened by the anthropogenic introduction of

suitable breeding habitat between previously isolated populations of each species (Gill

1980). In areas of overlap, a moving hybnd zone results where Blue-winged Warblers

predictably replace Golden-winged Warblers within 50 years of initial contact (Gill

1980). The exact causal mechanism for this replacement is yet unknown, however,

asymmetrical genetic introgression of mitochondrial DNA fiom Blue-winged Warblers to

Golden-winged Warblen has been identified in areas of sympatry, along with preferential

backcrossing of h a l e hybrids with male Golden-winged Warblers (Gill 1 997). Where

species introductions and anthropogenic habitat change have resulted in secondary

contact bebveen closely related species, this type of introgressive hybridization has

caused the decline and extinction of a number of bird and other animal species worldwide

(reviewed in Rhymer and Sirnberloff 1996). If the current range expansion of Blue-

winged Warblers continues, it is likely that the Golden-winged Warbler will become

extinct, with possible persistence limited to isolated populations geographically separated

from the main population body (Gill 1980). Recently, several such disjunct populations

of Golden-winged Warblers were discovered in western Manitoba and eastern

Saskatchewan, Canada (Cumming 1998). Maintenance of populations in these areas may

prove integral to the continued suMval of Golden-winged Warblers as a unique species.

Effective management of avian breeding habitat requires in depth knowledge of

species' nesting microhabitat requirements (Martin 1992). Contrary to previous

assumptions regarding the ubiquity of nesting habitat f3r open-cup nesting birds (Lack

197 l), studies documenting preferences for specific nesting habitats demonstrate that it

should be treated as a limiting resource for many species (eg. Walsberg 198 1, Sedgwick

and Knopf 1992, Martin 1993% Clark and Shutler 1999). A number of studies have also

suggested that the selection of breeding habitat by birds may be largely mediated by the

availability of suitable nesting habitat (Martin 1988, Martin and Roper 1988, Steele 1993,

Matsuoka et al. 1997a). Golden-winged Warblers are among those neotropical migratory

birds that appear to specialize in their use of edges as nesting habitat (Will 1986, Confer

1992) despite greater nest predation in these areas relative to intenor habitats (Gates and

Gysel 1978, Paton 1994). Golden-winged Warbler nests are generally located on the

ground, on the overgrown edges of fields, clearings and paths (Confer 1992), however

nest site characteristics remain largely unquantified, as nests are very dificult to locate

(Will 1986).

Nest site selection affects individual fitness due to its influence on reproductive

output (Martin and Roper 1988, Martin 1992, Martin 1993b). The dominant factor

affecting reproductive failure in birds is nest predation (Ricklefs 1969, Martin 1992), and

patterns of nest site selection are expected to have evolved to reduce nest predation

(Martin 1 998). Nest site characteristics that have generally been found to decrease nest

predation rates include increased nest concealment (29 of 36 studies reviewed in Martin

1992, but see Holway 199 1, Howlett and Stutchbury 1996), increased vegetation

heterogeneity and complexity in the nest patch (Bowrnan and Harris 1980, Martin

1993a), and increased density of potential nest sites surrounding the nest (Martin and

Roper 1988). Despite abundant research investigating nest predation patterns around

habitat edges, the effects of edge structure on these patterns remains poorly understood

(Paton 1994, Keyser et al. 1998). Of four artificial nest studies comparing nest predation

on gradua1 and abmpt edges, results are inconclusive; one study found lower predation on

sofi edses (Ratti and Reese 1988), hvo found no difference in predation rates between

edge structural types (Yahner et al. 1989, Huhta et al. 1998) and another found lower nest

predation on hard edges (Fenske-Crawford and Niemi 1997). In the only study testing

the effects of edge structure on the depredation of natural nests, Suarez et al. (1 997)

found that nest predation rates in the Indigo Bunting (Passerina cyanea) were almost

twice as high for those nests located on abrupt compared with gradua1 edges.

In this paper, 1 compared edge structural characteristics at Golden-winged

Warbler nest sites and at randomly chosen edges within tenitories to document patterns

of nesting microhabitat selection by this species. 1 then related nest site characteristics to

reproductive success by comparing habitat features of successfiil and depredated nests.

Based on my findings, 1 present management recornmendations to augment breeding

habitat quality for Golden-winged Warblers.

Methods

Al1 study sites were in the area surrounding the Queen's University Biological

Station (QUBS), near Chaffey's Lock, Ontario, Canada, (14'30'~: 76'23'~) . This

landscape is a patchy matrix of mature closed-canopy second p w t h deciduous forest,

interspersed with active and abandoned agriculturd fields in varying stages of succession

along with numerous srnaIl lakes and swrarnps. Forests in the area are ciominated by

Sugar iMaple (Acer- sacclzarttm), with other canopy species including American

Basswood (Ti[ia heterophylla), White Ash (Fraxinus americana), Bittemut Hickory

(C'cl~ya aqrratica), Shagbark Hickory (Carycl ovata), Amencan Elm (Ulmus americana),

Paper B irch (Betrtla papyr-vera), White Oak (Querais alba), and Red Oak (Qzterais

mbr-a). Understorey tree species include Ironwood (Osttya virginiana) and Blue Beech

(Ca~piuris cadiniana). Most clearings in the forest result fiom past anthropogenic land

clearing however, natural clearings created by exposed bedrock outcrops and beaver

ponds are also common in the study area. Species first colonizing abandoned fields and

clearings in the area include Common Pnckly Ash (Zartthoxylzrm americanum),

Arnerican Elm, Blue Beech, Gray Dogwood (Cornus mcemosa) and Red Raspberry

(Rribrrs idezis).

The study area at Q ü B S is near the northem extent of the Golden-winged

Warbler's range, and as such, there are no breeding Blue-winged Warblers in this area.

In recent years however, several sightings of Blue-winged Warblers have been recorded,

and phenotypic hybrids between the hvo species (Brewster's Warblers) represent a rninor

proportion of the population (approximately 5%; Demmons, pers. obs). This proportion

of Golden-winged Warblers and hybrids at QUBS corresponds with the fint phase (Phase

1) of five phases noted by Gill(1980) in the replacement of Golden-winged Warblers by

Blue-winged Warblers at a locality.

In this region, Golden-winged Warblers are commonly found in areas where field

or clearing edges have become overgrown with shmbby vegetation. Hereafter, this

shmbby vegetation at the edge will be referred to as the 'mantel" (Forman 1995), and

forest-field edges with a mante1 will be referred to as "gradua1 edges". Forest-field edges

without a mante1 will be referred to as "abrupt edges".

Potential nest predator species observed in the study area included the Comrnon

Raccoon (Procyon lotor-), Red Fox (Vttlpes mrlpes), Coyote (Canis latrans), Short-tailed

Weasel (Mzistela erminea), M i d (Mirstela vison), Red Squimel( Tamiasciunrs

hzidsoniczrs), Eastern Gray Squirrel (Scirrnrs cai-oliner~sis), Eastem Chipmunk (Tamias

straitns), Fisher (Martes pertnanti), Sniped Skunk (Mepltitis mephitis), Whi te-footed

Mouse (Peromyscrrs letrcoprrs), Deer iMouse (P. maniczrlatzrs), Meadow Vole (Microtirs

pet~tzs~~lva~iiczis), Woodland Jumping Mouse (Napaeorapw insignis), Cornmon Garter

snake (Thamnophis sirtalis), Eastern Ribbon snake (Thamnophis saurim), Black Rat

snake (Eiaphe o. obsoleta), Blue Jay (Cyanocifta cristata), and the American Crow

(COIWS brachyrhynchos). Brood parasitic Brown-headed Cowbirds (MoIothnrs ater)

were present at low densities within the study area.

From their arriva1 to the breeding grounds, 1 followed temtorial male Golden-

winged Warblers daily to assess pairing status. During this time, most males in the study

population were captured using mist nests and playback, and uniquely colour-banded to

allow individual identification. From the first tirne that a male was observed interacting

with a female, pairs were closely monitored, and females commenceci nest site searching

almost immediately upon pairing (flying fiom stem-to-stem along overgrown edges, and

hopping down to the bottom of the stem). This behaviour was quite conspicuous, and

females usually gave a distinctive sputtering chip note as they flew fiom stem to stem,

with males following nearby. Nest building commonly started one or two days after

pairing. Nests were most commonly located by following females with nesting material

to the nest site.

Before incubation, parents readily desert nests if disturbed (Confer 1992), so nest

status was not regularly checked until incubation started. From the onset of incubation,

al1 nests were checked every two days to monitor nest status. Nest contents were

observed by walking a path parallel to the edge, at a distance of at least 1 Sm fiom the

nest, so that the nest contents could be exarnined without creating a blind scent-trail to the

nest.

If nests were empty at the time of fledging, the area around the nest was intensely

surveyed to establish if pairs were feeding fledglings. Nesting atternpts were classified as

successful if parents were observeci feeding fledglings. Nests were classified as

depredated if nests were found empty prior to the expected fledging date (8-9 days p s t

hatch), or if building of a new nest commenced shortly after the previous nest was found

empty. Depredation dates for empty nests were titimated as half of the number of days

between nest checks (the previous day).

Vegetation measurements

Afier termination of nesting, habitat characteristics were sampled at nest sites and

random sites. Due to the inherent heterogeneity of the vegetation in this area of Golden-

winged Warbler nests (generally located on an edge), traditional cucular plot-type

sampling methodologies that assume homogeneity of vegetation immediately

surrounding nests could not be used. Instead, vegetative structure was measured within a

transect extending perpendicular to the edge fiom the field to the forest, and centered on

the nest site. Transects were 4m long and 2m wide, and marked at 50cm intervals,

forming a grid of points 5 wide, and 9 long (centered on the nest). For those nests that

were on an edge, but were further than 2m fiom the edge, transects were extended

towards the forest until the average vegetation height reached at least 1.25m. The

direction of exposure of the edge was measured using a compas at the location on the

edge closest to each nest site.

To identiQ edge attributes important in Golden-winged Warbler nest site

selection, nest site charactenstics were compared to those of random sites along the sarne

edges as nest sites. Random site locations were chosen by randomly selecting a direction

(lefi or right) and a distance (10-50m) along the same edge as the nest site. Vegetation

rneasurement transects were centered at the same distance fiom the edge (measured fiom

where edge height reached approximately lm) as nesi sites. Comparisons would be

inappropriate if there was a tendency for random sites to be closer or farther fiom the

edge than nest sites, so variation between groups in distance from the edge was tested

using a Pemutation test (described below).

Vegetation height along the transect was measured using a 2m long, 1 cm diarneter

copper pipe marked at 1 Ocm intervals. Sliding up and down this pipe was a 1Ocm

diarneter movable cardboard disk. This pipe was placed vertically at each gridpoint, and

vegetation height was measured as the highest woody vegetative contact to the disk as it

was moved along the pipe. Heights were rounded to the upper vaIue for each lOcm

interval (eg. 1 1 -2Ocm=20cm, 2 1 -3Ocm=30cm). Contacts fiom overhanging branches

fiom the forest edge were not included in the anaiysis. To generate a profile of edge

shape for each nest and random site, the average woody vegetation height was calculated

at each successive 50cm interval along the length of the transect from the field towards

the forest as the average of the 5 vegetation height measurements along the width of the

transect at each interval. This edge profile was then exarnined to determine at what

horizontal distance along the transect the average vegetation height reached 0. Sm,

0.75cm, 1 .Om and 1.25m. When these values did not fa11 directly on one of the 50cm

intervals, distances were approximated as the center point between the two intervals over

which the average vegetation height increased fiom below to above each value. Edge

shape was quantified by calculating the horizontal distance over which the vegetation

height increased from 0.50cm to 1.0m (H0.5- 1 .O), fiom 0.75m to L .25m (HO.75- 1.25),

and trom 0.50 cm to 1.25m (H0.5- 1 -25). Using this protocol, gradua1 edges have a

greater horizontal distance between each height interval than abrupt edges.

In addition to the edge shape measurements, the density of woody vegetation

and that of GoIdenrod (Solidago sp.; a cornmon herbaceous species known to be used as

nesting habitat by Golden-winged Warblers; Confer 1992) was also measured within a

central lm wide strip through each transect. Al1 woody species with stems greater than

20 cm in height, and al1 individual Goldenrod stems were counted within a lm2 area

centered on the nest (DwstN, DgolN), within a lm2 area adjacent to the nest towards the

field side (DwstFI, DgolFI), and the forest side of the nest (DwstFO, DgolFO), and

within the entire 2m by lm area centered on the nest (TDwst, TDgol).

Nest visibility ( N W S ) was assessed within one week of the expected fledging

date by measuring percent visibility of nests to the nearest 10% fiom a distance of lm at

8 cardinal and sub-cardinal points fiom an elevation of 45 degrees Fom the ground, and

also kom directly above the nests. Nest visibility was evaluated as the average of these

nine measurements. Timing visibility measurements relative to a particular period of the

nesting cycle removes potential biases resulting fiom changes in nest cover over t h e

(Burhans and Thompson 1998).

S tatis tical Analyses

The vegetative characteristics of nests and random sites, and those of successful

and depredated nests were compared using univariate and multivariate approaches. If

necessary, variables were transformed using the Box-Cox transformation to more closely

fit a normal distribution (JMP version 3.2.1, SAS Institute, Inc). Differences in daily

survival rates between nesting periods, years and general nesting habitat were evaluated

using Chi-square analyses (Mayfield 1975). Permutation tests (described below) were

emplo yed when sampIe sizes did not meet the sample size requirements of the Chi-square

test. ï h e Rayleigh test was used to test for nonrandom edge orientation at nest site

locations (Zar 1996).

Al1 other univariate cornparisons involved the use of two-tailed Permutation

(randomization) tests on untransformed data (Resampling Stats add-in for Excel version

1.1, Resampling Stats, Inc). These tests have no assumptions concerning data

distribution and have been shown to be more powerfùl than nonpararnetric tests, and

parametric tests when assumptions are not met (Crowley 1992, Ludbrook and Dudley

1998). Permutation tests used in this study calculated probabilities by repeatedly

shuffling the original data and re-assigning it at random into two groups (of the same size

as the original groups). Two-tailed probabilities were detemined by examining the

proportion of the cases of shuffled data for which the difference in group means equaled

or exceeded the difference in the observed group mean. For exarnple, if the differences

in shuffled group means were greater than or equal to the difference in the observed

group mean in 15.3% of cases, P(two-tailed)=O.153. Al1 Permutation tests were

conducted using 50,000 replications, as variance in p-values for similar randomization

tests have been found to stabilize well before this number of iterations (stable at 5000

iterations; Adams and Anthony 1996).

Stepwise discriminant function analysis (DFA) was used to identifi variables that

discriminated between nest sites and random sites, and between successful and

depredated nests (SPSS version 8.0, SPSS Inc.). Group sample sizes were equal in one

analysis (43 nests, 43 random sites) and almost equal in the other (20 successfül nests, 22

unsuccessfûl nests) therefore there was no need to use a chance correction (Titus et al.

1984). Equality of covariance matrices was tested using Box's M test (SPSS version 8.0,

SPSS Inc.). Covariance matrices did not differ between successful and depredated nest

vegetation @>0.05), however in the cornparison of nest and random site vegetation,

covariance matrices differed significantly (pc0.05). Discriminant function analysis is

robust to minor violations of this type (Lachenbruch 1975) and this violation should have

little effect on the classification ability of the model (Williams 1983). The contributions

of each variable to the model were interpreted by examining the strength of the

correlation of each variable with the discriminant function (Martin and Roper 1988).

Results

General nest site attributes

A total of 49 Golden-winged Warbler nests was found in 1998 (n=22) and 1999

(n=27), a sarnple size more than three times greater than the only other study quan t img

habitat structure at Golden-winged Warbler nests (n=i 5; Will 1986). Nests were located

predominantly within shmbby mante1 vegetation on field and clearing edges (43/49). A

minority of nests however, were not placed directly within mante1 vegetation, and were

instead located in low, shmbby vegetation at least 4m from an edge (6/49). Habitat types

lying adjacent to nest sites included abandoned agricultural fields (n=35), active

agricultural fields (n=7), bedrock clearings (n=5), a beaver swamp edge (n=l) and a path

edge (n=l ).

Typically, nests were constnicted at the base of a woody stem, or f5-m herbaceous

plant. Supporting stem species induded Prickl y Ash (Zanthoxylum americanum; n= 16),

Gray Dogwood (Cornus t-acemosc; n=8), Blue Beech (Carpinnu caroliniana) (n=5),

WiIlow (Salix sp.; n=4), Red liaspberry (Rubus ideirs; n=3), Sugar MapIe (Acer

sacchanun; n=2), American Elm (Ulmus americana; n=2), Blackberry (Rubus

occidentalis; n=2), lronwood (Ostrya virginiana; n=l), Shagbark Hickory (Carya ovata;

n= 1) and Riverbank Grape (Vitis riparia; FI). Remaining nests were supported by

herbaceous species including Cornmon Burdock (Arcfium rninnls; n=I), Goldenrod

(Solidago sp.; n=I ), Spreading Dogbane (Apocynum androsaemijiolium; n=l), and one

nest was located in the centre of a clump of grass. Of nests which were not destroyed

during a predation event, nest cup height m e a n f SE) was 9.73 + 0.58 cm (n=48). For

those nests with a supporting stem where the height could be measured (not vine or

grass), supporting stem height (Mean f SE) was 100.23 5 5.00 cm (n=47).

The orientation of exposwe of edges used as nest sites did not differ fiom a

random distribution (Rayleigh's test, 24.628, n=44, P>O. 1).

General nesting success

In total, 2 1/49 (42.8%) of nests successfully fledged Young. Of 39 nests found

during nest building, two nests (5.1%) were depredated prior to the start of incubation.

Nesr predation was the principal cause of nest failure in this study, resulting in 27 of 28

unsuccessfùl nesting attempts. Only one nest was found to have been parasitized by

Brown-headed Cowbirds, and this nest (which contained 2 Cowbird eggs, and 3 Golden-

winged Warbler eggs) was abandoned during incubation. It is interesting to note that one

nest depredated during the nestling stage contained one intact Cowbird egg on the first

nest check following depredation.

No di fferences were detected in daily survival between nests found in 1 998 and

those found in 1999 during incubation test, X2 =O. 195, d e l , P=0.659) or d u ~ g the

nestling period k2 test, X2=1.073, df=l, P=O.3). in combining suNival data from both

years, the daily survival rate during incubation (0.976) was found to be significantly

greater than the daily s u ~ v a l rate during the nestling period (0.940; Table 2.1).

Of the 6 nests that were not placed directly on an edge, only 1/6 (1 6.7%)

successfùlly fledged young, while 20/43 (46.5%) of nests located on edges successfÛlly

fledged young. This difference was not statistically significant (Permutation test,

p=O. 149).

Nest site selection

The vegetation structure of those nests that were not placed directly within mante1

vegetation (n=6) could not be examined using this vegetation measurement methodology,

and Our analysis is limited to nests placed directly on edges (n=43). No difference was

found in the distance of nests and randorn sites fiom the edge (Permutation test,

P=0.938), therefore the cornparison between nest and randorn site vegetation is

appropnate. Univariate tests showed significant differences in vegetation density and

edge shape between Golden-winged Warbler nest sites and random sites along the same

edges (probabilities less than Bonferroni corrected a=O.OO4 167; Table 2.2). Woody stem

density at nest sites was significantly greater in a lm square centered on the nest

(DwstN), and aII three measures of edge shape (HO.5- 1.25, H0.75- 1.25, H0.5- 1 .O)

revealed that vegetation height at the edge increased significantly more gradually at nest

sites than at random sites. Stem density measurements at other positions within the

transect showed no differences between nest sites and random sites, nor was the density

of Goldenrod found to difier behveen the two at any position.

Stepwise DFA using nest and edge structural variables (Table 2.2) significantly

discriminated between nest sites and random sites (eigenvaIue=0.908, Wilks'

Lambda=0.524, P<O.OO 1). Five variables entered into this model, generating the

classification function:

Discriminant score= 0.0232 + O.SO2(HO.S- 1.25) + 0.152(DwstN) + O.443(DwstFI)

+ 0.498(DwstFO) - 0.497(TDwst).

This model correctly classified 86.0% of nest sites and 76.7% of random sites (8 1.4%

overall success). In examining the strength of correlation of each variable with the

discriminant fûnction, the most important variable contributing to the model was the

horizontal distance over which edge height increased fiom 0.5m to 1.25m, with greater

distances (softer edges) associated with nest sites and lesser distances (abrupt edges)

associated with random sites (Table 2.3a). Following in order of importance, were the

density of woody stems at the nest, on the field side of the nest, on the forest side of the

nest and over the entire transect, with nest sites associated with greater stem densities for

each variable than random sites (Table 2.3a).

Factors affecting nesting success

Nest site vegetation and edge structural variables were similar at successfÙ1 and

depredated nests, and univariate tests revealed no differences between the two (al1

probabilities greater than Bonferroni adjusted a=0.003846; Table 2.4), although there

was a tendency for successful nests to have a greater density of Goldenrod sunounding

the nest than depredated nests. When al1 variables were included in a stepwise DFA, a

model was developed that significantly discriminated between successful and depredated

nests (eigenvalue=0.259, Wilks' Lambda=0.795, P=0.0 1 1). Two variables entered into

this model, forming the classification function:

Discriminant score= -3.78 i + 0.293(DgolFO) + 0.035(NVIS)

This model correctly classified 63.6% of successful nests and 70.0% of depredated nests

(66.7% overall success). The density of Goldenrod in the area towards the forest side of

the nest contributed most to the fit of the model, and successful nests were associated

with a greater density of Goldenrod in this area than depredated nests (TabIe 2.3b). Nest

visibility contributed a lesser arnount to the model, and successfÙl nests were surpnsingly

associated with greater nest visibility than depredated nests (Table 2.3b).

Discussion

The results of this study clearly demonstrate particular preferences in the structure

of the edges used by Golden-winged Warblers as nest sites. Nest sites were distinguished

fiom rmdom sites pnmarily by a more gradua1 edge dope, and secondarily by greater

woody stem density in the mantel. AIthough nests and random sites were located along

the same edges, the difference in edge shape was sûiking, and the average horizontal

distance over which edge height increased fiom 0.5m to 1.25m was more than twice as

great at nest sites than at random sites (Table 2.2). While other studies have shown

occasional use of Goidenrod clumps as nest sites for Golden-winged Warblers (Confer

1992), and one nest in this study was constmcted at the base of a Goldenrod stem, my

results did not show any tendency to construct nests dong edges with high Goidenrod

density. My results appear to indicate a propensity for nests to be constructed at the base

of Prickly Ash stems. However, this shmb species is a very common early successional

colonizer of abandoned fields and clearings in the study area (Demmons, pers. obs), and

it is likely that this finding does not represent a preference of this species in general, but

of the study population in particular.

General habitat types surrounding Golden-winged Warbler nests in this study area

(abandoned and active agricultural fieids, bedrock clearings, beaver swamp, and path

edge) were similar to those found in populations in New York (Confer 1992) and

Michigan (Will 1986). The importance of abandoned f m l a n d in the creation of

breeding habitat for Golden-winged Warblers (Confer and Knapp 198 1, Confer and

Larkin 1998) was also suggested by Our results, as more than 70% of nests were found in

overgrown abandoned agricul tural fields. Active agricultural fi eId edges were used for

nesting when areas adjacent to the edge remained unmowed for several years creating

pockets of mantel vegetation. The study area at QUBS was in the region affected by the

severe ice storm of January 1998, which caused extensive canopy darnage to forests in

this area. Following the ice stonn, there has been a recent emergence of a developing

mantel on many previously abrupt edges of active agriculturd fields where large

branches felled by the ice storm have obstructed the mowing of edges. It is likely that

afier severaf more years of regrowth, this mante1 vegetation will create additional

breeding habitat for Golden-winged Warblers in this area, and potentially throughout the

region affected by the ice stonn.

Although Golden-winged Warblers typically nested directly on edges,

approximateiy 12% of the nests found in this study were placed at a distance fiom the

edge within the shmbby vegetation of overgrown agricultwal fields. A similar result was

reported by Will(l986) where one of fifieen Golden-winged Warbler nests was found in

a comparabIe location away fiom an edge. This variation in general nest site

characteristics may simply indicate plasticity in general nest site selection by this species,

however it is also possible that +hs prevalence in use of edges as nest sites reflects the

lack of open shmbland in the study area. Overgrown edges are known to provide

breeding habitat for other early successional birds species in areas where extensive tracts

of shnibby vegetation are lacking (Gates and Gysel 1978, Morgan and Gates 1982,

Kroodsma 1984, Shalaway 1985). Use of these edges has been shown to decline when

open shmbby vegetation is available, indicating a preference for the latter by some

species (Kroodsma 1984). If Golden-winged Warblers showed similar preferences, one

would expect those nesting on edges to have lower reproductive success than those using

preferred nesting habitat (Martin 1998). Contrary to this prediction, my results show a

weak tendency for nests on edges to have greater success than those constmcted away

kom edges. Further study of nest site selection patterns by Golden-winged Warblers in

areas of varying landscape composition are required in order to M e r evaluate general

nesting habitat preference in this species.

In some Golden-winged Warbler populations, approximately one third of nests

are parasitized by at least one Brown-headed Cowbird egg (Will 1986, Confer and Coker

1990) leading some to suggest Cowbird removal as a method to increase nesting success

in Golden-winged Warblers (Confer 1992). Owing to low levels of agricultural

tiagmentation in my study area compared t the above studies, parasitism of Golden-

winged WarbIer nests by Brown-headed Cowbirds was very rare in my study (1/49 nests)

and had little eflect on nest success.

Overall nest success of Golden-winged Warblers was calculated to be 43.9%

using the Mayfield method (Mayfield 1975), which is similar to that of other neotropical

migrants (averaging 42%, as calculated in a review of Mayfield success estimates fiom

17 other species; Martin 1992). Daily nest survival was significantly greater during the

incubation period than during the nestling period (Table 2. l), indicating that more

fiequent parental movements, along with increased noise and scent fiom the nest may

serve as nest finding cues for potential nest predators (Kelly L993, Morton et al. 1993,

Matsuoka et al. 1997b). This is different fiom general patterns observed in most

neotropical migrants, where nest predation rates are oflen greater dunng incubation than

dunng the nestling period (22 of 28 studies reviewed by Martin 1992). Mead and Morton

(1 985) hypothesized that this trend exists because nests that are highly susceptible to

predation may be depredated eariy in the incubation penod, and nests that survive until

hatching may be more difficult for predators to locate. Matsuoka et al. (1997b) found

that daily nest survival was lower during the nestling period than during incubation in

their study of Townsend's Warblers (Dendroica townsendi) in Alaska, and attrïbuted this

to different predation regimes in their study area comparai with 0 t h studies of more

southerly breeding passennes. Nesting guild may also play a role in affecting relative

predation rates during the incubation and nestling period. Ground nests and above-

ground nests are subject to different suites of nest predators, with avian nest predators

playing a comparatively larger role in depredating above-ground nests than ground nests,

and most rnamrnalian nest predators showing the opposite trend (Yahner et al. 1 989,

Soderstrom et al. 1998, Rangen et al. 1999). Variation in nest finding cues used by

different nest predator species cornrnoniy depredating nests of each nesting guild may

affect relative predation rates during the incubation and nestling period. For example,

evidence of the differential influence of nest noise on nest predation between nesting

guilds c m be found in a study by Haskell(1994), where begging calls broadcasted from

artificial nests increased predation on ground nests, but not those placed in trees.

A cornparison of vegetation characteristics at successful and depredated Golden-

winged Warbler nests yielded unexpected results, especially with respect to the influence

of nest visibility on nesting success. Successfûl nests were associated with greater nest

visibility (lower nest cover) than depredated nests. Studies have commonly found a

negative relationship between nest concealment and nesting success (reviewed in Martin

1992), however many exceptions exist where no relationship was found (eg. Holway

199 1, Howlett and Stuchbury 1996). Although variation in nest cover may not always

predictably affect the magnitude of nest predation intensity, it likely affects the

susceptibility of the nest to predation by different types of predators. By directly

camouflaging the nest, and obscuring parental movements around the nest, increased

cover at the nest would tend to reduce predation pressure by nest predators that are

prirnarily visually onented (eg. corvids; Andrén 1992). For nest predators such as

Comrnon Raccoons and Striped Skunks that rely primady on olfactory cues to find nests

(Whelan et al. 1993, Keyser et al. 1998), increased nest cover would only be expected to

reduce predation pressure if cover at the nest impeded predator movernents around the

nest (Bowman and Harrris 1980), or affected the transmission of scents or auditory

signals fiom the nest (Martin 1993a). Small rnammals such as mice and voles have

recently been identified as playing a much larger role as nest predators than previously

assumed, largely due to the use of small eggs or plasticindclay eggs to detect predators in

current artificia1 nest studies (Maxson and Oring 1978, Guillory 1987, Bayne and Hobson

1997, Fenske-Crawford and Niemi 1997, Hannon and Cotterill 1998, DeGraaf et al.

1999). Instead of being hampered by the presence of high vegetative cover, habitat use

studies of mice and voles show increased abundance of both groups in areas of high

cover (Stapp and Van Home 1997, Pasitschniak-Arts and Messier 1998), as dense cover

provides protection &om predators while foraging (Baker and Brooks 1982). In a recent

study using artificiaI nests containing plasticine eggs to examine the differential effects of

nest site characteristics on predation by individuai groups of nest predators, Rangen et al.

(1 999) found that mice and voles tended to depredate nests that were placed in areas of

high nest concealment. With (1991) also demonstrated that nests of McCown's Longspur

(Ca/acarizcs mccownii) were more susceptible to predation by Thirteen-lined Ground

Squirrels (Spermophihrs tridecemlineatirs) when placed near shrubs in open habitat, as

activities of this species are concentrated in such patches of high cover. The association

of depredated Golden-winged Warbler nests with high vegetative cover may be related to

29

high predation by smaller cover-loving mammals such as chipmunks, mice and voles,

although the ability of the latter two to depredate natural nests (with parental nest

defence) is currently unknown (Bayne and Hobson 1997, Rangen et al. 1999). If mice

and voles do play a major role as nest predators for ground-nesting passerines, population

fluctuations that are cornmon in many of these small mamrnals (eg. Wolff 1996, Boonstra

et al. 1998) are likely to affect nest predation rates (Sieving and Willson 1998) potentially

resulting in temporal variation in adaptive nest site placement with respect to cover.

My results show some indication that the type or structure of cover may also

affect nest predation patterns, as successful nests were associated with areas of high

Goldenrod density, but woody stem density did not influence success. Particularly for

opportunist or generalist nest predators, vegetation composition surrounding the nest may

mediate use of a patch if cover types differ in their effects on (for example) food supply,

movement, protection fiom predators, and thermoregulation. The comparative influence

of cover provided by Goldenrod and that of woody stemmed species on these processes is

unknown.

1 found no evidence that edge shape affected nesting success of Golden-winged

Warblers in this study. This result appears to differ fkom the findings of Suarez et al.

( 1997), where Indigo Bunting nests placed on gradual edges were more successful than

those placed on abrupt edges. However, unlike Indigo Buntings, which appear quite

plastic in their choice of nest edges (with respect to edge shape), virtually al1 nest edges

used by Golden-winged Warblers in this study would be classified as "gradual" (Table

2.2). The narrow range of edge structure at Golden-winged Warbler nest sites in this

study limits my ability to generalize about the effects of edge structure on nest predation,

and 1 conclude only that among gradua1 edges, variation in shape does not appear to

affect nesting success in Golden-winged Warblers.

Within the constraints of this study, there was no evidence of ongoing selection

reducing nest predation in Golden-winged Warblers, as there was no correspondence

between those nest site characteristics correlated with nesting success, and those differing

between used and unused sites. Factors which may explain this lack of agreement

include the possibilities that my methodology ignored important habitat variables (eg.

edge linearity; Demmons, following chapter), and that current patterns of nest predation

are too recent to yet show an effect on nest site selection (Clark and Shutler 1999), that

the sarnple size did not allow sufficient power to adequately compare success of typical

and atypical nest sites (Martin 1998), especially with respect to nests placed on and away

fiom edges, or that unpredictable variation in nest predator communities throughout the

breeding range result in temporal and spatial fluctuations in selection regimes and thus

"optimal" nest site characteristics (see Martin 1993a). Adaptive preferences for larger-

scale habitat characteristics by Golden-winged Warblers wi11 be evaluated in a further

study examining the effects of edge structure on nest predation patterns at forest-field

edges (Chapter 3).

Conservation Implications

If Blue-winged Warblers replace Golden-winged Warblers throughout their main

population body, it may be possible to enhance the persistence of geographically isolated

popuIations (such as those reported by Cumrning 1998) through breeding habitat

management. This study showed that Golden-winged Warblers were able to use the

3 1

edges of active agricultural fields as breeding habitat providing that adequate shmbby

regrowth was present. Relaxation of mowing around the edges of agricultural fields

within isolated populations may create additional breeding habitat and increase

population size in these areas. Over time, regrowth of mante1 vegetation wiil advance

beyond that which can be used by Golden-winged Warblers as nesting habitat, and edges

could be mowed on a rotating schedule to maintain appropriate habitat characteristics in

perpetuity. The conversion of abrupt agiculturai field edges to gradua1 edges would also

be expected to increase avian diversity (Morgan and Gates 1982, Kroodsma 1984) and

potentially increase nest success of birds currently breeding on the edge (Yahner 1988,

Suarez et al. 1997). In order to maintain allopatric populations of Golden-winged

Warblers, care must be also taken to avoid the creation of suitable breeding habitat in the

areas between isolated populations and the main population body1Blue-winged Warbler

populations, as this may accelerate gene flow and secondary contact (Gill 1980).

Table 2.1. Nesting success of Golden-winged Warblers in southeastern Ontario

caiculated using the methods of Mayfield ( 1 975).

Period Number Number Exposure days Daily survival Pro bablility of swvived depredated (#nes ts) rate successa

Incubation 33 10 412 (43) 0 -976 0.763 Nestling 21 Overall success

'Probability of success calcuiated for an incubabstion period of 1 1 days and a neslling paiod of 9 &YS. "Daily survival rare significuitly grextrr during incubation than during the nestling paiod (%' test. ~~4.773. d e l . P-0.0289).

Table 2.2. Comparison of nest site and edge stnictwal variables (Mean + SE) at

Golden-winged Warbler nests (n=43) with those at random sites (n=43) along the sarne

edges. Vegetation density variables measure the number of woody stems and Goldenrod

stems within a 1 m2 area centered on the nest (DwstN, DgolN), within a 1 m2 squared area

adjacent to the nest towards the field side (DwstFI, DgolFI), and the forest side of the

nest (DwstFO, DgolFO), and within a 2m by Im area centered on the nest (TDwst,

TDgol). Edge shape variables were calculated as the horizontal distance over which the

vegetation height increased from 0.50cm to 1 .Om (H0.5-1 .O), fiom 0.75m to 1.25m

(H0.75-1 .Z), and from 0.50 cm to 1.25m (H0.5-1.25).

Variable Nest Sites Random Sites Pa

Vegetation density TDwst DwstN DwstFi DwstFO Tdgol DgolN DgolFl DgolFO

Edge shape H0.5-1.25 H0.75-1.25 H O 5 1 .O

'Rrponrd probabililies were cslculated using Permutation tests. B o n f m n i adjusteci a=O.O04167.

Table 2.3. Correlations of nest and edge structural variables to discriminant functions

resulting from stepwise DFA's comparing (a) Golden-winged Warbler nest sites (n=43)

and random sites dong the same edges (n=43) and (b) successfùl (n=20) and depredated

nests (n=22). See Tables 2.2 and 2.4 for descriptions of variables.

Variable Correlation coefficient

(a)Nests and random sites HO.5- 1.25 DwstN DwstFI DwstFO TDwst

(b)Successful and depredated nests DgolFO 0.734* NVIS 0.43 1 *

*. P<O.OS; ***P<O.OO 1

Table 2.4. Comparison of nest site and edge structural variables (Mean + SE) at

successful (n=20) and depredated (n=22) Golden-rvinged Warbler nests. Refer to Table

2.2 for description of variables.

Variable SuccessfÙl De~redated Pa

Vegetation density Wisb TDwst DwstN DwstFl DwstFO TDgol DgolN Dgol FI DgolFO

Edge shape H0.5-1.25 H0.75-1.25 H0.5-1 .O

'Reportrd probabilities werc calcuhtd wing Permutation tests. Bonferroni sdjustd a=0.003846. ?ou1 nest visibility fmm a disance of lm cxpmsd as a percent,

Chapter 3

The Effects of Edge Linearity and Edge Shape on Nest Predation at Forest-field Edges

Abstract

The structure of habitat boundaries is known to have widespread effects on

ecological processes, and yet quantification or standardization of edge structure is often

absent in studies of edge-related nest predation. In this paper, 1 tested the novel

prediction that edge linearity affects predation intensity, and also exarnined the effects of

edge shape and nest visibility on artificial nest predation at forest-field edges around the

Queen's University Biological Station in ChafTey 's Lock, Ontario. Forest-field edge

structural types investigated in this experiment were chosen a priori as combinations of

edge shape (abrupt or gradual) and edge linearity (linear or cunilinear). Artificial ground

nests containing C hinese Painted Quail (Xexcalfactoris chinensis) and plasticine eggs

were pIaced on each of the four resultant edge structurai types, and nest visibility was

later assessed. After ten days of exposure, 55% of artificial nests showed signs of

depredation. Edge shape and nest visibility did not affect nest predation intensity,

however my results demonstrated that edge linearity influenced nest predation patterns.

Artificial nests located on linear edges tended to be depredated more eequently than

those on curvilinear edges, and I hypothesized that reduced travel dong c u ~ l i n e a r

boundaries by nest predators was responsible for this trend. Because edge shape had no

effect on nest predation intensity at forest-field edges, this study did not demonstrate

adaptive preference by Golden-winged Warblers in their use of gradua1 forest-field edges

as nest sites. Classification of predator types at artificial nests using marks on plasticine

eggs demonstrated that each identified group (mice/voles, chipmunkskquirrels, larger

marnmals) was responsible for an approximately equai mmber of depredated artificial

nests. Edge structural characteristics and nest visibility differed among artificial nests

depredated by each predator type, indicating that individual members of a nest predator

assemblage respond differently to variation in habitat structure. These results suggest

that nest predator cornmunity differences between sites may explain the inconsistent

results of recent studies examining the effects of edge structure on nest predation

patterns.

Introduction

Avian nests situated near habitat edges generally experience elevated rates of nest

predation compared to those located within intenor habitats (reviewed in Paton 1994).

Factors known to influence this adverse edge effect include the distance of the nest fiom

an edge (Gates and Gysel 1978, Small and Hunter 1988, Johnson and Temple 1990),

habitat types on either side of the edge (Angelstam 1986, Bayne and Hobson 1997,

Saracco and Collazo 1999), nest height (Bayne and Hobson 1997, Soderstrom et al.

1998), and degree of landscape fiagmentation (Andrén et al. 1985, Bayne and Hobson

1997, Donovan et al. 1997, Tewksbury et al. 1998). One component that is ofien ignored

in studies of edge-related nest predation is the structure of the edges themselves, despite

its likely influence on this process (Keyser et al. 1998).

One of the most frequently cited explanations for higher nest predation rates near

habitat edges is the use of edge habitats as travel-lanes by nest predatoa, increasing the

encounter rate of nests near edges (eg. Gates and Gysel 1978, Ratti and Reese 1988,

Suarez et al. 1997, Huhta et al. 1998, Keyser et ai. 1998). Most of these studies refer to

Bider (1 968), a landmark paper demonstrating that anirnals encountering a linear habitat

feature (eg. a forest-field edge) tend to move parallel to this feanue for a certain distance,

thus increasing the use of edges relative to habitat intenors. One might assume that

curvilinearity of edges would affect the movement patterns of animais around these

features however, this topic has received surpnsingly little scientific attention. Results of

a study in progress by Forman, Smith, and Collinge presented in Forman (1995) of the

movements of large mammals (mainly cervids) around straight and cunrilinear

boundaries demonstrated that movement along boundaries decreased with curvilinearity,

and that mamrnals tended not to follow boundaries on curvilinear edges and instead

rnoved from lobe to lobe (extensions or peninsulas of one habitat type into the other).

Given these results, it is possible that the effects of boundaxy curvilinearity also extend to

the movements of nest predator species, potentially affecting nest predation patterns on

edges, however, this hypothesis has yet to be tested empincally.

Another element of edge structure, edge shape (defined here as the abrupt to

gradua1 transition behveen adjoining habitat types based on the growth of the vegetation

at the edge), has widespread effects on ecological processes around habitat boundaries

(Morgan and Gates 1982, Matlack 1993, Murcia 1995). Gradua1 edges with shmbby,

complex growth, should lead to reduced nest predation intensity (Yahner 1 988) since

predator movement will be impeded compared to abrupt edges (Bowman and Harris

1980) and detection of nests by predators may be impaired (Martin 1992, 1993a). This

prediction was supported by Suarez et al. (1997), where predation on indigo Bunting

(Passerina cyanea) nests placed on abrupt edges was much greater than those placed on

gradual edges. The results of four published artificial nest studies examining predation

rates at abrupt and gradual edges however are inconsistent, and one study found lower

predation on gradual edges (Ratti and Reese 1988), two found no effect of edge shape

(Yahner et al. 1989, Huhta et al. 1998), and another found lower predation on abrupt

edges (Fenski-Crawford and Niemi 1997).

Nest predation is the dominant cause of reproductive failure in birds (Riclclefs

1969, Martin 1992), and evolved pattems of nest site selection by individual species

reflect directional selection towards microhabitats with low predation intensity (see

Martin 1998). Nest site selection of edge-nesting birds would thus be expected to reflect

general differences in nest predation pressure among edge types. In one such edge-

nesting species, the Golden-wïnged Warbler (Vermivora chtysoptera), nests are placed

almost exclusively on gradual forest-field edges although abrupt edges are more abundant

within their territories (Dernmons, previous chapter). This pattern of nest placement by

Golden-winged Warblers may suggest that gradual edges confer a reproductive advantage

to individuals nesting on these edges, potentially through reduced nest predation pressure.

In order to evaluate the influence of edge structure on nest predation, 1 placed

artificial nests on forest-field edges to examine the effects of edge linearity and shape on

nest predation patterns. Nest concealment was also measured in this study due to its

frequently demonstrated influence on nest predation (reviewed in Martin 1992). 1 predict

that Iinear edges will have higher nest predation than cunilinear edges due to their

40

increased use as travel lanes for nest predatoa. Furthemore, 1 predict that selection of

gradual edges as nest sites by Golden-winged Warblers will reflect reduced predation

rates on gradual compared with abrupt forest-field edges. Because habitat structure

surrounding nests differs in its effect on individual nest predator species (eg. Haskel

1995a, Rangen et al. L999), and it has been suggested that individual predator species

may respond differently to variation in edge structure (Fenske-Crawford and Niemi 1997,

Soderstrom et al. L998), plasticine eggs will be used to identiQ predators of artificial

nests placed on different edge types.

Methods

This study was conducted in June 1999, in the area surrounding the Queen's

University Biological Station (QUBS) near Chaffey 's Lock, ON, Canada (44'30'~:

7 6 O 2 3 ' ~ ) . This landscape mostly forested with scattered active and abandoned

agrkultural fields, bedrock clearings, lakes and swamps. Forests in the area are

predominantly mature, closed-canopy deciduous forests dominated by Sugar Maple (Acer

sacchanrm). Other canopy species include Arnerican Basswood (Tilia heterophylla),

White As h (Fraxinus americana), Bittemut Hickory (Cava aqiratica), S hagbark Hickory

(Caya ovata), American Elm (Ulmirs americana), Paper Birch (Betirla papyrifra),

White Oak (Querars alba), and Red Oak (Qirercns nrbra). Understory tree species

include Ironwood (Osîrya virginiana) and Blue Beech (Carpinus cardiniana).

Abandoned agricultural fields of various sizes and successional stages can be found

throughout this area, as well as numerous active agricultural fields, mostly growing crops

of hay. Where abandoned agricultural fields or the edges of active fields have been left

to regrow, common early-successional colonizers include Common Prickly Ash

(Zarrthoxylum americanirm), American Eh, Blue Bcech, Gray Dogwood (Cornus

r-acemosa) and Red Raspberry (Rubu ideus).

Potential nest predators which occur within the study area include the Common

Raccoon (Procyon loror), Red Fox (Virlpes vzdpes), Coyote (Canis latrarts), Short-tailed

Weasel (Mristela erminea), Mink (Matela vison), Red Squirrel ( Tamiasciunu

hudsonicus), Eastern Gray Squirrel (Sciunrs carohensis), Eastern Chipmunk (Tamias

straitus), Fisher (Martes pennanti), S triped S kunk (Mephitis mephiris), Whi te-footed

Mouse (Pemmysczrs lerrcopirs), Deer Mouse (P. manicirlahrs), Meadow Vole (Microtus

pemsylvanictrs), Woodland Jumping Mouse (Napaeozapus insignis), Common Garter

Snake (Tharnnophis sirtalis), Eastern Ribbon Snake (Tharnnophis sairritus), Black Rat

Snake (Elaphe o. obsoleta), Blue Jay (Cyanocitta cristata), and the American Crow

(Corvtis brachyrhynchos).

Artificial nests were commercially-available wicker basket canary nests (Hagen

Art.# 8- 1980). Artificial nests were placed outdoors for at least 10 days prior to

placement on edges to reduce human or other foreign scents on nests. In the experiment,

two Chinese Painted Quai1 (Xacalfactoris chinensis) eggs, and two plasticine eggs were

placed in each artificial nest. Chinese Painted Quai1 eggs were used instead of more

commonly-used Japanese Quai1 (Coturnix japonica) eggs due to their smaller size.

Chinese Painted Quai1 eggs are roughly the size of a large passerine egg, although they

are still slightly larger than the eggs of smaller sparrow and warbler species (Lewis and

Montevecchi 1999). Quai1 eggs were refngerated and nnsed with well-water prior to

placement in artificial nests. Plasticine eggs were molded using rubber gloves and rinsed

with well-water prior to use.

Predation of artificial nest contents was used to examine the effects of edge shape

(abrupt or graduai) and edge linearity (linear or curvilinear) on nest predation at forest-

field edges. Gradua1 edges were considered to be those forest-field edges with low

shrubby growth ("mantel"; Forman 1995) of at least 0.5m in height extending at least lm

into the field from the vertical edge wall (measured fiom a height of approximately lm)

and abrupt edges were those with no shrubby growth extending into the field fkom the

vertical edge wall. Edges were considered linear when the forest-field border foilowed a

relatively straight line over a distance of at least 3 h , and curvilinear edges were those

edges where the border was noticeably wavy (qualification of selections described

below). Forest-field edges of each of the four combinations of the above edge structural

variables were not equally prevalent in Our study area. To ensure a balanced

experimental design, similar numbers of forest-field edges of each combination were

chosen a priori as artificial nest sites. Of 140 artificial nests used in this study, 40 were

placed on abruptflinear edges, 40 on abrupt/curvilinear edges, 30 on graduaMinear edges

and 30 on gradual/curvilinear edges. Due to the effects of side canopy closure on

environmental variables at the edge (Matlack 1993, 1994), only forest-field edges with a

closed side canopy were used in this study.

Exact sites for nest placement within a selected edge were randomly chosen, and

artificial nests were placed on the ground at the base of the fust woody stem greater than

0.75m in height as the experimenter advanced fiom the field towards the forest. Artificial

nests were rnarked for relocation at a distance of at least 5m using labeled flags.

Artificial nests were placed at least 5ûm apart and edge types used in the experiment

were scattered haphazardly throughout the study area in an effort to ensure spatial

independence of predation events at each artificial nest. Aithough many nest predator

species have home ranges that could potentially encompass several artificial nests in this

study, this distance is among the largest between-nest distances used in most artificial

nest studies (eg. 50m apart in Donovan et al. 1997, King et al. 1998, Sieving and Willson

1999). Accordingly, I make the assumption that the potential for each predation event

was independent.

Al1 nests were checked 4 and 8 days after placement, and collected after a total

exposure time of 10 days. These fiequent nest checks were conducted to reduce the

Iikelihood of multiple predation events on single nests (Keyser et al. 1998). During each

nest check, quai1 eggs were examuied closely for signs of predation, and marks on

plasticine eggs were examined for evidence of disturbance. Artificial nests were

considered depredated if quail or plasticine eggs were damaged, missing, or outside of

the nest cup. Following depredation, notes were taken on the condition of the quai1 eggs

in the nest, and plasticine eggs were collected for predator identification. Nests and eggs

were always handled while wearing rubber gloves, and rubber boots were worn during al1

nest checks.

After artificial nest contents were collected, nest visibility was assessed by

measuring the percent visibility of each nest to the nearest 10% from a distance of lm at

each of the 4 cardinal directions h m an elevation of 45 degrees fiom the ground, and

also tiom directly above the nests. Total nest visibility was evaluated as the sum of these

five measurements, and Iater transformed to a nomina1 scale by dividing nests into two

categories of high and low visibility (high= greater than median visibility, low= less than

or equal to median visibility). Sarnple sizes of artificial nests placed on each of the

resultant eight categories of edge typdvisibility were ten or greater.

To identiv predators on artificial nests, tooth marks on plasticine eggs from

depredated nests were compared to impnnts made fiom museum skulls and collected

predator species. These marks were used to divide predators into four different groups.

Incisor imprints could be used to distinguish smaller mamrnalian predators into two

groups "mice/voles" and "chipmunks/squirrels", and those depredated nests where

plasticine eggs showed imprints of large tooth marks were grouped as "larger mammals".

Depredated nests were classified as 6'unknown predators" when no discemable marks

remained on plasticine eggs following damage to quai1 eggs or removal of any egg.

Artificial nests where contents remained undisturbed were classified as "intact".

Artificial nests where predator type was unknown were excluded fiom later analyses as it

is likely that predators in this group include members of the other classified predator

groups which could have removed or destroyed eggs without leaving marks on plasticine

eggs-

In order to ensure that my designations of Iinear and curvilinear edges were "real"

and to present a scale for these appointments, a subset of edges were mapped using a

Trimble (ProXL) GPS with an RTCM receiver, capable of sub-metre accuracy. Eighty

edges used in the experiment were randomly selected for measurernent (20 fkom each of

the four categories of edge structure). Over a strip roughly 30m long centered on the

artificial nest location, edge linearity was mapped by following the edge at a height of

approximately lm, and stopping to map points at approximately 3m intervals along the

edge. To improve accuracy of measurements, 15 positions were taken at each point using

a PDOP (Position Dilution of Precision) mask of 7, with a minimum of 4 satellites

available for triangulation. The average positions of each point were calculated and

piotted using AutoCad Map 2000 (Autodesk Inc.), and the total distance hetween each

consecutive point along the edge, and the distance between the first and 1 s t points were

calculated. The "cunilinearity" of the edge was then calculated as the ratio of the sum of

the total distances between each consecutive point divided by the straight-line distance

from the beginning to the end of the transect.

The dependency of artificial nest success (intact or depredated) on each binary

variable (edge linearity, edge shape and nest visibility) was tested using a logistic

regression mode1 (JMP version 3.2.1, SAS hstitute, inc.). For those artificial nests

where predator types could be identified and for intact nests, painvise logistic regression

analyses were conducted to detennine whether edge structural characteristics or nest

visibility affected the probability of an artificial nest experiencing a specific fate.

Lack of fit tests were conducted on al1 logistic regression models to examine if the

inclusion of interaction or polynomial terms would improve the fit of the model (SAS

1995). Odds ratios were used to evaluate the direction of influence of each variable.

This term shows how the probability of a success is multiplied as each variable changes

from its minimum to its maximum (Knoke and Burke 1980, SAS 1995). For binary

response and effect variables (eg. O=depredated, 1 =intact; linear=O, curvilinear= 1 ), JMP

version 3.2.1 treats the higher number as a success or the maximum (SAS 1995).

Likelihood-Ratio %' tests were used to evaluate the importance of each variable in the

model (Ishii-Kuntz 1994, SAS 1995).

Two-tailed Permutation tests were used to compare differences in the proportion

of nests depredated by each predator group where quail eggs rernained intact

(Resampling Stats add-in for Excel version 1.1, Resarnpling Stats, Inc.), and also to

examine differences in curvilinearity between edge types. Permutation tests calculate

probabilities by repeatedly shuffling the original data and re-assigning it into two groups

(of the same size as the original groups). Two-tailed probabilities were calculated as the

proportion of cases of shuffled data for which the difference between groups (eg. in the

number of depredated nests where quail eggs remained intact) equaled or exceeded the

difference between the observed groups. To ensure stable P-values, Permutation tests

were conducted using 50,000 iterations (Adams and Anthony 1996). For al1 statisticai

tests, probabilities less than or equai to 0.05 were considered significant, and those

between 0.05 and 0.1 0 were considered suggestive of a trend.

Results

Mapping and measurement of randomly seIected forest-field edges designated as

Iinear (n=40) and curvilinear (n=40) confirmed o w appointments, and there was no

overlap between the two groups. Mean curvilinexity (range) was calculated as 1 .O468

(1 -0047- 1.1 150) for linear edges and 1 S447 (1.1454-4.5 156) for c u ~ l i n e a r edges

(Permutation test, n=40, 40, P=O). There was no difference in the "curvilinearity" of

abniptlcurvilinear and gradual/curvilinear edges (Permutation test, n=20,20, P=0.5290),

or that abrupt/linear and graduaiAinex edges (Permutation test, n=20, 20, P=0.602 1).

Of 140 artificial nests placed on forest-field edges, data fiom one artificial nest

was omitted because the nest was not recovered during nest checks. After 10 days of

exposure, 77 (55.4%) of the remaining nests displayed signs of depredation. Logistic

regession analysis did not detect an effect of edge shape or nest visibility on the

probability of nest depredation (both P>0.25; Table 3.11, however the probability of a

nest remaining intact tended to be greater for artificial nests located on curvilinear

compared with linear edges (P<O. IO; Table 3.1 ).

Examination of tooth marks on plasticine eggs allowed identification of predators

at 7 1 % of depredated nests (Table 3.2). Each ciassified predator group (mice/voles,

chipmunks/squirrels, larger mamrnals) was responsible for roughly the same number of

depredated nests for which the predator could be indentified (Table 3.2). None of the

recovered plasticine eggs showed marks characteristic of avian predation. Presumably as

a result of frequent nest checks, no plasticine eggs showed evidence of multiple predation

events on single nests, although this would have been impossible to detect in cases where

original and secondary predators were similar, or if plasticine eggs were removed by a

secondary predator.

In some artificial nests that were considered depredated, quail eggs were intact,

but plasticine eggs were marked. Predators on the 1 1 artificial nests found in this

condition, were identified as mice/voles (n=5), chipmunks/squirrels (n=5), and larger

mammals (n=l). The proportion of depredated nests where quai1 eggs remained intact

did not differ between mice/voles and chipmunks/squirrels (Permutation test, P=0.6489),

however the proportion of depredated nests where quai1 eggs remained intact for those

nests depredated by larger mammals tended to be less than both other groups

(Permutation tests, P=O.O8 152, P=0.07 104 respectively).

Logistic regession analyses revealed that the probability of a nest falling Mctim

to a particular predator group or remaining intact was dependent on edge linearity, edge

shape or nest visibility in three of six painvise cornparisons (Table 3.3). The probability

of a nest remaining intact compared with being depredated by a mouse/vole was greater

for artificial nests located on curvilinear compared with linear edga. The probability of

a nest remaining intact compared with being depredated by a larger mammal was greater

for artificial nests located on gradual edges compared with abrupt edges and tended to be

greater for those nests with low compared with high visibility. Finally, the probability of

a nest being depredated by a larger mammal compared with a mouse/vole tended to be

greater for those nests located on cunilinear compared with linear edges and those on

abrupt compared with gradual edges.

Discussion

Contrary to my prediction, the overall predation rate of artificial nests was not

affected by edge shape, nor did nest visibility affect the likelihood that a nest would be

preyed upon. My results however, supported my prediction that nests located on linear

edges tend to be depredated more fiequently than nests on c u ~ l i n e a r edges (P<0.10).

When the identity of predators at depredated nests was considered, there was evidence

that nest predator groups responded differently to variation in edge structure or nest

visibility.

General patterns of predation intensity

Greater depredation of artificial nests on Iinear edges compared with curvilinear

edges suggests that edge linearity might affect the activity of nest predators around

forest-field edges. Movements of some nest predator species may follow similar general

patterns to those of cervids, which tend to follow linear habitat boundanes but move

between lobes along curvilinear boundaries (Forman, Smith, and Collinge presented in

Forman 1993, although rnovement patterns of the predators found in this study are yet to

be investigated. Elevated predation on edge nests resulting fiom the use of linear habitat

features as travel lanes by nest predaton (Bider 1968) would therefore be diminished on

curvilinear edges as movements of nest predators would tend not to be as concentrated

along the edge. Compared with habitat edges of natural origin, anthropogenic edges are

generally considered to be of lower quaiity for birds because their reduced structurai

diversity provides fewer nesting microhabitats (Morgan and Gates 1982, Shalaway 1985)

and potentially increases nest predation intensity (Suarez et al. 1997). The results of this

study suggest that the linearity of most anthropogenic edges may also reduce habitat

quality through decreased reproductive success of birds nesting on these edges, especially

if the appearance of this edge type is too recent to have yet pemitted adaptive responses

in nest si te selection patterns (Gates and Gysel 1978, Hansson and Angelstam 199 1 ).

Other factors related to edge Iinearity that potentially affect nest predation but were not

quantified in this study include placement of nests relative to lobes along curvilinear

edges, the degree of cu~lineari ty of the edge, and the scale of curvilinearity

measurements. Future studies examining nest predation around habitat edges are

recornrnended to control for edge Iinearïty or include a curvilinearity terni in models

relating predation rates to habitat characteristics in these areas.

This study shows no evidence that edge shape affects artificial nest predation

intensity at forest-field edges, consistent with the findings of Yahner et al. (1 989), and

Huhta et al. (1 998) but diffenng frorn those of Ratti and Reese (1 988) and Fenski-

Crawford and Niemi (1 997). The s h b b y , heterogeneous growth at gradual edges may

decrease nest predation compared with more abrupt edges (Ratti and Reese 1988, Yahner

1988) because increased vegetative complexity is known to reduce the search efficiency

of some nest predator species (Procyon lotor, Bowman and Harris 1980). At forest-field

edges, the consequences of this effect on differential artificial nest predation intensity

between edge types might be balanced by increased use of gradual edges by nest

predators due to increased prey (nest) density around gradual edges (Morgan and Gates

1982, Kroodsma 1984, Shalaway 1985) and because smaller rnammalian nest predators

fiequent areas of high cover as protection fiom predators (Baker and Brooks 1982,

Pasitschniak-Arts and Messier 1988, Stapp and Van Home 1997).

My results showed no evidence that edge shape affects nest predation intensity at

forest-field edges, and consequently this smdy did not demonstrate adaptive preference

by Golden-winged Warblers in their use of gradual forest-field edges as nest sites. In

order to evaluate adaptive habitat preference, it was necessary to make the assumption

that the effects of edge structure on nest predation patterns for artificial nests reflected

those on Golden-winged Warbler nests. One shortcoming of artificial nest studies in their

capacity to address this type of question is their inability to account for predation

resulting fkom detection of parental movements around the nest (Haskell 1995b,

Soderstrom et al. 1 998, Wilson et al. 1998). While the nests of most passerines tend to be

depredated more frequently during the incubation compared with the nestling period

(Martin 1992), predation on Golden-winged WarbIer nests is significantly greater during

the nestling period (Demrnons, previous chapter), indicating that factors associated with

this stage (including more frequent parental visits to the nest) may play a large role in

revealing nest locations to nest predators. Despite my results demonstrating equal

likelihood that a predator would discover a nest 'bnaided by parental cues on gradual

and abrupt edges, selection of gradual edges for nesting by Golden-winged Warblers

would prove adaptive if the greater vegetative cover at a larger scale offered by gradual

edges inhibited nest discovery by visually oriented nest predators cueing in on parental

movements to find nests.

Nest predator identification

Al1 three identified types of nest predators (mice/voles, chipmunks/squirrels,

larger marnmals) were responsible for approximately equal numbers of depredated

artificial nests in this study (Table 3.2). Similar to other research demonstrating that

birds rarely depredate artificial ground nests (eg. Bayne and Hobson 1997, Sôderstrom et

al. 1998, Rangen et al. 1999), marks characteristic of avian predation were absent from

al1 depredated nests in this study. Chinese Painted Quai1 eggs tended to be more

frequently darnaged in nests depredated by large mammals than in those depredated by

mice/voles or chipmunks/squi~els, indicating that members of the latter two groups may

have difficulty breaking through the egg shells (see Haskel 1995b) or that they were more

likely to abandon a depredation attempt afier biting a plasticine egg.

Identification of predator types at depredated artificial nests supported previous

suggestions that individual species of a nest predator assemblage respond differently to

variation in edge structure (Fenske-Crawford and Niemi 1997, Sôdentr6m et al. 1998;

Table 3.3). Nest visibility did not differ between intact nests and those depredated by

chipmunks/squi~~els and, comparable to the findings of several other studies, visibility

was similar at intact nests and those depredated by miceholes (With 1994, Rangen et al.

1999). The tendency for nests depredated by larger mammals to be more visible than

intact nests indicates that vegetative cover inhibited nest predation by this group by

visually concealing nests (Martin 1992, 1993) or impeding predator movements around

nests (Bowman and Hanis 1980). Differences in edge shape between intact nests and

those depredated by larger mammals provides fiirther indication that larger marnrnal

predation may be reduced in areas of greater vegetative heterogeneity (Bowman and

Hams 1980), as intact nests were located on more gradua1 edges than those falling victim

to larger mammalian predators. The results of this study showing increased linearity of

edges at nests depredated by micdvoles compared with intact nests was unexpected

because the large scale of cunilinearity examined in this study might not be predicted to

affect the use of edges by such small animals. Despite their size, some of these species

have relatively large home ranges (eg. Microrus pennsylvanicus home range can exceed

690m2 in size; Gaulin and FitzGerald 1988), and it is possible that travel along forest-

field edges would be reduced by edge curvilinearity, even at this large scale. Painvise

cornparisons of habitat charactenstics at nests depredated by known predator groups

showed that nests depredated by mice/voles tended to be found on edges that were more

linear and more gradua1 than those depredated by larger mammals, echoing the results of

the above cornparisons of each group to intact nests.

The differential responses of predator groups to variation in edge stmcture

suggests that the composition of the predator assemblage at a study location plays a large

role in determining general patterns of nest predation around edges at that locale (see

Martin 1987). Predator community differences may explain the inconsistent results of

other studies investigating the effects of edge shape on nest predation, (Ratti and Reese

1988, Yahner et al. 1989, Fenski-Crawford and Nierni 1997, Suarez et al. 1997, Huhta et

al. 1 W8), suggesting that generalizations between landscapes of edge structural effects on

nest predation may be problematic (see Murcia 1995), especially in regions which

contain unique predator communities (eg. Newfoundland; Lewis and Montevecchi 1 999).

Patterns of nest predation intensity at forest-field edges appear to be driven by the

unique effects of edge structural variation on individual predator species. Given the

predator assemblage present in this study area, my results imply that the linear

configuration of most anthropogenic edges may cause reduced reproductive success for

birds nesting in edge habitats. If further study demonstrates widespread agreement with

this pattern, edge-related nest predation intensity could be reduced if landuse practices

typically generating linear edges (eg. clearcut timber harvesting, agricultural land

clearing) altered the application of these practices to create cunilinear edges. Providing

that the reduction in predation around curvilinear edges in this study is a result of

decreased travel along the edge by nest predators, the ability of other methods to

accomplish similar functions (eg. fences perpendicular to the edge) should also be

investigated.

Table 3.1. Logistic regression model examining the effefts of edge shape, linearity and

nest visi bili ty on depredation of arti ficial nests placed on forest- field edges

(Depredated=O, intact= 1 ). This model was not significant (Logistic regression, n= 139,

x'=5. 147, P=O. 1613).

Tenn Estimate (SE) Odds Ratio L-R x" P

Edge shape 0.1985 1.4874 1.262 0.2612 (O=abrupt, 1 =graduai) (O. 1769)

Edge linearity 0.297 1 1.8 1 15 2.919 0.0875 (O=linear, 1 =cuFrilinear) (O. 1750)

Nest visibility -0.1650 0.7 190 0.867 0.35 18 (O=low, l=high) (O. 1774)

Table 3.2. Predatoa of artificial nests containhg Chuiese Painted Quai1 (CPQ) eggs

and plasticine eggs placed on forest-field edges based on identification of marks lefi

plasticine eggs.

% of % nests with marked Predator type total plascicine eggs but n

nests intact CPO enns

Chipmunkked squirrel 12-9 29.4 18 Mouselvole i 2.2 27.8 17 Larger mammal 14.3 5 .O 20 Unknown predator 15.8 - 22 intact nests 44.6 - 62 Total 139

Table 3.3. Painvise logistic regressions comparing edge shape, linearity and nest

visibility at intact nests and depredated nests where predator types were identified

(M-mouse/vo le, C=chipmunk/squirrel, L=larger mamrnal, I=intact). In eac h cornparison,

predator types were entered into the mode1 as "O" vs "1". Binary effects in the model

were coded as the following: edge Iinearity (Iinear-O, cu~l inear=l) , edge shape

(abrupt=O, gradual=l), and nest visibility (low=O, high=l ). Effects in each model were

considered important if PcO. 10.

Cornparison Whole mode1 test important effect(s) Odds 1

n 'I - P in mode1 Ratio L-R xb P

1 vs M 80 5.504 0.1383 edge linearity 0.268 5.392 0.0202

1 vs L 82 8.354 0.0392 edge shape 0.292 4.407 0.0358 aest visibility 2.696 2.946 0.086 1

M vs C 35 2.524 0.4710 noae - - - ,M vs L 38 8,080 0.0444 edge linearity 4.097 3.568 0.0589

edge shape 0.25 1 2.9 18 0.0876

'Likelihood-Ratio x' me lack of f i t test for this modcl \bas significant. howevaail additive models w a c unstable and mults o f the rcduccd mode1 are displayed.

Chapter 4

General Discussion

Al1 edges are not created equal. In this study, nest site selection pattems of one

edge-nesting bird species were closely tied to a specific edge structural type, and patterns

of general and predator-specific nest predation intensity were both affected by variation

in edge structural characteristics. Further reconciliation of the lack of understanding of

the effects of edge structure on nest predation was possible by contrasting the findings of

a nantral nest study and an artificial nest study investigating nest predation patterns at

forest-field edges.

In Chapter 1 , I identified the importance of documenting the nesting habitat

requirements of declining neotropical migratory bird species and in Chapter 2, this was

accomplished for one such species, the Golden-winged Warbler. Golden-winged

Warbler nests were constructeci alrnost exclusively dong edges with a developed mantel,

despite a much greater abundance of abrupt edges within their temtories. Their ability to

utilize active agriculhiral field edges as nesting habitat when adequate mante1

development was present was of particular interest, as this allowed the development of a

simple method by which edge structure could be manipulated on farmed edges to

promote Golden-winged Warbler nesting habitat. As a species of conservation concem,

the Golden-winged Warbler presents somewhat of an unusual case, in that the pnmary

cause of their decline (hybridization with Blue-winged Warblers) is virtually impossible

to directly address using traditional management methods. This fact demands that

breeding habitat management efforts proceed cautiously, as creation of breeding habitat

in areas of syrnpatry or between the main Golden-winged Warbler population body and

isolated populations could bring about the opposite of the desired effect. When

conducted in appropnate areas however (isolated populations or areas of complete

allopatry), the conversion of abrupt to gradual edges will not only benefit Golden-winged

Warbler populations, but will likely have corollary benefits to overall avian diversity

(Morgan and Gates 1982, Kroodsma 1984) and perhaps reproductive success across

species (Suarez et al. 1997, Yahner 1988), although 1 found no support for reduced nest

predation on gradual edges in my artificial nest experiment.

In designing this study, Golden-winged Warblers were beIieved to be an

appropriate mode1 for which to test the effects of edge structure on nest predation on

natural nests. However, the results of this stridy showed that they use a very narrow

range of edge structural types (with respect to edge shape) as nest sites, and my analysis

of the effects of edge structure on nest predation in this species was limited. In

association with their relatively unique nesting habitat preferences, patterns of predation

on Golden-winged Warblen nests also proved different fiom most studied passennes.

Notably, nest predation was greater during the nestling period than during incubation, and

increased nest visibility tended to reduce nest predation intensity.

To examine the evidence for ongoing selection to reduce nest predation in

Golden-winged WarbIers, 1 compared the similarity between habitat features selected as

nest sites with those demonstrated to reduce nest predation intensity, however, there was

no visible correspondence. This analysis was subject to several listed caveats, probably

the most important of which related to the inadequacy of my sarnple size in addressing

this type of question. In Chapter 3, my discovery of the effects of edge linearity on nest

predation intensity, a factor that was not quantified in my study of Golden-winged

Warbler nest site selection, also presented the possibility that selection was ongoing for

habitat features that were missed by this analysis.

In my artificial nest study documenting the effects of edge structural variation on

nest predation patterns on forest-field edges, 1 was able to test adaptive selection of

gradual edges as nest sites by Golden-winged Warblers using a much greater samp1e size

than was possible using natural nests. Despite my expecîations, artificial nests placed on

graduai edges suffered similar predation intensity to those on abrupt edges, and adaptive

nest site selection was not demonstrated. This study also found no evidence of the effects

of nest visibility on nest predation intensity however, artificial nests located on linear

edges tended to be depredated more fiequently than those on curvilinear edges. Pnor to

this study, the effects of edge linearity on nest predation have never been tested, so this is

an entirely new finding. The relationship between edge linearity and nest predation

intensity warrants further attention, especially due to the implication that the linear nature

of most anthropogenic edges contributes to reduced reproductive success for birds

nesting in edge habitats.

In Chapter 2, nest predation was treated mainly as a "black box" due to the fact

that nest fate could only be analyzed in terms of two possible outcornes; successful or

depredated. The opening of this black box was accomplished in Chapter 3 by using

plasticine eggs to identify nest predators in my artificial nest experiment. The results of

this portion of my study indicated that the mechanisms underlying nest predation patterns

at forest-field edges are very cornplex. A wide variety of nest predators depredated

art i ficial nests, and of three classi fiable groups (mice/voles, chipmunks/squirrels, larger

mammals), each responded individually to variation in edge stmcture or nest visibility.

The primary implication of these findings is that the local predator assemblage strongly

affects general pattems of nest predaîion around forest-field edges.

In interpreting the results of Chapters 2 and 3, one of the main obstacles that 1

faced was my inability to concretely explain mechanisms for encountered patterns o f nest

predation due to the lack of available background information regarding predator

behaviour. Compared to the nwnber of studies that examine the effects of habitat

structure on nest predation pattems, there is an extreme paucity of studies examining

habitat structural effects on nest predators. Ofien, researchen must rely on

interpretations presented in other nest predation studies to explain their results instead of

direct information fiom empincal studies of predator species. Uncornmon studies such as

Bowman and Harris' (1980) enclosure experiment with raccoons and Bider's (1968)

investigation of animal movement pattems around edges provide much better insight into

mechanistic explanations for predation pattems. Important areas of fiiture research in this

field include further experiments (eg. predator enclosure experiments) testing the effects

of structural variables on the ability of a variety of nest predator species to find nests,

studies tracking the movement of nest predators in areas of varying habitat configuration,

and investigations of the cues used by individual predator species to locate nests.

As illustrated by several of my findings that conflicted with prevalent notions

regarding pattems of nest predation, generalizations concerning nest predation patterns,

especially those around edges, fiequently do not hold between studies. For virtually

every trend that is discovered based on sound research (eg. decreased nest cover increases

nest predation intensity, Martin and Roper 1 988; gradua1 edges have lower nest predation

intensity than abrupt edges, Suarez et al. 1997; nest predation is greater near habitat

61

edges, Gates and Gysel 1978), there are other studies conducted with equivalent scientific

ngor demonstrating a different result (eg. decreased cover has no efTect on nest predation

intensity, Howlett and Stuchbury 1996; edge shape has no effect on nest predation

intensity, Huhta et al. 1998; distance to edge does not affect nest predation intensity,

Keyser et al. 1998). This disagreement arnong studies can be attributed to three main

factors which are frequently ignored in compansons between investigations of nest

predation patterns.

First, as mentioned above, the findings of this study and others show that nest

predator species react individually to di fferences in habitat structure (eg. Hannon and

Cotterill 1998, Rangen et al. 1999). Predator divenity and abundance vary regionaily,

and general predation patterns would be expected to shifi dong with predator assemblage

variation (Murcia 1995). For example, in study areas with one dominant predator species

(Cornrnon Raccoons, Whelan et al. 1994; Red Squirrels, Lewis and Montevecchi 1999),

nest predation patterns are driven by the responses of that species to habitat structurai

variation. Where the nest predator assemblage is more diverse, predation pattems result

from the combined reactions of a number of species to habitat structure. Second, ground

nests and above-ground nests are subject to different suites of nest predators (eg. Yahner

et al. 1 989, Soderstrom et al. 1 998, Rangen et al. 1 999), thus general patterns of nest

predation should not be expected to hold between nesting guilds in view of the

individuality of predator responses to habitat structure.

My third explanation for between-study discrepancies in nest predation patterns,

and one of particular relevance to my study, is the tendency to compare the effects of

habitat structure on the depredation of natwal nests and artificial nests. Inconsistent

findings can result f?om such comparisons if the relative contribution of each habitat

variable on nest predation intensity differs between the two groups. For example, in

Chapter 3,1 hypothesized that although predation of artificial nests did not differ between

gradual and abrupt edges (due to equivalent predator "traffic" on both edges types), the

greater vegetative cover at a larger scale offered by gradual edges may inhibit discovery

of naturaI nests by visually oriented nest predators cueing in on parental movements.

Parental defence (Montgomerie and Weatherhead 1988), the tendency for incubating

females to increase nest crypsis (Martin 1993a), and differing egg size between natural

and artificial nests (Haskel 1995b) may also result in varying effects of habitat structure

on predation in each group. For the most part, these differences in the effects of

structural features on predation between nest types operate once a predator is in the

vicinity of the nest. While artificid nests may be appropriate for tests of larger-scale

predation patterns that are dependent chiefly on overall predator abundance (eg. patch

size, degree of landscape fragmentation) they may not be as useful in tests of the effects

of smaller-scale habitat structure on nest predation (eg. nest visibility, stem density

around the nest) because these factors likely differ in their effect on predation of each

nest type.

This study makes an important contribution to the subject of avian ecology for

three main reasons. First, this is the only study where an adequate number of nests were

found to document the nesting habitat requirements of the Golden-winged Warbler;

information that is crucial in the development of management strategies to prrvent the

loss of this species. Second, this study identified a previously unknown factor affecting

nest predation on edges, edge linearity. The discovery of this novel factor affecting nest

predation is important both in allowing a greater understanding of edge-related nest

predation patterns, and in its potential application in land management decisions. Finally,

this is the first study documenting differential responses of individual predator groups to

variation in edge structure, which demonstrates the importance of predator assemblages

in determining patterns of edge-related nest predation, and may explain the conflicting

resufts of other studies investigating such patterns.

Summary

(1) Golden-winged Warblers placed their nests along edges with a more gradua1

edge dope and greater stem density around the nest than unused sites along the same

edges.

(2) Golden-winged Warbler nests were less susceptible to nest predation when

placed in areas of hi& Goldenrod density and high nest visibility.

(3) Artificial nests located on linear forest-field edges were subject to greater

predation intensity than those placed on cwilinear edges, but edge shape and nest

visibility did not affect predation intensity.

(4) Variation in edge structure and nest visibility affected the type of predator

that depredated artificial nests, demonstrating the importance of the local predator

assemblage in determining general predation trends.

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Appendix A. Correlation of variabln measured nt nests (n=43) and random sites (n=43) using Pcnrson product-momcot correlotions (JMP version 3.2.1, SAS Institute, Inc.). For ticscription of variables, sec Table 2.2.

Vari:iblc w 'TDgol DwstN Dwstl~l DwsîFO DgolN DgolFI DgolFO 1-10.5- 1.25 1-10.5- I .O MO.75- 1 .25 'I'Dwsi 1 .O000 0.2 166 0.91 08 0.8532 0.8944 0.2 170 0.22 19 0.2206 0.4877 0.4 127 0.3670 TDgol 0.261 6 1 .O000 0.2690 0.1093 0.3072 0.9240 0.8852 0.8934 0.2264 0.1740 0.2340 DwstN 0.9 1 08 0.2690 1.0000 0.8 196 0.7958 0.2052 0.1970 0.2556 0.4970 0,4246 0.3572 Dwst FI 0.8532 O, 1093 0.8 196 1 .O000 0.557 1 0.052 1 O, 105 1 0.0834 0.5290 0.4436 0,4034 Dwst FO 0.8944 0.3072 0.7958 0.557 1 1 .O000 0.2980 0.2489 0.2767 0.3546 0.28 10 0.2757 DgolN 0.2 170 0.9240 0.2052 0,052 1 0.2980 1.0000 0.8868 0.7938 0.2244 0.2062 0.2230 Dg01 FI 0.22 19 0.8852 O. 1970 0.1052 0.2489 0.8868 1 .O000 0,6528 0,2207 0.2647 O. 1671 Dgol FO 0,2206 0,8934 0.2556 0,0834 0.2767 0.7938 0.6528 1 .O000 O. 1 O 13 0.0263 0.1030 H0.5- 1.25 0.4877 0.2264 0.4970 0.5290 0.3546 0.2244 0.2207 O. 101 3 1 .O000 0.8722 0.7361 140.5- 1 .O 0.4 127 0.1740 0.4246 0,4436 0.28 10 0.2062 0.2647 0.0263 0.8722 1 .O000 0.4884 Ji0.75- I ,25 0.3670 0.2340 0.3572 0,4034 0.2757 0.2230 O. 167 1 O, 1030 0.736 1 0.4884 1 .O000