Forest Songbirds Research Articles

Effects of spatial habitat heterogeneity on habitat selection and annual fecundity for a migratory forest songbird

Understanding how spatial habitat patterns influence abundance and dynamics of animal populations is a primary goal in landscape ecology. We used an information-theoretic approach to investigate the association between habitat patterns at multiple spatial scales and demographic patterns for black-throated blue warblers (Dendroica caerulescens) at 20 study sites in west-central Vermont, USA from 2002 to 2005. Sites were characterized by: (1) territory-scale shrub density, (2) patch-scale shrub density occurring within 25 ha of territories, and (3) landscape-scale habitat patterns occurring within 5 km radius extents of territories. We considered multiple population parameters including abundance, age ratios, and annual fecundity. Territory-scale shrub density was most important for determining abundance and age ratios, but landscape-scale habitat structure strongly influenced reproductive output. Sites with higher territory-scale shrub density had higher abundance, and were more likely to be occupied by older, more experienced individuals compared to sites with lower shrub density. However, annual fecundity was higher on sites located in contiguously forested landscapes where shrub density was lower than the fragmented sites. Further, effects of habitat pattern at one spatial scale depended on habitat conditions at different scales. For example, abundance increased with increasing territory-scale shrub density, but this effect was much stronger in fragmented landscapes than in contiguously forested landscapes. These results suggest that habitat pattern at different spatial scales affect demographic parameters in different ways, and that effects of habitat patterns at one spatial scale depends on habitat conditions at other scales.

First Confirmed Record of Breeding Brown Creepers in the Northwest Territories, Canada

The Brown Creeper (Certhia americana) is a small, bark-foraging songbird found throughout the forested regions from Central America north to south-central Canada and Alaska. Its avoidance of edges, cryptic plumage, and highpitched songs and calls contribute to a low detection rate for this species on most surveys (Hejl and others 2002; Alberta Sustainable Resource Development 2003). The abundance and distribution of the species at the northern edge of its range is poorly understood and is likely currently underestimated (Hejl and others 2002; Alberta Sustainable Resource Development 2003). Although creepers are known to occur along the Pacific coast to south-central Alaska, no confirmed breeding records have been reported above 606N latitude east of the continental divide (Hejl and others 2002). In this note, we document the 1st known occurrences of breeding Brown Creepers in the Northwest Territories, Canada. We observed breeding activity at 2 Brown Creeper territories within 100 m of point-count stations initiated for long-term forest songbird monitoring by the Canadian Wildlife Service (CWS) in the Liard Valley of the Northwest Territories. The study area for regional songbird monitoring spans over 700 km, and includes over 195 point-count stations and several line transects and census routes (Machtans and Latour 2003). The Liard Valley is in the far southwest corner of the Northwest Territories, just north of the British Columbia border and east of the Yukon border. Our observations were made in the low relief boreal forest region east of the Liard River. The Liard Valley typically has higher average temperatures and more precipitation than many regions of the northern boreal forest (Machtans and Latour 2003). Forests are typically old and, on average, taller than in other areas of the Northwest Territories. Vegetation and site characteristics for the Liard Valley are described in Machtans (2000) and Machtans and Latour (2003). From 15 June to 12 July 2008, several observations of Brown Creepers were made at 2 locations near Fort Liard, Northwest Territories (606139N, 1236229W) that confirmed breeding by this species in the area. On 15 June 2008, 2 Brown Creepers were heard and observed foraging during point-count surveys in the area (UTM Zone 10: 480493E 6697423N, NAD1983; hereafter location A). Upon further investigation, at least 3 juvenile creepers were flushed from the trunk of a tree, which provoked distraction displays from one or both parents. This display consisted of several aggressive flights to within 2 m of the observer’s head. On 21 June 2008, an audio recording of a singing Brown Creeper was obtained using a high quality hand held digital recorder (Edirol R-09, Roland Corporation) near the same location. On 6 July 2008, at least 4 Brown Creepers were foraging approximately 300 m from the original location. Several pictures were taken of the birds observed on 6 July (Fig. 1). On 12 July 2008, these birds were again observed and an inactive nest was found near the original sighting at location A (Fig. 2). The nest was 5.5 m above the ground in a 35-m-tall Trembling Aspen (Populus tremuloides) snag with a diameter at breast height (dbh) of 39 cm. On 15 June 2008, a Brown Creeper was heard and observed for about 10 min during a pointcount survey at a location approximately 4.6 km away from location A (UTM Zone 10: 480601E 6692793N, NAD1983; hereafter location B). On NORTHWESTERN NATURALIST 90:156–159 AUTUMN 2009 156 NORTHWESTERN NATURALIST 90(2)

Short- and long-term implications of clearcut and two-age silviculture for conservation of breeding forest birds in the central Appalachians, USA

Two-age (deferment or leave tree) harvesting is used increasingly in even-aged forest management, but long-term responses of breeding avifauna to retention of residual canopy trees have not been investigated. Breeding bird surveys completed in 1994–1996 in two-age and clearcut harvests in the central Appalachian Mountains of West Virginia, USA allowed us to document long-term changes in these stands. In 2005 and 2006, we conducted point counts in mature unharvested forest stands and in 19–26 year-old clearcut and two-age harvests from the original study and in younger clearcut and two-age stands (6–10 years old). We found differences in breeding bird metrics among these five treatments and temporal differences in the original stands. Although early-successional species are typically absent from group selection cuts, they were almost as common in young two-age stands as clearcuts, supporting two-age harvests as an alternative to clearcutting. Although older harvests had lower species richness and diversity, they were beginning to provide habitat for some species of late-successional forest songbirds that were absent or uncommon in young harvests. Overall, late-successional forest-interior species were more flexible in their use of different seral stages; several species used both age classes and harvest types in addition to mature forest, which may reflect the lack of edges in our heavily-forested landscape. Consequently, two-age management provides habitat for a diverse group of species as these stands mature and may be an ecologically sustainable alternative to clearcutting in landscapes where brown-headed cowbirds ( Molothrus ater) are uncommon.

Comparison of Methods for Estimating Bird Abundance and Trends From Historical Count Data

Abstract: The use of bird counts as indices has come under increasing scrutiny because assumptions concerning detection probabilities may not be met, but there also seems to be some resistance to use of model‐based approaches to estimating abundance. We used data from the United States Forest Service, Southern Region bird monitoring program to compare several common approaches for estimating annual abundance or indices and population trends from point‐count data. We compared indices of abundance estimated as annual means of counts and from a mixed‐Poisson model to abundance estimates from a count‐removal model with 3 time intervals and a distance model with 3 distance bands. We compared trend estimates calculated from an autoregressive, exponential model fit to annual abundance estimates from the above methods and also by estimating trend directly by treating year as a continuous covariate in the mixed‐Poisson model. We produced estimates for 6 forest songbirds based on an average of 621 and 459 points in 2 physiographic areas from 1997 to 2004. There was strong evidence that detection probabilities varied among species and years. Nevertheless, there was good overall agreement across trend estimates from the 5 methods for 9 of 12 comparisons. In 3 of 12 comparisons, however, patterns in detection probabilities potentially confounded interpretation of uncorrected counts. Estimates of detection probabilities differed greatly between removal and distance models, likely because the methods estimated different components of detection probability and the data collection was not optimally designed for either method. Given that detection probabilities often vary among species, years, and observers investigators should address detection probability in their surveys, whether it be by estimation of probability of detection and abundance, estimation of effects of key covariates when modeling count as an index of abundance, or through design‐based methods to standardize these effects.

Breeding Bird Responses to Silvicultural Treatments In the Klamath Province of Oregon

Abstract Silvicultural practices such as thinning are performed both for commodity production and to help achieve fuel reduction and biodiversity goals. Although there are several studies that have examined effects of clearcut harvest techniques on forest songbirds, few have examined the effects of thinning. We studied the response of songbirds to different silvicultural approaches to forest management. We experimentally manipulated three 80 to 110 y-old Douglas-fir stands to evaluate influences of commercial thinning and clearcut harvest silvicultural systems on populations of diurnal breeding birds in southwestern Oregon. We conducted 10 to 12 bird point count surveys in each stand between 1992 and 1994 (prior to treatment), and point count surveys of birds 6 times each year from 1998 through 1999 (subsequent to treatment). We found that relative abundance and species richness were highest in the uncut control plots, slightly lower in the thinned plots, and lowest in the clearcut plots. A species-by-spe...

SURVIVAL OF FLEDGLING HOODED WARBLERS (WILSONIA CITRINA) IN SMALL AND LARGE FOREST FRAGMENTS

Studies on the effects of forest fragmentation on the reproductive ecology of forest songbirds have focused almost exclusively on the egg and nestling stages. Little is known about survival or habitat use of fledglings in their first weeks out of the nest. We radiotracked adult Hooded Warblers (Wilsonia citrina) attending fledglings from large (>150-ha) and small (<30-ha) forest fragments during 2002 and 2003. Mark–recapture models were used to assess the effects of forest-fragment size and age of the attending adult on the survival of young. We also examined the structure of nesting- versus postfledging-habitat used by fledglings. Only 19% of fledgling Hooded Warblers survived the 28-day fledgling period prior to independence, and fledglings' daily survival probability was lowest in the first four days after they left the nest (1–2 days postfledging: 0.72; 3–4 days postfledging: 0.69). Fledgling survival did not differ between large and small forest fragments. Large fragments were more likely to contain older adults, and age of parent had a positive influence on fledgling survival, independent of fragment size. As in many other studies, overall productivity was higher within larger fragments; this was largely attributable to differences in number fledged rather than fledgling survival. Habitat used by fledglings was more structurally complex than habitat at nest sites, which indicates that fledglings may seek habitat with high cover to avoid predation. Fledgling survival was much lower than estimates used in demographic models, which suggests that for many species, these models may need re-evaluation. Supervivencia de Volantones de Wilsonia citrina en Fragmentos de Bosque Pequeños y Grandes

Comparing geographic boundaries in songbird demography data with vegetation boundaries: a new approach to evaluating habitat quality

To plan for the habitat needs of forest songbirds of conservation concern, managers need to understand how spatial heterogeneity in forest conditions influences habitat quality. I used difference boundary detection (wombling) and spatially constrained clustering to delineate boundaries in various combinations of four forest vegetation variables (understory height, understory density, percent deciduous vs. conifer understory, and percent canopy closure) in two Michigan northern hardwood forests. My goal was to identify vegetation boundaries that corresponded with boundaries in an understory-dependent songbird’s distribution, and with boundaries in demographic measures for this songbird that indicate habitat quality (e.g., occupancy by older vs. yearling males, reproductive success). Both forests were actively-managed, mature stands: The first site (78 ha) was heavily deer-browsed (HB), with many browse-resistant conifers in the understory, and the second (62 ha) was less-browsed (LB), with deciduous-dominated understory. I compared the vegetation difference and cluster boundaries to difference boundaries based on 6 years of distribution and demographic data for black-throated blue warblers (Dendroica caerulescens). At the HB site, warbler boundaries overlapped strongly with vegetation boundaries that included all four variables, and clustering effectively divided the habitat into areas with different warbler occupancy and demographic characteristics. At the LB site, warbler distribution showed high overlap with difference and cluster boundaries based on just the height and density of understory vegetation, and cluster boundaries again effectively partitioned the study area into sites that varied in habitat quality. Thus, geographic boundary analysis is likely to be a useful tool for identifying key vegetation variables for management, and for delineating clusters (habitat patches) within sites that capture differences in habitat quality.

Managing breeding forest songbirds with conspecific song playbacks

AbstractRecent studies confirm that several territorial songbird species use conspecific cues, including song, when selecting habitat. We conducted a controlled experiment with a population of black‐throated blue warblers Dendroica caerulescens, a species of concern in boreal hardwood transition forests, to determine whether song playbacks influence settlement patterns. We tested whether conspecific song broadcast during spring arrival attracts black‐throated blue warblers during territory establishment, and whether song playbacks shift territories from the previous year's breeding distributions. The results of our experiment demonstrate that song playbacks significantly increased territorial occupancy and shifted territories closer to the speakers. Loss of breeding habitat is implicated in population declines of many songbirds, and our results suggest that song playbacks should be further investigated as a tool for mitigating the effects of disruptive management actions. Additional work should consider the spatial arrangement of playback speakers to avoid attracting birds from safe areas. The demographic consequences of conspecific attraction must also be explored before this technique is widely implemented.

Consequences of Habitat Utilization by Nest Predators and Breeding Songbirds Across Multiple Scales in an Urbanizing Landscape

Abstract Abstract. Nest predation may influence habitat selection by birds at multiple spatial scales. We blended population and community ecology to investigate this possibility for 15 species of forest songbirds and their diurnal nest predators (corvids and sciurids) in 28 1 km2 sites near Seattle, Washington, from 1998 to 2004. We determined whether songbirds were positively or negatively associated with nest predators at three spatial scales, and whether their co-occurrence affected reproductive success. At the largest ‘neighborhood’ scale (1 km2 areas that included suburban and exurban development and second-growth forest remnants), nest predators and their prey were positively or negatively correlated according to general species-specific habitat associations. At the intermediate ‘forest patch’ scale (among remnant forested areas 0.5 to 70 ha), associations between predators and prey were generally weak. At the smallest ‘within patch’ scale (multiple 50 m radius survey plots within each forest patch), some songbird species avoided areas with greater predator use, particularly by Steller's Jays (Cyanocitta stelleri). Failed nests and territories tended to be in locations of higher predator occurrence (especially of corvids) than successful ones, but at the largest 1 km2 neighborhood scale relative abundance of nest predators was not correlated with the fate of nesting attempts or annual reproductive success. Reproductive success was generally high, with 52% of all nests and 49% of all territories fledging at least one young (for all species and years combined). Nest predation influenced some species' use of resources, but was not a strong influence on overall reproductive success or community structure.

Factors affecting nest predation on forest songbirds in North America

Nest predation is an important factor in the ecology of passerines and can be a large source of mortality for birds. I provide an overview of factors affecting nest predation of passerines in North America with the goal that it may provide some insight into the ecology and management of woodland birds in the United Kingdom. Although several factors influence productivity, nest success is perhaps the most widely measured demographic characteristic of open‐cup‐nesting birds, and nest predation is usually the largest cause of nest failure. The identity of predator species, and how their importance varies with habitat and landscape factors, must be known for managers and scientists to design effective conservation plans and place research on nest predation in the appropriate context. Recent studies using video surveillance have made significant contributions to our understanding of the relative importance of different predator taxa in North America. Spatial and temporal variation in nest predation can be better understood when landscapes are placed in a biogeographical context and local habitat and nest‐site effects are placed in a landscape context. Low productivity resulting from high nest predation is one of several potential causes of bird population declines in North America and the UK. Although the ‘forest fragmentation paradigm’ from the eastern US may not apply directly to the UK, thinking about avian demographics from a multiscale perspective, and consideration of factors affecting nest predation with knowledge of the dominant predator species, may provide insight into population declines.

CONSEQUENCES OF HABITAT UTILIZATION BY NEST PREDATORS AND BREEDING SONGBIRDS ACROSS MULTIPLE SCALES IN AN URBANIZING LANDSCAPE

Abstract Abstract. Nest predation may influence habitat selection by birds at multiple spatial scales. We blended population and community ecology to investigate this possibility for 15 species of forest songbirds and their diurnal nest predators (corvids and sciurids) in 28 1 km2 sites near Seattle, Washington, from 1998 to 2004. We determined whether songbirds were positively or negatively associated with nest predators at three spatial scales, and whether their co-occurrence affected reproductive success. At the largest ‘neighborhood’ scale (1 km2 areas that included suburban and exurban development and second-growth forest remnants), nest predators and their prey were positively or negatively correlated according to general species-specific habitat associations. At the intermediate ‘forest patch’ scale (among remnant forested areas 0.5 to 70 ha), associations between predators and prey were generally weak. At the smallest ‘within patch’ scale (multiple 50 m radius survey plots within each forest patch...

Influence of Forest Vegetation Structure on Avian Community Composition in West-Central Idaho

Abstract Locally derived information is needed if management guidelines intended to improve wildlife habitat are to be relevant, reliable, and applicable. Within the Intermountain West, few studies have examined habitat relationships of forest songbirds in mixed-conifer forests. During 1996–1997, we studied breeding bird communities in a mixed-conifer forest in west-central Idaho, USA, to describe how relative avian abundance was related to forest structure. Our study objectives were to 1) describe bird use of 3 distinct forest age-classes, 2) identify groups of species that expressed similar relationships with forest structural attributes, and 3) model species-specific bird–habitat relationships. We sampled 46 study sites (forest stands) classified into 1 of 3 common “vegetation growth stages” (seral stages): 1) shrub/seedling/sapling (n = 15 stands), 2) small tree (n = 16 stands), and 3) old growth (n = 15 stands). In each stand we surveyed breeding birds using fixed-radius point counts and measured for...

Social cues facilitate habitat selection: American redstarts establish breeding territories in response to song

For migratory songbirds nesting in northern temperate forests, a short breeding season demands that males rapidly establish territories. Because critical insect and vegetation resources are unavailable during spring arrival, we suggest that conspecifics serve as settlement cues for males new to a local population. To test conspecific attraction, we conducted playback experiments with American redstarts Setophaga ruticilla. Experimental results demonstrate that song playbacks strongly attract conspecifics, recruiting an average of 4.2 additional males per plot; adult males new to our sites increased, while yearling males failed to respond. Yearlings arrived 6 to 10 days later than adults, raising the possibility that yearlings responded to songs of early arriving adults rather than to playbacks. Our work indicates that conspecific attraction is an important mechanism for breeding habitat selection in an established population of a migratory forest songbird, but the effect is moderated by age, reproductive experience and arrival timing.

Factors Influencing Predation on Ovenbird (Seiurus Aurocapilla) Nests in Northern Hardwoods: Interactions Across Spatial Scales

AbstractMicrosite, patch, and landscape conditions may interact to influence nest predation. In northern Minnesota, silvicultural and agricultural practices may be involved in recent increases in nest predators and regional declines in several ground-nesting songbirds. To examine this problem, we evaluated 17 hierarchical models of predation on Ovenbird (Seiurus aurocapilla) nests that included microsite variables, distances to edges, and amount of core forest within a 2-km radius surrounding six study plots. During 2000 and 2001, 157 Ovenbird nests were monitored to estimate nest predation rates. A model that included the main effects of litter depth and core forest area and an interaction term between the two best described variation in predation on Ovenbird nests (AICc weight = 0.83). The nest predation rate from this model was 0.51 ± 0.01 (mean ± SE), assuming mean values of litter depth and amount of core forest. Shallow litter was associated with higher nest predation in three plots surrounded by less core habitat (40–60 ha), whereas there was no relationship in three plots surrounded by more core area (100–150 ha). Management that promotes deep leaf litter and the maintenance of large, intact forest tracts will likely benefit Ovenbirds and other forest songbirds.Factores que Influencian la Depredación de Nidos de Seiurus aurocapilla en los Bosques del Norte: Interacciones a través de las Escalas Espaciales

A New Species of Bush-warbler from Bougainville Island and a Monophyletic Origin for Southwest Pacific Cettia

We describe a new species of Cettia from the Crown Prince Range, Bougainville Island, North Solomons Province, Papua New Guinea. By combining morphometric and molecular phylogenetic techniques, we attempt to broaden our understanding of evolutionary processes within the genus Cettia in the southwest Pacific. The new species proves to be distinct with respect to several morphological characteristics that are most probably related to a more terrestrial lifestyle than that of its congeners. Molecular data agree with morphological data in establishing that these birds are at least as distinct from the other island forms of Cettia as those forms are from each other, far exceeding intraspecific variation. These data and the restricted distribution of the population on Bougainville strongly support recognition of a new species. The application of molecular phylogenetic techniques also supports the idea that the new species and other island forms of Cettia confined to mountains on southwest Pacific islands are derived from a single common ancestor rather than being independently derived from one or more mainland forms. In addition, the relatively recent discovery of two new species of Cettia suggests that additional forms await discovery in other poorly known areas of the southwest Pacific. Our results point to the need for further molecular studies and for additional field research into the distribution and ecology of forest songbirds on islands.

Spatial Aggregation of Forest Songbird Territories and Possible Implications for Area Sensitivity

Bourque, J. and A. Desrochers 2006. Spatial aggregation of forest songbird territories and possible implications for area sensitivity. Avian Conservation and Ecology - Écologie et conservation des oiseaux 1(2): 3.http://dx.doi.org/10.5751/ACE-00043-010203

Gamebird vulnerability to forest fragmentation: patch occupancy of the crested guineafowl (Guttera edouardi) in Afromontane forests

AbstractThe forest‐restricted and social crested guineafowlGuttera edouardiis believed to be declining in Afromontane forests due to the fragmentation of its habitat. Incidence functions, derived from presence/absence data from 82 forests, were used to investigate area, isolation and environmental/anthropogenic effects on guineafowl patch occupancy and persistence. The incident probability of the guineafowl increased significantly with increasing area, but was invariant with increasing isolation distance from a putative mainland source patch. Most large patches (&gt;24 ha, mean of occupied patches=192 ha), comprising 22% of those surveyed, were occupied. From incidence functions the estimated minimum viable patch size, at which the extinction probability was unity, was 33 ha. Guineafowl were less likely to be present in forests close (&lt;0.5 km) to human dwellings. Apart from the proximity of a patch to dwellings, all other environmental factors had no significant effect on occupancy patterns. However, because of their large home‐range area requirements, the guineafowl are more vulnerable to fragmentation effects, especially area effects, than most forest songbird species. Although reluctant dispersers, crested guineafowl appear to persist in mainland–island metapopulations with low immigration rates. The crested guineafowl is not under immediate threat of regional extinction and current populations are not too isolated to create a sound mainland–island metapopulation network.

Habitat occupancy patterns of a forest dwelling songbird: causes and consequences

We examined patterns of habitat use and reproductive performance of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens (Gmelin, 1789)), within a 3160-ha forested landscape. We surveyed 371 sites over a 3-year period. Some sites were never occupied, while others were occupied for 1, 2, or 3 years. For these 3 years we found that warbler abundance increased with frequency of occupancy. Additionally, we found that (i) deciduousness and understory shrub density increased with frequency of occupancy; (ii) in 1 of 3 years, food abundance was higher at the most frequently occupied sites; and (iii) nest predators exhibited predator-specific abundance patterns across occupancy categories. We next used occupancy patterns documented in the first 3 years of the study to predict settlement, age structure, and reproductive performance at a subset of sites in the final year of the study. We found that males settled earlier in the breeding season at sites with a high frequency of occupancy. There were no differences in arrival times of females. Additionally, age structure did not vary for either males or females across sites with different occupancy levels. Although we found no difference in mean reproductive output across sites with different occupancy levels, over 50% of the young produced fledged from territories overlapping the high occupancy sites.

Avian Mobbing Response is Restricted by Territory Boundaries: Experimental Evidence from Two Species of Forest Warblers

AbstractPredator mobbing has been viewed as an adaptation to reduce the risk of predation, however, factors influencing mobbing behaviour are still debated. We report on the results of an experiment with Dendroica caerulescens and Dendroica virens designed to determine (1) whether mobbing response by forest songbirds during the breeding season is restricted by territory boundaries, (2) the distance songbirds will move in response to anti‐predator mobbing calls, and (3) whether reproductive status, age, and time of the breeding season determine the distance moved to mob. We did not detect an effect of reproductive status, age, or time of breeding season on the distance moved by birds to mob. All birds responded to the mobbing playback within their territory (defined by territorial defence in relation to specific song playbacks). The maximum distance moved within a territory to engage in mobbing ranged from 25 to 175 m ( = 72 ± 6 m). Three of 37 birds responded to playbacks outside their territory boundaries. In all three cases, maximum movement distances outside territories were short (25 m). Thus, for two species of warblers, mobbing is highly constrained by territory boundaries during the breeding season. This finding is congruent with arguments that mobbing is primarily a selfish behaviour, at least with respect to conspecifics. Our results also provide support for the ‘move‐on’ hypothesis.

Edge and Area Effects on the Occurrence of Migrant Forest Songbirds

Abstract: Concerns about forest fragmentation and its conservation implications have motivated numerous studies that investigate the influence of forest patch area and forest edge on songbird distribution patterns. The generalized effects of forest patch size and forest edge on animal distributions is still debatable because forest patch size and forest edge are often confounded and because of an incomplete synthesis of available data. To fill a portion of this gap, we incorporated all available published data (33 papers) in meta‐analyses of forest edge and area effects on site occupancy patterns for 26 Neotropical migrant forest‐nesting songbirds in eastern North America. All reported area effects are confounded or potentially confounded by edge effects, and we refer to these as “confounded” studies. The converse, however, is not true and most reported edge effects are independent of patch area. When considering only nonconfounded studies of edge effects, only 1 of 17 species showed significant edge avoidance and 3 had significant affinity for edges. In confounded studies, 12 of 22 species showed significant avoidance of small patches and edges, and 1 had an affinity for small patches and edges. Furthermore, average effect sizes averaged across studies or species tended to be higher for confounded studies than for edge studies. We discuss three possible reasons for differences in results between these two groups of studies. First, studies of edge effects tended to be carried out in landscapes with greater forest cover than studies of confounded effects; among confounded effects studies, as forest cover increased, we observed a nonsignificant trend towards decreasing strength of small patch or edge avoidance effects. Thus, the weaker effects in edge studies may be due to the fact that these studies were conducted in forest‐dominated landscapes. Second, we may have detected strong effects only in confounded studies because area effects are much stronger than edge effects on bird occurrence, and area effects drive the results in confounded studies. Third, edge and area effects may interact in such a way that edge effects become more important as forest patch size decreases; thus, both edge and area effects are responsible for results in confounded studies. These three explanations cannot be adequately separated with existing data. Regardless, it is clear that fragmentation of forests into small patches is detrimental to many migrant songbird species.