Empidonax Virescens Research Articles

Nesting Success of a Songbird in a Complex Floodplain Forest Landscape in llinois, USA: Local Fragmentation vs. Vegetation Structure

Measuring edge effects in complex landscapes is often confounded by the presence of different kinds of natural and anthropogenic edges, each of which may act differently on organisms inhabiting habitat patches. In such landscapes, proportions of different habitats surrounding nests within patches often vary and may affect nesting success independently of distance to edges. We developed methods to measure and study the effects of multiple edges and varying habitat composition around nests on the breeding success of the Acadian flycatcher (Empidonax virescens), an understory, open-cup nesting songbird. The Kaskaskia River in Southwestern Illinois was our study area and consists of wide (>1000-m) floodplain corridors embedded in an agricultural matrix with a variety of natural (wide rivers, backwater swamps, and oxbow lakes) and anthropogenic (internal openings, and agricultural) habitats. We also measured vegetation structure around each nest. Nest survival increased with increasing nest concealment, and probabilities of brood parasitism increased with increasing distances from anthropogenic and natural water-related openings surrounding nests. The magnitude of these effects was small, probably because the landscape is saturated with nest predators and brood parasites. These results illustrate the importance of considering both larger landscape context and details of natural and anthropogenic disturbances when studying the effects of habitat fragmentation on wildlife.

Factors Influencing Acadian Flycatcher Nesting Success in an Intensively Managed Forest Landscape

We examined factors affecting the nesting success of a migratory songbird, the Acadian flycatcher (Empidonax virescens), in loblolly pine plantations in the coastal plain of South Carolina, USA. From 1997–2000, we located and monitored 163 Acadian flycatcher nests in loblolly pine stands and corridors that were 18–27 years old. We used Mayfield logistic regression (Aebischer 1999, Hazler 2004) to model the effects of edge and stand-level vegetation structure on nest daily survival rate. There was no evidence of an effect of edge on nest survival, but nest survival was positively related to the height of the deciduous subcanopy and to the density of shrub cover. Although Acadian flycatchers are generally regarded as habitat specialists requiring mature hardwood forests, our data suggest that pine plantations can support breeding populations, provided that a substantial hardwood component is present. We believe that maintaining multiple vegetation strata and increasing the length of harvest rotations would improve the habitat value of pine plantations for Acadian flycatchers and presumably other species more typically associated with deciduous forests. Maintenance of a corridor network, as practiced by some industrial forest managers, is one means of providing more mature forest habitat, thereby fostering higher nesting success. Concern that these corridors might act as ecological traps seems to be unwarranted in our study area. Corridors thus appear to be a valuable management tool for promoting wildlife values within the context of an industrial forest landscape.

Scale-Dependent Habitat use of Acadian FlyCatcher (Empidonax Virescens) in Central Ohio

AbstractWe used a multiscale approach to examine use of riparian forests by Acadian Flycatchers (Empidonax virescens) at three distinct scales: microsite (nest or territory), local stand (3-ha patch of forest), and landscape (1-km-radius area). We examined the extent to which understory arthropod biomass, vegetation structure, and breeding productivity affected habitat use by Acadian Flycatchers across 36 mature riparian forests along an urbanization gradient in central Ohio, 2001–2002. Using an information-theoretic approach, we found that although vegetation structure and understory arthropod biomass were associated with nest and territory location at small spatial scales, they failed to explain habitat use across stands. Instead, percentage of urban cover in the landscape best explained variation in the abundance of Acadian Flycatchers among stands. In fact, Acadian Flycatchers were >3× more abundant in the most rural (≤1% urban) riparian forests than in the more urban (≥10% urban) riparian forests. Nest survival and productivity also declined as urban development increased within the landscape. We suggest that increased nest predation in urban riparian forests, coupled with either reduced productivity or site fidelity or both, may be an underlying mechanism of landscape-scale patterns. Results suggest that explicit consideration of multiple spatial scales, and especially landscape matrix composition, is critical for understanding habitat use.Uso del Hábitat Dependiente de la Escala por parte de Empidonax virescens en el Centro de Ohio

SCALE-DEPENDENT HABITAT USE OF ACADIAN FLYCATCHER (EMPIDONAX VIRESCENS) IN CENTRAL OHIO

Abstract We used a multiscale approach to examine use of riparian forests by Acadian Flycatchers (Empidonax virescens) at three distinct scales: microsite (nest or territory), local stand (3-ha patch of forest), and landscape (1-km-radius area). We examined the extent to which understory arthropod biomass, vegetation structure, and breeding productivity affected habitat use by Acadian Flycatchers across 36 mature riparian forests along an urbanization gradient in central Ohio, 2001–2002. Using an information-theoretic approach, we found that although vegetation structure and understory arthropod biomass were associated with nest and territory location at small spatial scales, they failed to explain habitat use across stands. Instead, percentage of urban cover in the landscape best explained variation in the abundance of Acadian Flycatchers among stands. In fact, Acadian Flycatchers were >3× more abundant in the most rural (≤1% urban) riparian forests than in the more urban (≥10% urban) riparian forests. N...

Factors Affecting Avian Species Richness and Density in Riparian Areas

Riparian areas are an important part of conservation efforts directed toward minimizing the effects of habitat loss on birds because riparian areas host a large number of bird species. We compared species richness and densities of breeding songbirds among 3 narrow (55–95 m) and 3 wide (400–530 m) forested-riparian areas with adjacent grassland-shrub buffer strips and 3 narrow and 3 wide forested-riparian areas without adjacent grassland-shrub buffer strips, in northeastern Missouri, USA. More bird species occurred in wide than in narrow forested-riparian areas (mean difference = 8.58, 95% CI = 4.82–12.35). Wide forested-riparian areas with and without adjacent buffer strips had similar numbers of species (mean difference = 0.50, 95% CI = −2.77–3.77). Narrow forested-riparian areas with adjacent buffer strips had more species than narrow forested-riparian areas without adjacent buffer strips (mean difference = 9.67, 95% CI = 5.43–13.90). A mixed model with a fixed effect of width had the most support for predicting density of ovenbird (Seiurus aurocapillus), Acadian flycatcher (Empidonax virescens), Kentucky warbler (Oporornis formosus), gray catbird (Dumetella carolinensis), common yellowthroat (Geothlypis trichas), northern cardinal (Cardinalis cardinalis), and indigo bunting (Passerina cyanea). The null model had the most support for wood thrush (Hylocichla mustelina), Louisiana waterthrush (Seiurus motacilla), and field sparrow (Spizella pusilla). Wide forested-riparian areas provided breeding habitat for more bird species than narrow forested-riparian areas, especially forest area–sensitive species. The addition of grassland-shrub buffer strips adjacent to forested-riparian areas increased species richness in those areas and provided habitat for gray catbird, common yellowthroat, northern cardinal, field sparrow, and indigo bunting.

Individuality in songs of Acadian flycatchers and recognition of neighbours

Acadian flycatchers, Empidonax virescens , like many other tyrant flycatchers (Tyrannidae), sing complex songs with little variation among individuals. Measurements of frequency and timing revealed that individuals' songs included consistent but slight individual differences. A comparison of differences between songs of territorial neighbours and between more distant males revealed no evidence that neighbours learn the features of each other's songs. Playbacks of neighbours' and strangers' songs for 30 min at predetermined distances from singing subjects provided marginal evidence that territorial birds recognize these differences. Playbacks for only 2 min provided no such evidence, although similar experiments with hooded warblers, Wilsonia citrina , and Kentucky warblers, Oporornis formosa , in the same forest had provided clear evidence for recognition of individual neighbours. The slight differences in individuals' songs and the degradation of songs during propagation in a forest must make this discrimination difficult for Acadian flycatchers. In tyrant flycatchers, complex songs with innate developmental constraints might evolve by sexual selection for species recognition despite disadvantages for individual recognition.

Reproductive success of Acadian Flycatchers in the Blue Ridge Mountains of Virginia

Abstract Conservation strategies for Neotropical migratory birds have emphasized identification and preservation of habitats in which populations are reproducing above replacement rates. In 2000–2001, we monitored 141 Acadian Flycatcher (Empidonax virescens) nests in the extensively forested Blue Ridge Mountains in Virginia to assess the site's potential to host “source” populations of this common species. Our estimates of reproductive success, unlike most studies, incorporated the nesting behavior of marked females followed throughout the breeding season. Despite the presence of Brown-headed Cowbirds (Molothrus ater) in a nearby 15-y-old clearcut and in open areas of a summer camp, we found no parasitized nests. Overall, the predation rate was 41%, 2.5 fledglings were produced per successful nesting attempt, and seasonal fecundity approached two female fledglings per adult female per season. Given return rates (minimum survival rates) of 52% at our study site, our measured levels of reproductive success ...

Male Acadian flycatchers, Empidonax virescens, obtain extrapair fertilizations with distant females

We used genetic parentage analysis and radiotracking to determine how far male Acadian flycatchers travel from their territory in search of extrapair matings. Acadian flycatchers on our study site have a linear spatial distribution of territories associated with watercourses. With this type of spatial arrangement, males have only one or two immediate neighbouring females within 100 m and therefore have limited opportunities for extrapair (EP) fertilizations unless they travel far from their territory. In our sample of 53 nests collected over 4 years, 58% of nests contained EP young and 40% of nestlings were EP young. We identified the EP sire and the location of his territory at 13 nests with EP young, and over half (7/13) were not immediate neighbours. The mean distance between the territory of the EP sire and the territory where he attained EP fertilizations was 526 m. We were also able to exclude all immediate neighbours as EP sires for 12 additional nests containing EP young. The minimum average distance between an EP sire's territory and the territory containing an EP young that he sired was estimated to be 910 m. Overall, the EP sire was a neighbour in only 24% of nests with EP young. Radiotelemetry observations showed that six of seven mated males made off-territory forays and averaged 0.4 forays/h. Males travelled an average of 202 m off-territory (range 50–1500 m) and were away from their territories an average of 10.2 min/foray. Because our findings contrast sharply with those of other studies examining extrapair fertilization in passerines, in which most extrapair sires are neighbours, we suggest that when males have few adjacent neighbouring females, the benefits of searching widely to increase the number of potential extrapair mates can outweigh the costs of EP forays.

FOREST MANAGEMENT ALTERNATIVES AND SONGBIRD BREEDING HABITAT ON THE CHEROKEE NATIONAL FOREST, TENNESSEE

We used a 60-yr forest simulation of the Cherokee National Forest, Tennessee, USA, to model the effects of timber harvest and natural disturbance upon habitat availability for 6 songbird species: Acadian flycatcher (Epidonax virescens), blue-headed vireo (Vireo solitarius), chestnut-sided warbler (Dendroica pensylvanica), tufted titmouse (Parus bicolor), yellow-billed cuckoo (Coccyzus americanus), and yellow-throated warbler (Dendroica dominica). Forest simulations, based on expected harvest intensities and historic levels of natural disturbance, were used to update a stand inventory database at 10-yr intervals between 1993 and 2053. Habitat models for the 6 bird species were applied to the updated stand inventory and available habitat quantified for each decade. Late-successional species showed substantial increases in habitat availability over the 60-yr period at most harvest intensities, whereas habitat for early-successional species was stable or declined at most harvest intensities. Acadian flycatcher, yellow-throated warbler, and blue-headed vireo habitat increased by 200%, 213%, and 40%, respectively, whereas tufted titmouse habitat remained relatively constant at expected harvest levels. Chestnut-sided warbler habitat was stable at expected harvest levels but declined at lower harvest intensities, and yellow-billed cuckoo habitat declined by 37% at expected harvest levels. Natural disturbance had little effect on habitat availability for any bird species compared to the effects of timber harvests and increasing forest age. Our models suggest that anthropogenic disturbance, and lack thereof, can play a definitive role determining habitat availability and population viability for forest songbirds.

Width of Forest Streamside Zones and Breeding Bird Abundance in Eastern Texas

Abstract We evaluated breeding bird communities in forested streamside zones in eastern Texas to determine threshold widths of riparian forest that were associated with the addition of mature-forest-breeding birds and loss of shrub-breeding birds. We observed an association of shrub-breeding birds with narrow streamside zones and an increasing number of mature forest species within wider streamside zones. Streamside zones also provided song perches for many shrub breeding species. Although many bird species increased or decreased in a generally linear pattern as streamside zone width increased, some species appeared to have threshold widths associated with their presence. The Acadian Flycatcher (Empidonax virescens), Yellow-throated Vireo (Vireo flavifrons), and Yellow-billed Cuckoo (Coccyzus americanus) seemed to require at least 70 m of forest width before their abundance increased. In contrast, the Blue Grosbeak (Passerina caerulea), Painted Bunting (Passerina ciris), and Prairie Warbler (Dendroica dis...

Effects of prescribed burning on distribution and abundance of birds in a closed-canopy oak-dominated forest, Missouri, USA

Prescribed, biennial burning in forest understory started in Cuivre River State Park, Missouri, USA, in the late 1980s to help restore the forest to conditions that existed prior to European settlement. Bird surveys were started in 1996 on two burned and two unburned sections of the park to determine what effects the burning and subsequent changes in vegetation were having on bird populations. Birds were sampled at 17 60-m radius point counts on each study area; each point was sampled twice per year during the main breeding period from 1996 through 2002. Total abundance and species richness differed among the four areas but no differences could be attributed to burning. Some individual species, however, differed in abundance and frequency of occurrence between burned and unburned areas. For example, Indigo Bunting ( Passerina cyanea), Kentucky Warbler ( Oporornis formosus), and several species of woodpeckers were more abundant on burned areas; Ovenbird ( Seiurus aurocapillus), Worm-eating Warbler ( Helmitheros vermivorous), Wood Thrush ( Hylocicla mustelina) and Acadian Flycatcher ( Empidonax virescens) were among the species more abundant on unburned areas. As a consequence, overall community composition differed significantly between burned and unburned areas of the park, but did not differ between burned areas or between unburned areas. Prescribed burning was instituted to restore vegetation to presettlement conditions and has started to achieve that objective. Restoration also has affected and likely will continue to affect bird populations. Future maintenance of a full complement of bird species, including a number of neotropical migrants, will be dependent on presence of both burned and unburned forest habitat.

Factors Affecting Songbird Nest Survival in Riparian Forests in a Midwestern Agricultural Landscape

Abstract We investigated factors affecting nest success of songbirds in riparian forest and buffers in northeastern Missouri. We used an information-theoretic approach to determine support for hypotheses concerning effects of nest-site, habitat-patch, edge, and temporal factors on nest success of songbirds in three narrow (55–95 m) and three wide (400–530 m) riparian forests with adjacent grassland-shrub buffer strips and in three narrow and three wide riparian forests without adjacent grassland-shrub buffer strips. We predicted that temporal effects would have the most support and that habitat-patch and edge effects would have little support, because nest predation would be great across all sites in the highly fragmented, predominantly agricultural landscape. Interval nest success was 0.404, 0.227, 0.070, and 0.186, respectively, for Gray Catbird (Dumetella carolinensis), Northern Cardinal (Cardinalis cardinalis), Indigo Bunting (Passerina cyanea), and forest interior species pooled (Acadian Flycatcher [Empidonax virescens], Wood Thrush [Hylocichla mustelina], Ovenbird [Seiurus aurocapillus], and Kentucky Warbler [Oporornis formosus]). The effect of nest stage on nest success had the most support; daily nest success for Gray Catbird and Indigo Bunting were lowest in the laying stage. We found strong support for greater nest success of Gray Catbird in riparian forests with adjacent buffer strips than in riparian forests without adjacent buffer strips. Patch width also occurred in the most-supported model for Gray Catbird, but with very limited support. The null model received the most support for Northern Cardinal. Riparian forests provided breeding habitat for area-sensitive forest species and grassland-shrub nesting species. Buffer strips provided additional breeding habitat for grassland-shrub nesting species. Interval nest success for Indigo Bunting and area-sensitive forest species pooled, however, fell well below the level that is likely necessary to balance juvenile and adult mortality, which suggests that when riparian forests are located within agricultural landscapes, the potential even for wide riparian forests with adjacent buffer strips to provide high-quality breeding habitat is severely diminished for some species.

Breeding density affects point-count accuracy in Missouri forest birds

Abstract We compared 50-m fixed-radius point-count detections for four bird species with density data from spot-mapping in order to evaluate the relationship between monitoring methodologies. We conducted this study in five Missouri oak-hickory forest study sites over eight years. There were significant positive correlations between monitoring methods, but the strength of these relationships varied for Acadian Flycatchers (Empidonax virescens), Ovenbirds (Seiurus aurocapilla), Worm-eating Warblers (Helmitheros vermivorus), and Wood Thrushes (Hylocichla mustelina). There was significant bias associated with the point-count detections for Acadian Flycatchers, Worm-eating Warblers, and Wood Thrushes. The direction and degree of bias varied; for example, Acadian Flycatchers and Worm-eating Warblers were on average significantly overestimated by point counts, whereas the Wood Thrush was significantly underestimated and Ovenbirds were slightly underestimated. The magnitude and direction of the bias error varied...

FACTORS AFFECTING SONGBIRD NEST SURVIVAL IN RIPARIAN FORESTS IN A MIDWESTERN AGRICULTURAL LANDSCAPE

Abstract We investigated factors affecting nest success of songbirds in riparian forest and buffers in northeastern Missouri. We used an information-theoretic approach to determine support for hypotheses concerning effects of nest-site, habitat-patch, edge, and temporal factors on nest success of songbirds in three narrow (55–95 m) and three wide (400–530 m) riparian forests with adjacent grassland-shrub buffer strips and in three narrow and three wide riparian forests without adjacent grassland-shrub buffer strips. We predicted that temporal effects would have the most support and that habitat-patch and edge effects would have little support, because nest predation would be great across all sites in the highly fragmented, predominantly agricultural landscape. Interval nest success was 0.404, 0.227, 0.070, and 0.186, respectively, for Gray Catbird (Dumetella carolinensis), Northern Cardinal (Cardinalis cardinalis), Indigo Bunting (Passerina cyanea), and forest interior species pooled (Acadian Flycatcher [...

PERCH CHARACTERISTICS OF ACADIAN FLYCATCHERS (EMPIDONAX VIRESCENS) IN SOUTHERN BOTTOMLAND HARDWOOD FORESTS

Perch-site selection is thought to be an important component of foraging success for sit-and-wait avian predators, such as flycatchers. Perch selection has further consequences for territorial advertisement and display. We quantified perch characteristics of Acadian Flycatchers in three bottomland hardwood forests in Arkansas, Louisiana, and South Carolina. We compared measurements made at flycatcher perches with those made at randomly selected trees and branches. In two of our study areas, flycatchers perched on dead limbs more often than expected by chance, and foliage volume was less around flycatcher perches than at randomly selected branches. Both of these results suggest a preference for foraging and displaying sites that provide a relatively unobstructed view. Discriminant function analysis revealed substantial overlap in perch-site characteristics among study areas, indicating that species-wide preferences were at least as important as local conditions in perch-site selection by Acadian Flycatchers.

Avian response to removal of a forest dominant: consequences of hemlock woolly adelgid infestations

Abstract Aim This study examines changes in avian community composition associated with the decline and loss of eastern hemlock [Tsuga canadensis (L.) Carr.] resulting from chronic hemlock woolly adelgid (HWA; Adelges tsugae Annand) infestations.Location The study was conducted in a 4900‐km2 study region extending from Long Island Sound northward to the southern border of Massachusetts and including the Connecticut River Valley in Connecticut, USA.Methods Bird surveys were conducted at 40 points in 12 hemlock stands varying in HWA infestation and overstory mortality levels during the avian breeding seasons of 2000 and 2001. Ten‐minute, 50‐m radius point counts were used to survey all birds seen or heard at each point. Overstory and understory vegetation were sampled at each point. Indicator species analysis and non‐metric multidimensional scaling were used to examine relationships between avian community composition and vegetation structure.Results Overstory hemlock mortality was highly correlated with avian community composition. Abundance of eastern wood‐pewee (Contopus virens), brown‐headed cowbird (Molothrus ater), tufted titmouse (Baeolophus bicolor), white‐breasted nuthatch (Sitta carolinensis), red‐eyed vireo (Vireo olivaceus), hooded warbler (Wilsonia citrina), and several woodpecker species was highest at points with >60% mortality. Black‐throated green warbler (Dendroica virens), Acadian flycatcher (Empidonax virescens), blackburnian warbler (Dendroica fusca), and hermit thrush (Catharus guttatus) were strongly associated with intact hemlock stands that exhibit little or no mortality from HWA.Conclusions Eastern hemlock has unique structural characteristics that provide important habitat for numerous bird species in the north‐eastern US. As a result, removal of hemlock by HWA has profound effects on avian communities. Black‐throated green warbler, blackburnian warbler, and Acadian flycatcher are very strongly associated with hemlock forests in southern New England and appear to be particularly sensitive to hemlock removal. The hooded warbler, a species whose status is of regional concern, may actually benefit from the development of a dense hardwood seedling layer associated with high hemlock mortality.

Barking up the wrong tree: climbing performance of rat snakes and its implications for depredation of avian nests

Nest depredation is the leading cause of nest failure in Neotropical–Nearctic migratory birds, which are of interest because of their declining populations. In a recent study in a bottomland hardwood forest, Acadian Flycatchers (Empidonax virescens) experienced higher nest success in Nuttall oak (Quercus nuttallii), a tree species with relatively smooth bark at maturity. To determine if variation in bark-surface irregularities may influence the ability of a predator species to access the contents of avian nests, we examined the climbing abilities of rat snakes (Elaphe obsoleta) on trees having three different bark types. None of the subjects was able to ascend large Nuttall oaks in the absence of vines; with vines present, subjects still required more time to climb Nuttall oaks than to climb other species. A few of the subjects successfully climbed smaller Nuttall oaks lacking vines, but ascent time was longer and climbing behavior was modified from that observed in the other trials. Our results indicate that the likelihood of nest predation by rat snakes decreases in this forest when birds nest in trees with smooth bark and without vines. Investigators need to consider differences among nest substrates that are important to both the prey and the predator.

A Removal Model for Estimating Detection Probabilities from Point-Count Surveys

Use of point-count surveys is a popular method for collecting data on abundance and distribution of birds. However, analyses of such data often ignore potential differences in detection probability. We adapted a removal model to directly estimate detection probability during point-count surveys. The model assumes that singing frequency is a major factor influencing probability of detection when birds are surveyed using point counts. This may be appropriate for surveys in which most detections are by sound. The model requires counts to be divided into several time intervals. Point counts are often conducted for 10 min, where the number of birds recorded is divided into those first observed in the first 3 min, the subsequent 2 min, and the last 5 min. We developed a maximum-likelihood estimator for the detectability of birds recorded during counts divided into those intervals. This technique can easily be adapted to point counts divided into intervals of any length. We applied this method to unlimited-radius counts conducted in Great Smoky Mountains National Park. We used model selection criteria to identify whether detection probabilities varied among species, throughout the morning, throughout the season, and among different observers. We found differences in detection probability among species. Species that sing frequently such as Winter Wren (Troglodytes troglodytes) and Acadian Flycatcher (Empidonax virescens) had high detection probabilities (∼90%) and species that call infrequently such as Pileated Woodpecker (Dryocopus pileatus) had low detection probability (36%). We also found detection probabilities varied with the time of day for some species (e.g. thrushes) and between observers for other species. We used the same approach to estimate detection probability and density for a subset of the observations with limited-radius point counts.

RELATIONSHIPS AMONG NORTH AMERICAN SONGBIRD TRENDS, HABITAT FRAGMENTATION, AND LANDSCAPE OCCUPANCY*

Fragmentation of breeding habitat has been hypothesized as a cause of population declines in forest-nesting migratory birds. Negative correlations between the degree of fragmentation and bird density or fecundity at local or regional scales support the fragmentation hypothesis. Yet, in spite of reduced fecundity and densities in fragmented systems, many forest-nesting passerine species have increased in numbers over time. We hypothesized that range-wide population change in species for which habitat fragmentation negatively affects reproductive success should depend on the proportion of the population that actually occupies fragmented landscapes. We predicted that fragmentation-sensitive species (e.g., species that occur in reduced densities in fragmented landscapes) should increase globally in numbers at a greater rate than species that readily occupy fragmented landscapes, because fragmentation-sensitive distributions place a large proportion of the global population in contiguous landscapes that are superior for breeding. We used Breeding Bird Survey (BBS) data and associated landscape metrics to test this prediction for 10 species of forest-nesting passerines in the United States that experience reproductive dysfunction associated with habitat fragmentation: Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo olivaceus), Wood Thrush (Hylocichla mustelina), Northern Cardinal (Cardinalis cardinalis), Worm-eating Warbler (Helmitheros vermivorus), Kentucky Warbler (Oporornis formosus), Indigo Bunting (Passerina cyanea), Scarlet Tanager (Piranga olivacea), Acadian Flycatcher (Empidonax virescens), and Hooded Warbler (Wilsonia citrina). Our approach was to: (1) quantify landscape features associated with BBS routes across the eastern United States, (2) classify landscapes around BBS routes as “fragmented” or “contiguous,” (3) estimate the proportion of detected individuals that occurred in fragmented landscapes on a species-by-species basis, and then (4) associate 10-yr trends for each species with the proportion of breeding individuals occupying fragmented landscapes. Regression analysis indicated a significant, negative relationship between the proportion of the breeding population occupying fragmented landscapes and the population trend from 1970 to 1980. Although this result links habitat fragmentation to population change and provides support for the fragmentation hypothesis, other factors (e.g., land use change, weather, varying life history traits, varying winter survivorship, habitat amount thresholds) could generate similar results. More work is needed to partition the relative influence of these factors on regional bird population dynamics if conservationists are to understand more clearly the effects of fragmentation on the distribution and abundance of species across their ranges.

A Removal Model for Estimating Detection Probabilities From Point-Count Surveys

AbstractUse of point-count surveys is a popular method for collecting data on abundance and distribution of birds. However, analyses of such data often ignore potential differences in detection probability. We adapted a removal model to directly estimate detection probability during point-count surveys. The model assumes that singing frequency is a major factor influencing probability of detection when birds are surveyed using point counts. This may be appropriate for surveys in which most detections are by sound. The model requires counts to be divided into several time intervals. Point counts are often conducted for 10 min, where the number of birds recorded is divided into those first observed in the first 3 min, the subsequent 2 min, and the last 5 min. We developed a maximum-likelihood estimator for the detectability of birds recorded during counts divided into those intervals. This technique can easily be adapted to point counts divided into intervals of any length. We applied this method to unlimited-radius counts conducted in Great Smoky Mountains National Park. We used model selection criteria to identify whether detection probabilities varied among species, throughout the morning, throughout the season, and among different observers. We found differences in detection probability among species. Species that sing frequently such as Winter Wren (Troglodytes troglodytes) and Acadian Flycatcher (Empidonax virescens) had high detection probabilities (∼90%) and species that call infrequently such as Pileated Woodpecker (Dryocopus pileatus) had low detection probability (36%). We also found detection probabilities varied with the time of day for some species (e.g. thrushes) and between observers for other species. We used the same approach to estimate detection probability and density for a subset of the observations with limited-radius point counts.