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The use of song by wood-warblers (Parulidae) with multiple song types has received little attention during migration. On the breeding area, the two song types serve to attract mates and defend territory, but the function(s) during migration, if any, are unclear. I studied the singing of Townsend's Warblers (Setophaga townsendi) during spring migration through southern Oregon and northern California. Males preferentially used Type II songs (79%) whether alone or in flocks of up to ~20 individuals. Type 1 songs of wood-warblers tend to be acquired early in the hatch year while Type II songs appear to be subject to modification over an extended period of time. The preference for Type II songs in migration may reflect an important period in the development of Type II song. More than one dialect of Type I song was recorded within flocks indicating that flocks were composed of individuals from different breeding populations. Type II songs are recognizably similar in breeding populations from central Oregon to Alaska. The similarity of Type II songs across the range of the species may result from song refinement through interaction with other singing males while in transit. [ABSTRACT FROM AUTHOR]

The records of species in new localities are of particular interest to develop integral conservation strategies. The distribution of bird species is usually well-known, but the lack of information about some species, climatic change, land-use modifications, among other factors lead to variations in their distribution and their subsequent detection in different localities. The Black-throated Blue Warbler (Setophaga caerulescens) is a species of the Parulidae family that can be found in Mexico mostly as a fall transient, and sparsely as a winter resident, in particular along the coast of the Gulf of Mexico and the eastern coast of the Yucatán Peninsula. There are additional records in Oaxaca, Nuevo León, Sonora, Baja California, and Baja California Sur. In November 2012, we recorded a single female of S. caerulescens in a disturbed habitat area in Tabasco. This is the first record of the species in the state. Based on previous knowledge about the species wintering range, it is interesting the time of year when the record was made because it would be expected to find the observed individual predominantly in the Antilles. Our observation indicates that S. caerulescens increases its habitat-use versatility during its fall-winter migration period. We believe that there is an important need to increase the knowledge about the distribution and habitat use of this species in Mexico. [ABSTRACT FROM AUTHOR]

North American wood-warblers (Parulidae) are well known for exhibiting two distinct singing modes: first category song, in which a single song type is sung repeatedly, usually before dawn, and second category song, in which several song types ate sung in irregular sequence. Studies suggest that first category song types have higher performance characteristics and that second category song types are shared preferentially among territorial neighbors. Here we present the first formal description of two-category singing in the Pine Warbler (Setophaga pinus). Similar to other species in the genus Setophaga, Pine Warblers produce second category song before dawn and first category song primarily during daylight hours. First category songs also had significantly higher trill rates, suggesting that they ate more challenging to perform. Unlike many congeners, however, Pine Warblers regularly alternate between first and second category singing throughout the day, and first category song types often appear intermixed in second category singing, including singing before dawn. Furthermore, comparisons among territorial males showed that individuals do not share song types more with neighbors than with non-neighbors. Our results suggest that Pine Warblers have two song categories similar to other Setophaga warblers, but singing patterns in this species differ from congeners in interesting ways that warrant further investigation. [ABSTRACT FROM AUTHOR]

Aim Our objective was to identify the distribution of the endangered golden-cheeked warbler ( Setophaga chrysoparia) in fragmented oak-juniper woodlands by applying a geoadditive semiparametric occupancy model to better assist decision-makers in identifying suitable habitat across the species breeding range on which conservation or mitigation activities can be focused and thus prioritize management and conservation planning. Location Texas, USA. Methods We used repeated double-observer detection/non-detection surveys of randomly selected ( n = 287) patches of potential habitat to evaluate warbler patch-scale presence across the species breeding range. We used a geoadditive semiparametric occupancy model with remotely sensed habitat metrics (patch size and landscape composition) to predict patch-scale occupancy of golden-cheeked warblers in the fragmented oak-juniper woodlands of central Texas, USA. Results Our spatially explicit model indicated that golden-cheeked warbler patch occupancy declined from south to north within the breeding range concomitant with reductions in the availability of large habitat patches. We found that 59% of woodland patches, primarily in the northern and central portions of the warbler's range, were predicted to have occupancy probabilities ≤0.10 with only 3% of patches predicted to have occupancy probabilities >0.90. Our model exhibited high prediction accuracy (area under curve = 0.91) when validated using independently collected warbler occurrence data. Main conclusions We have identified a distinct spatial occurrence gradient for golden-cheeked warblers as well as a relationship between two measurable landscape characteristics. Because habitat-occupancy relationships were key drivers of our model, our results can be used to identify potential areas where conservation actions supporting habitat mitigation can occur and identify areas where conservation of future potential habitat is possible. Additionally, our results can be used to focus resources on maintenance and creation of patches that are more likely to harbour viable local warbler populations. [ABSTRACT FROM AUTHOR]

Powerline rights-of-way (ROWs) often provide habitat for early successional bird species that have suffered long-term population declines in eastern North America. To determine how the abundance of shrubland birds varies with habitat within ROW corridors and with land use patterns surrounding corridors, we ran Poisson regression models on data from 93 plots on ROWs and compared regression coefficients. We also determined nest success rates on a 1-km stretch of ROW. Seven species of shrubland birds were common in powerline corridors. However, the nest success rates for prairie warbler (Dendroica discolor) and field sparrow (Spizella pusilla) were <21%, which is too low to compensate for estimated annual mortality. Some shrubland bird species were more abundant on narrower ROWs or at sites with lower vegetation or particular types of vegetation, indicating that vegetation management could be refined to favor species of high conservation priority. Also, several species were more abundant in ROWs traversing unfragmented forest than those near residential areas or farmland, indicating that corridors in heavily forested regions may provide better habitat for these species. In the area where we monitored nests, brood parasitism by brown-headed cowbirds (Molothrus ater) occurred more frequently close to a residential area. Although ROWs support dense populations of shrubland birds, those in more heavily developed landscapes may constitute sink habitat. ROWs in extensive forests may contribute more to sustaining populations of early successional birds, and thus may be the best targets for habitat management. [ABSTRACT FROM AUTHOR]

Several animal species have recently been shown to have hybrid origins, but no avian examples have been documented with molecular evidence. We investigate whether the Audubon's warbler ( Dendroica auduboni), one of four visually distinct species in the yellow-rumped warbler complex, has originated through hybridization between two other species in this group, the myrtle warbler ( D. coronata) and black-fronted warbler ( D. nigrifrons). Analysis of nuclear amplified fragment length polymorphism (AFLP) and sequence markers shows that Audubon's warblers are genetically intermediate and carry a mixture of alleles otherwise found only in one or the other of their putative parental species. Audubon's warblers also carry two deeply divergent mitochondrial DNA lineages, each shared with only one putative parental form. Broad clines between Audubon's and black-fronted warblers in AFLP markers call into question the validity of these two forms as full species; nevertheless, our results suggest that the Audubon's warbler probably originated through hybridization between two long-diverged species. It is likely that more cases of avian species of hybrid origin will be revealed by surveys of variation in nuclear DNA and other traits. [ABSTRACT FROM AUTHOR]

In this issue of Molecular Ecology, present strong evidence for a case of hybrid speciation within the yellow-rumped warbler complex. Although homoploid hybrid speciation has now been documented in many animals (), it seems rare in tetrapods () and it has barely even been mentioned in birds (). Brelsford and colleagues thus present the first detailed molecular evidence suggesting that hybrid speciation can occur in birds. posit that Audubon's warbler ( Dendroica auduboni) constitutes a hybrid species originating from the admixture of two distinct parental lineages, represented today by myrtle warbler ( D. coronata) and black-fronted warbler ( D. nigrifrons). The authors present three major lines of molecular evidence suggesting that this is not simply a case of a hybrid swarm or limited introgression. [ABSTRACT FROM AUTHOR]

We studied a newly established population of Kirtland's Warbler (Dendroica kirtlandii) in Adams County, Wisconsin, nesting in a red pine (Pinus resinosa) plantation. We found eight males and five females in Adams County in 2008 and 10 males and 10 females in 2009. Five of seven (71%) males color-banded in 2008 returned in 2009, and at least eight successful nests produced an estimated 33 young over the 2 years. Red pine comprised 66.9% of trees on the main site, 20.6% were nonhero pin oak/black oak (Quercus ellipsodalis/Q, velutina), and 12.5% were jack pine (Pinus banksiana). Total tree density at the main site was 1.876 trees/ha, lower than generally reported in Michigan. Percent canopy cover and ground cover types were similar to Michigan sites. Lowest live branch height of jack pine was similar to Michigan sites, but lowest live branches of red pine at our site were closer to the forest floor. Significant red pine die-off at our site combined with substantial natural jack pine recruitment created a landscape matrix of openings and thickets that produced suitable Kirtland's Warbler habitat. We suggest young red pine-dominated plantations should be searched when surveying for Kirtland's Warblers as some lower-density red pine plantations could provide important supplemental habitat as the species expands its range. [ABSTRACT FROM AUTHOR]

Cerulean Warblers (Dendroica cerulea) have experienced one of the highest population declines of any neotropical-Nearctic migratory species in North America. We performed point counts and habitat assessments in areas used and unused by Cerulean Warblers in northern Alabama during the 2005 and 2006 breeding seasons to examine their avian associations and identify microhabitat features that best explained their occurrence. We detected on average ~50 Cerulean Warbler males (total) in three disjunct populations during each breeding season. Areas used by Cerulean Warblers were characterized by avian communities with significantly higher species richness, diversity, and abundance compared to areas where they were not detected. Correspondence analysis related Cerulean Warblers to inhabitants of riparian, bottomland deciduous forests (e.g., Kentucky Warbler [Oporornis formosus], Acadian Flycatcher [Empidonax virescens], and Northern Parula [Parula americana]) and two edge specialists (Blue-winged Warbler [Vermivora cyanoptera] and Indigo Bunting [Passerina cyanea]) suggesting Cerulean Warblers in our study areas may be tolerant of some habitat disturbance within an otherwise largely forested landscape. Information theoretic criteria and canonical correspondence analysis indicated Cerulean Warblers preferred bottomland forests containing tall (> 29 m), large diameter, well-spaced (> 27 m²/ha) deciduous trees with greater canopy cover (≥ 90%), closer (< 20 m) canopy gaps, fewer snags (≤ 25/ha), and a moderately complex canopy structure. [ABSTRACT FROM AUTHOR]

Traits that converge in appearance under similar environmental conditions among phylogenetically independent lineages are thought to represent adaptations to local environments. We tested for convergence in nest morphology and composition of birds breeding in two ecologically different locations in Canada: Churchill in northern Manitoba and Elgin in southeastern Ontario. We examined nests from four families of passerine birds (Turdidae: Turdus, Parulidae: Dendroica, Emberizidae: Passerculus and Fringillidae: Carduelis) where closely related populations or species breed in both locations. Nests of American Robins, Yellow Warblers, and Carduelis finches had heavier nest masses, and tended to have thicker nestwalls, in northern Manitoba compared with conspecifics or congenerics breeding in southeastern Ontario. Together, all species showed evidence for wider internal and external nest-cup diameters in northern Manitoba, while individual species showed varying patterns for internal nest-cup and external nest depths. American Robins, Yellow Warblers, and Carduelis finches in northern Manitoba achieved heavier nest masses in different ways. American Robins increased all materials in similar proportions, and Yellow Warblers and Common Redpolls used greater amounts of select materials. While changes in nest composition vary uniquely for each species, the pattern of larger nests in northern Manitoba compared to southeastern Ontario in three of our four phylogenetically-independent comparisons suggests that birds are adapting to similar selective pressures between locations. [ABSTRACT FROM AUTHOR]

Stable isotope analysis of feathers may help track populations of migratory birds throughout their annual cycle. We use deuterium isotope ratios in feathers (δD) to confirm that Yellow Warblers Dendroica petechia go through both a prebasic molt on or near their breeding grounds and a prealternate molt on their wintering grounds, and evaluate whether δD analysis of different feather types can help assign birds to breeding and winter origins. We show that primary coverts and tail feathers have δD values that reflect long-term average deuterium isotope ratios in precipitation (δD) on the breeding grounds. Most greater coverts and crown feathers, however, have δD values that are more enriched than predicted based on the δD of their wintering distribution. We also found significant interannual variation in the δD values of tail feathers, greater coverts and crown feathers. Interannual variation in δD of winter-grown feathers may be explained by El Niño/La Niña conditions that influence rainfall and thus δD; feathers were more depleted following an El Niño year (2005) than a La Niña year (2006). Gender had no effect on the δD values of any feathers sampled. Age influenced the δD values of crown feathers but not greater coverts. This study therefore confirms that δD of summer-grown feathers can be used to identify breeding locations, but suggests that the ability to use δD of winter-grown feathers to assign individuals to winter locations is limited by weak spatial gradients and temporal variation in δD. [ABSTRACT FROM AUTHOR]

Many animal signals given during agonistic interactions are thought to convey levels of aggressive motivation of signallers, but few studies have experimentally tested whether such signals actually predict subsequent aggressive behaviour. In songbirds, the predictive content of most putatively aggressive vocal signals remains largely unresolved. Prior studies of agonistic vocal signalling in birds have generally assessed the reliability of vocal displays using indirect response measures such as approach to a playback speaker. However, this method does not allow aggression to be assessed directly, because subjects have no opportunity to engage in actual attacks. In this study, we assessed the aggressive content of vocal signals in black-throated blue warblers, Dendroica caerulescens, using a direct measure of aggression. We presented territorial males with a brief period of conspecific song playback, recorded their vocal responses, and then revealed a taxidermic mount, giving subjects an opportunity to attack. We found that one song feature, low-amplitude ‘soft’ song, was an extremely reliable predictor of attack; a logistic regression model in which the occurrence of soft song was the sole predictor correctly classified 93% of males as attackers or nonattackers. By contrast, four other vocal behaviours (total number of songs, rate of song type switching and numbers of two nonsong vocalizations) failed to predict whether birds would attack the mount. These data provide evidence that soft song in black-throated blue warblers conveys extremely reliable information about impending aggression. We suggest that the reliability of soft song observed in this study and recent studies in emberizid sparrows may be stabilized by vulnerability costs because the limited distance with which it can be transmitted forces signallers to place themselves at risk of retaliatory attack. [ABSTRACT FROM AUTHOR]

The article presents a study on the broad scale movements of juvenile songbirds prior to migration. The study used radio telemetry and the topographical characteristics of the study sites to assess four hypothesis including the beginning of migration, habitat optimization, and the selection of breeding territory. The result showed that the movement of Yellow-rumped Warbler (Dendroica coronata) was directed toward down river valleys while Balckpoll Warbler (D. striata) moved toward the southeast.

The article presents information on a study which analyzed an adult male Yellow Warbler that had replaced all secondary coverts and six inner secondaries on both wings, the innermost primary and its corresponding primary covert on the right wing, and the innermost two primaries and their corresponding primary coverts on the left wing, but no rectrices during the prealternate molt. It notes that it is the only species among Dendroica warblers that can regularly include up to all greater coverts and five inner secondaries during this molt.

This article discusses a study which investigated the taxonomic status of the Bahama subspecies of the Yellow-throated Warbler (Dendroica dominica flavescens). By examining morphological, song and genetic data, researchers found significant differences between flavescens and continental dominica in wing chord and in bill, tarsus and tail lengths. They recommended that flavescens be reclassified as a separate species, the Bahama Warbler (Dendroica flavescens).

Variation in sexual signals across populations is a common phenomenon, and most research to date has found that individuals discriminate against nonlocal signals. However, the strength of nonlocal signal discrimination can vary asymmetrically across populations, a possibility that has received less attention. Such asymmetries can be due to recognition errors in some populations, variation across populations in the ‘quality’ of local individuals, such that populations with high-quality individuals respond more strongly, and/or variation in the perception of signals, with signals from some populations being perceived as more intimidating or aggressive. Here, we examine song differences and male responsiveness to local and nonlocal songs in two populations of the black-throated blue warbler, Dendroica caerulescens, and also explore possible reasons for geographical variation in responsiveness. In the northern population, male responses were stronger to local songs than to nonlocal songs, whereas in the southern population, male responses to local and nonlocal songs did not differ. Overall responsiveness did not differ between the populations, and songs from one population were not responded to more weakly (or strongly) across populations. Overall, our results fit best with a model of asymmetric recognition error. Asymmetries in nonlocal responsiveness across populations may be a common phenomenon, and can have profound effects on patterns of dispersal, mate choice and gene flow. [Copyright &y& Elsevier]

A topic of recurring interest in ecological research is the degree to which vegetation structure influences the distribution and abundance of species. Here we test the applicability of remote sensing, particularly novel use of waveform lidar measurements, for quantifying the habitat heterogeneity of a contiguous northern hardwoods forest in the northeastern United States. We apply these results to predict the breeding habitat quality, an indicator of reproductive output of a well-studied Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). We found that using canopy vertical structure metrics provided unique information for models of habitat quality and spatial patterns of prevalence. An ensemble decision tree modeling approach (random forests) consistently identified lidar metrics describing the vertical distribution and complexity of canopy elements as important predictors of habitat use over multiple years. Although other aspects of habitat were important, including the seasonality of vegetation cover, the canopy structure variables provided unique and complementary information that systematically improved model predictions. We conclude that canopy structure metrics derived from waveform lidar, which will be available on future satellite missions, can advance multiple aspects of biodiversity research, and additional studies should be extended to other organisms and regions. [ABSTRACT FROM AUTHOR]

The article presents a study on the effects of an experimental inducement of Black-throated Blue Warblers (Dendroica caerulescens) to colonize a new environment. The study employed dynamic occupancy modeling to examine the dynamics of colonization and local extinction. It reveals that there is an evidence for dynamic bird occupancy during breeding season. It also states that three of the four species tended to vacate an area once the Black-throated Blue Warblers occupied it.

The article discusses a study on the use of radiotelemetry in investigating patterns of habitat associations in juvenile Yellow-rumped Warblers and Blackpoll Warblers in a managed boreal forest system in Newfoundland. Vegetation characteristics such as foliage density and the quantity of vertical woody stems are measured for their potential correlation to prey availability. The density and rug plots showing bimodal distribution of movement rates for the birds studied are illustrated.

Birds concentrated in riparian habitats are poorly sampled by traditional survey methods because of the difficulties associated with accessing these habitats. Our objectives were to test the effectiveness of river-based surveys to determine the status, distribution, and relative abundance for riparian bird species in Missouri and northern Arkansas, with special emphasis on Dendroica cerulea (Cerulean Warbler). Our canoe-based surveys revealed an average of 2.3 and 0.8 singing male Cerulean Warblers/river km along the Current River (128 river km surveyed), MO, and the Buffalo National River (96 river km), AR. Nonparametric estimates for repeated surveys of the same river stretches indicate that 69-79% of singing male Cerulean Warblers were detected. However, the bias associated with the estimate methodology and independent song rate data suggest those are conservative estimates. In comparison with land-based point-counts, this river-based protocol offers a quick and efficient assessment of Cerulean Warblers in riparian areas. [ABSTRACT FROM AUTHOR]

The Yellow-Headed Warbler (Teretistris fernandinae) and the Olive-Capped Warbler (Dendroica pityophila) are two bird species that co-exist in Western Cuba. Their spatial distribution has not been studied. This study evaluated their vertical distribution in pine forests during 2007. Six field trips were conducted to the Tibisí pine groves, Minas de Matahambre, Pinar del RÞo, Cuba, to determine the differential use of forest strata, with the fixed-radio round parcel method. A total of 30 counting points were located in every forest formation. The forest stratum was recorded for each detected bird according to three categories: low stratum (0-2m over ground), medium stratum (2-6m) and high stratum (6m or more). From the 360 counts carried out, the Olive-Capped Warbler reached greater abundances (3.33±0.14ind/UM) than the Yellow-Headed Warbler (2.32±024ind/UM). No abundance differences were found among both forest formations. The Olive-Capped Warbler had the highest frequency in the high stratum and it was observed in the low stratum only during the reproductive season. The Yellow-Headed Warbler had a preference for the low and medium strata, and was seen less frequently in the high stratum. Rev. Biol. Trop. 57 (4): 1263-1269. Epub 2009 December 01. [ABSTRACT FROM AUTHOR]

Hybrid zones between recently diverged taxa are natural laboratories for speciation research, allowing us to determine whether there is reproductive isolation between divergent forms and the causes of that isolation. We present a study of a classic avian hybrid zone in North America between two subspecies of the yellow-rumped warbler ( Dendroica coronata). Although previous work has shown very little differentiation in mitochondrial DNA across this hybrid zone, we identified two nuclear loci (one sex-linked and one autosomal) that show fixed differences across the hybrid zone, in a close concordance with patterns of plumage variation. Temporal stability and limited width of the hybrid zone, along with substantial linkage disequilibrium between these two diagnostic markers in the center of the zone, indicate that there is moderate reproductive isolation between these populations, with an estimated strength of selection maintaining the zone of 18%. Pairing data indicate that assortative mating is either very weak or absent, suggesting that this reproductive isolation is largely due to postmating barriers. Thus, despite extensive hybridization the two forms are distinct evolutionary groups carrying genes for divergent adaptive peaks, and this situation appears relatively stable. [ABSTRACT FROM AUTHOR]

Isolates of Cladobotryum dendroides from Serbian mushroom farms and Agaricus bisporus F56 were tested for sensitivity to selected fungicides in vitro. Chlorothalonil was the most toxic fungicide to C. dendroides isolates (EC50 values were below 1.68 mg L- 1). Trifloxystrobin and kresoxim-methyl were not effective in growth inhibition of C. dendroides isolates (EC50 values exceeded 300 mg L- 1). Metalaxyl-M+mancozeb was the most toxic fungicide to strain F56 of A. bisporus, and iprodione the least toxic. The fungicide selectivity indexes for both C. dendroides and A. bisporus indicated that iprodione, chlorothalonil, captan and metalaxyl-M+mancozeb had satisfactory selective fungitoxicity. Iprodione had the best selectivity to both the pathogen and the host, although inferior than prochloraz manganese and carbendazim, fungicides officially recommended for mushroom cultivation in European Union (EU) countries. [ABSTRACT FROM AUTHOR]

The article discusses research on the genetic population structure of the Black-throated Blue Warbler or Dendroica caeruslescens which breeds in the forests in eastern North America. The researchers analyzed 333 base pairs of the mitochondrial control region from 287 birds from 14 populations in Michigan and central Appalachian forests. Results of their study revealed relatively high haplotype diversity, low nucleotide diversity, and limited but significant phylogeographic structure in the birds' breeding range and populations.

Delineating habitat requirements and preferences of species is essential for conservation planning. We studied nest habitat use and effects of microsite vegetation characteristics on breeding success of yellow-rumped warblers (Dendroica coronata (L., 1766)), blackpoll warblers (Dendroica striata (J.R. Forster, 1772)), and white-throated sparrows (Zonotrichia albicollis (Gmelin, 1789)) in an area with a low extent (<6% of available land) of forest harvest in northwestern Newfoundland. During 2004 and 2005, 99 nests were located and monitored, and the characteristics of nest sites measured. Vegetation at yellow-rumped and blackpoll warbler nest sites differed from random sites; however, within used sites, no vegetation characteristics were significantly associated with success. White-throated sparrow nest sites contained more downed wood and less ground vegetation than did random sites; however, successful nests were associated with different variables than those that distinguished them from random sites, including less canopy cover and less woody debris. Thus, whereas yellow-rumped and blackpoll warblers used specific nest-site characteristics and white-throated sparrows had higher nest success associated with certain characteristics, the nest characteristics these birds appeared to choose did not have demonstrable fitness benefits. [ABSTRACT FROM AUTHOR]

Landscape genetic approaches offer the promise of increasing our understanding of the influence of habitat features on genetic structure. We assessed the genetic diversity of the endangered golden-cheeked warbler ( Dendroica chrysoparia) across their breeding range in central Texas and evaluated the role of habitat loss and fragmentation in shaping the population structure of the species. We determined genotypes across nine microsatellite loci of 109 individuals from seven sites representing the major breeding concentrations of the species. No evidence of a recent population bottleneck was found. Differences in allele frequencies were highly significant among sites. The sampled sites do not appear to represent isolated lineages requiring protection as separate management units, although the amount of current gene flow is insufficient to prevent genetic differentiation. Measures of genetic differentiation were negatively associated with habitat connectivity and the percentage of forest cover between sites, and positively associated with geographic distance and the percentage of agricultural land between sites. The northernmost site was the most genetically differentiated and was isolated from other sites by agricultural lands. Fragmentation of breeding habitat may represent barriers to dispersal of birds which would pose no barrier to movement during other activities such as migration. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the pattern of variation in morphological traits related to flight of migratory and sedentary yellow-rumped warbler populations. The study proposed a graphic model to determine the role of antagonistic selective pressure and body size difference on migratory flight against maneuverability. Results determine that morphological differentiation between migratory and sedentary species is caused by divergent natural selection.

Ecological research sometimes entails animal suffering and even animal killing. The ethical appropriateness of animal suffering and killing in conservation research may entail considerations that differ from many other kinds of research. This is true, insomuch as conservation research is specifically motivated by an ethical premise: an appreciation for non-human life. In striking contrast with other academic fields (e.g. medicine), however, the ethical dimension of conservation research is only rarely discussed. When it is discussed, it tends to be characterized by logical errors. These errors are important because they are general (i.e. both common and with far-reaching implications), and they are easily made by intelligent people; especially those with no formal training in ethics or logic. Failure to recognize these errors could stymie efforts to increase the ethical quality of ecological research conducted in the name of conservation. We take advantage of a recently published dialogue concerning the ethical appropriateness of a specific field experiment that entailed killing black-throated blue warblers, Dendroica caerulescens. Both sides of this debate exemplify the kinds of errors to which we refer. In this paper we briefly review the arguments presented on each side of this debate, highlight their mistakes, and indicate necessary corrections. We argue that: (1) compliance with animal research regulations, while important, inadequately accommodates the ethical aspects of animal research, and (2) individual ecologists ought to understand themselves what does and does not represent sound and valid arguments for ethical decisions. Finally, we discuss how any ecological researcher might begin to apply our analysis to his or her own research. [ABSTRACT FROM AUTHOR]

Acute corticosterone secretion helps individuals survive perturbation, particularly when faced with unpredictable conditions. However, elevated corticosterone is often incompatible with reproduction. Male Yellow Warblers (Dendroica petechia) breeding at high latitude avoid this conflict by exhibiting a reduced adrenocortical response while provisioning nestlings. To determine if this reduction is cued by nestlings, in June 2000 we switched clutches between nest pairs to manipulate hatching date. We sampled males with early-hatching clutches for the adrenocortical response 24 hr after nestlings hatched, but 24 hr prior to the premanipulated (original) hatching date. Similarly, we sampled males with late-hatching clutches 24 hr after the original hatching date, but prior to nestlings hatching. Males with early-hatching clutches (n = 6) showed no reduced adrenocortical response, while males with late-hatching clutches (n = 6) appeared to show a reduced response even without nestlings, suggesting that nestlings do not cue the reduction. An endogenous clock, day-counting mechanism, or hormonal changes may be associated with the reduced adrenocortical response. [ABSTRACT FROM AUTHOR]

I used an information-theoretic approach to determine support for hypotheses concerning the effects of edge and temporal factors on Golden-checked Warbler (Dendroica chrysoparia) nest survival on Fort Hood Military Reservation, Texas, during 2003-2006. I predicted that nest survival would be greater earlier in the breeding season than later in the breeding season, in areas with less forest edge than in areas with more forest edge, and during the laying and incubation stages than during the nestling stage. I used the logistic-exposure method to model nest survival as a function of the explanatory variables and to produce model-based estimates of daily and period survival. The overall daily survival rate was 0.96 (95% CI: 0.94, 0.97) and overall period survival was 0.34 (95% CI: 0.23, 0.46). Forest edge density had the strongest effect on nest survival. Nest survival decreased as forest edge density increased. Period survival tended to decrease as the breeding season progressed, but the 95% confidence intervals of the model-averaged parameter estimates overlapped. These results demonstrate that effective conservation strategies designed to provide high-quality breeding habitat for this endangered species should include measures to reduce predation by edge-adapted predators. [ABSTRACT FROM AUTHOR]

Reproduction is physiologically stressful for many animals. Female birds often lose body mass after their young hatch, which has been commonly attributed either to an energetic deficit incurred while breeding (the energetic stress hypothesis) or to an adaptive reduction in wing-loading to save energy during flight (the wing-loading hypothesis). We tested these two hypotheses for an open-cup nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens), using a food-supplementation experiment. We found that females provided with extra food lost less body mass between the incubation and nestling periods than did control females and that mass loss varied seasonally, with females in both treatments that nested earlier in the season losing more mass. We conclude that greater mass loss in control females supports the energetic stress hypothesis in Black-throated Blue Warblers. [ABSTRACT FROM AUTHOR]

We studied breeding dark-eyed juncos (Junco hyemalis), yellow-rumped warblers (Dendroica coronata), and spruce nesting birds from 1997 to 1998 among forests with different levels of spruce (Picea spp,) mortality following an outbreak of spruce beetles (Dendroctonus rufipennis) in Alaska, USA. We identified species using five and beetle-killed spruce for nest sites and monitored nests to determine how the outbreak influenced avian habitat selection and reproduction. We tested predictions that 1) nesting success of ground nesting juncos would increase with spruce mortality due to proliferation of understory vegetation available to conceal nests from predators, 2) nesting success of canopy-nesting warblers would decrease with spruce mortality due to fewer five spruce in which to conceal nests, and 3) both species would alter nest-site selection in response to disturbance. Juncos did not benefit from changes in understory vegetation; nesting success in highly disturbed stands (46%) was comparable to that in undisturbed habitats throughout their range. In stands with low spruce mortality, nesting success of juncos was low (5 %) and corresponded with high densities of red squirrels (Tamiasciurus hudsonicus). Yellow rumped warblers nested exclusively in spruce, but success did not vary with spruce mortality. As disturbance increased, nesting warblers switched from selecting forest patches with high densities of live white spruce (Picea glauca) to patches with beetle-killed spruce. Warblers also placed nests in large diameter live or beetle-killed spruce, depending on which was more abundant in the stand, with no differences in nesting success. Five of the 12 other species of spruce nesting birds also used beetle-killed spruce as nest sites. Because beetle-killed spruce can remain standing for >50 years, even highly disturbed stands provide an important breeding resource for boreal forest birds. We recommend that boreal forest managers preserve uncut blocks of infested forest within managed forest landscapes mad practice partial harvest of beetle-killed spruce rather than commercial clear-cutting of infested stands in order to sustain breeding bird populations until natural reforestation occurs, Because breeding densities do not always reflect illness, assessing impacts of a massive natural disturbance should include measuring impacts of changes in vegetation on both reproductive success and predator-prey dynamics. [ABSTRACT FROM AUTHOR]

The relative importance of the Pleistocene glacial cycles in driving avian speciation remains controversial, partly because species limits in many groups remain poorly understood, and because current taxonomic designations are often based on phenotypic characteristics of uncertain phylogenetic significance. We use mtDNA sequence data to examine patterns of genetic variation, sequence divergence and phylogenetic relationships between phenotypically distinct groups of the yellow-rumped warbler complex. Currently classified as a single species, the complex is composed of two North American migratory forms (myrtle warbler Dendroica coronata coronata and Audubon's warbler Dendroica coronata auduboni), and two largely sedentary forms: Dendroica coronata nigrifrons of Mexico, and Dendroica coronata goldmani of Guatemala. The latter are typically considered to be races of the Audubon's warbler based on plumage characteristics. However, mtDNA sequence data reveal that sedentary Mesoamerican forms are reciprocally monophyletic to each other and to migratory forms, from which they show a long history of isolation. In contrast, migratory myrtle and Audubon's warblers form a single cluster due to high levels of shared ancestral polymorphism as evidenced by widespread sharing of mtDNA haplotypes despite marked phenotypic differentiation. Sedentary and migratory forms diverged in the early Pleistocene, whereas phenotypic differentiation between the two migratory forms has occurred in the Holocene and is likely the result of geographical isolation and subsequent range expansion since the last glaciation. Our results underscore the importance of Quaternary climatic events in driving songbird speciation and indicate that plumage traits can evolve remarkably fast, thus rendering them potentially misleading for inferring systematic relationships. [ABSTRACT FROM AUTHOR]

Balsam fir (Abies balsamea) understory may be an important predictor of Black-throated Blue Warbler (Dendroica caerulescens) and Veery (Catharus fuscescens) distributions in northern hardwood forests that are heavily browsed by white-tailed deer (Odocoileus virginianus). We examined the abundance and age ratios of Black-throated Blue Warblers, and the abundance of Veerys, in 16 plots of hardwood forest with different understory composition within a heavily browsed region of the Hiawatha National Forest in Michigan's eastern Upper Peninsula. Four of these 36-ha plots had minimal understory and 12 had dense understory with variable amounts of balsam fir. Black-throated Blue Warbler abundance was significantly greater in plots with an average of 27% balsam fir understory cover than in plots dominated by deciduous understory; no Black-throated Blue Warblers were detected on the minimal understory plots. Age ratios did not differ significantly relative to balsam fir understory density. Veery abundance also did not vary with balsam fir understory density, but it increased with overall understory density. In forests such as these, where deer are abundant but rarely browse balsam fir, active management of balsam fir understory could provide key habitat for sustaining populations of Black-throated Blue Warblers and Veerys. We recommend that managers consider the presence of balsam firs in the understory when planning forest harvests in deer-impacted areas, so that they leave some balsam fir and stagger the cutting of stands with balsam fir over time to create and maintain heterogeneous understory structure. [ABSTRACT FROM AUTHOR]

The Cerulean Warbler (Dendroica cerulea) is currently the focus of considerable management interest; however, our ability to develop effective management strategies is hampered by a dearth of life history and basic behavioral data. Here, we present information on male-female interactions of Cerulean Warblers and parental nest attentiveness that is, to our knowledge, among the first such rigorously collected data for this species. Males feed females during nest building and on the nest during incubation; the relative infrequency of these events suggests that they play more of a role in pair-bond maintenance than they do in enhancing female energetics. Female incubation rhythms were not significantly influenced by temperature, time of day, or egg age. Compared with other Dendroica warblers, we observed relatively infrequent female departures during incubation, perhaps in response to a high risk of nest predation. As the nestlings aged, females spent less time brooding nestlings, presumably to allow for more frequent feeding; however, both males and females exhibited relatively low rates of food delivery compared with other Dendroica warblers. Despite the low rates of food delivery, feeding trips were more frequent at successful nests than unsuccessful nests. Our results suggest that Cerulean Warblers are tightly constrained by the competing pressures of predation risk and sufficient food provisioning for nestlings. [ABSTRACT FROM AUTHOR]

Like many other migratory birds, the black‐throated blue warbler (Dendroica caerulescens) shows pronounced differences in migratory behaviour and other traits between populations: birds in the southern part of the breeding range have darker plumage and migrate to the eastern Caribbean during the winter, whereas those in the north have lighter plumage and migrate to the western Caribbean. We examined the phylogeography of this species, using samples collected from northern and southern populations, to determine whether differentiation between these populations dates to the Pleistocene or earlier, or whether differences in plumage and migratory behaviour have arisen more recently. We analysed variation at 369 bp of the mitochondrial control region domain I and also at seven nuclear microsatellites. Analyses revealed considerable genetic variation, but the vast majority of this variation was found within rather than between populations, and there was little differentiation between northern and southern populations. Phylogeographic analyses revealed a very shallow phylogenetic tree, a star‐like haplotype network, and a unimodal mismatch distribution, all indicative of a recent range expansion from a single refugium. Coalescent modelling approaches also indicated a recent common ancestor for the entire group of birds analysed, no split between northern and southern populations, and high levels of gene flow. These results show that Pleistocene or earlier events have played little role in creating differences between northern and southern populations, suggesting that migratory and other differences between populations have arisen very recently. The implications of these results for the evolution of migration and defining taxonomic groups for conservation efforts are discussed. [ABSTRACT FROM AUTHOR]

Like many other migratory birds, the black-throated blue warbler ( Dendroica caerulescens) shows pronounced differences in migratory behaviour and other traits between populations: birds in the southern part of the breeding range have darker plumage and migrate to the eastern Caribbean during the winter, whereas those in the north have lighter plumage and migrate to the western Caribbean. We examined the phylogeography of this species, using samples collected from northern and southern populations, to determine whether differentiation between these populations dates to the Pleistocene or earlier, or whether differences in plumage and migratory behaviour have arisen more recently. We analysed variation at 369 bp of the mitochondrial control region domain I and also at seven nuclear microsatellites. Analyses revealed considerable genetic variation, but the vast majority of this variation was found within rather than between populations, and there was little differentiation between northern and southern populations. Phylogeographic analyses revealed a very shallow phylogenetic tree, a star-like haplotype network, and a unimodal mismatch distribution, all indicative of a recent range expansion from a single refugium. Coalescent modelling approaches also indicated a recent common ancestor for the entire group of birds analysed, no split between northern and southern populations, and high levels of gene flow. These results show that Pleistocene or earlier events have played little role in creating differences between northern and southern populations, suggesting that migratory and other differences between populations have arisen very recently. The implications of these results for the evolution of migration and defining taxonomic groups for conservation efforts are discussed. [ABSTRACT FROM AUTHOR]

Although most habitat characteristics are known to be continuously variable in space, practicality dictates that most habitat-selection studies at the spatial scale of the territory treat within-territory habitat as essentially homogeneous. However, the limitations associated with such a compromise have remained largely unexamined. Male Cerulean Warblers (Dendroica cerulea) exhibit nonrandom space-use patterns within their territories, in that all territories contain areas of intensive use or core areas. In addition to documenting territory-wide habitat and behavioral use patterns in this species, we asked two specific questions about core-area structure and function. (1) Are core-area habitats distinct in their vegetative composition and structure from the rest of the territory? (2) What behavioral mechanisms underlie the nonrandom space-use patterns? On a territory-wide basis, males used trees in proportion to their availability; however, core areas were predominantly composed of bitternut hickory (Carya cordiformis), which was a highly preferred song-post tree. Core areas were not consistently associated with any other habitat feature, including canopy gaps. Core areas were singing centers; song-post densities within core areas were 10x higher than in noncore areas. In our study area, bitternut hickories have significant delayed leaf-out patterns, potentially offering minimal acoustic hindrance to song transmission until late in the breeding season. These singing centers may be strategically placed to simultaneously maintain vigilance over social nests and maximize communication with conspecifics. Core areas are potentially as important to males as nesting habitat is to females, and their provision should be taken into account when implementing conservation or management strategies. [ABSTRACT FROM AUTHOR]

The Cerulean Warbler (Dendroica cerulea) is a Nearctic-Neotropical migratory bird species that has declined significantly over the long-term. Poor reproductive success may be an important factor contributing to the observed decline, but reproductive output has been measured for very few breeding populations. From 2003 to 2005, I intensively monitored 22-23 breeding territories/year in each of two large forest habitats in southwestern Michigan: oak- (Quercus spp.) hickory (Carya spp.) (2003: Barry State Game Area) and black locust- (Robinia pseudoacacia) black cherry (Prunus serotina) (2004-2005: Fort Custer U.S. Army Michigan National Guard Reservation). I also gathered descriptive data on nonsong vocalizations and age of territorial males. I describe four distinctive call notes, by sex, including the social and environmental contexts in which they were used. Using two independent methods of aging, there was a strong preponderance of after-second-year males at both study sites. Only 9 (n = 7 nests), 12 (n = 14), and 30 (n = 25) fledglings were produced during the 2003, 2004, and 2005 breeding seasons, respectively. Nest heights were the highest recorded for this species (mean = 19-20 m). During the same period, male reproductive success was 0.30, 0.32, and 0.80 male fledglings/breeding male and 0.60, 0.63. and 1.58 fledglings/breeding pair. Productivity estimates, not thought to be self-sustaining, were even lower than those of a well-studied Cerulean Warbler population in southern Ontario. Thus, reproductive output was low in two geographic regions--representing three different forest types--in the northern portions of the Cerulean Warbler's breeding range. The preponderance of after-second-year males at the Michigan study sites and in southern Ontario suggests a need for regional models of Cerulean Warbler population dynamics. [ABSTRACT FROM AUTHOR]

We examined Cerulean Warbler (Dendroica cerulea) abundance and occurrence in southwestern West Virginia, where the coal-mining technique of mountaintop removal mining-valley fill converts large contiguous tracts of deciduous forest to forest patches surrounded by early successional habitats. Our study objectives were to quantify abundance and occurrence of Cerulean Warblers relative to (1) distance from the edge of extensive reclaimed grasslands and (2) habitat structure and landscape characteristics. Cerulean Warbler abundance increased with distance from the edge and edge effects extended 340 m into the forest. Percent occurrence did not vary with distance from mine edge, suggesting a degree of tolerance to the extensive edge occurring at the interface of forest and reclaimed lands. Abundance and occurrence were greater on ridges and midslopes than in bottomlands; consequently, disturbances such as mountaintop mining in which ridges are removed may have a greater impact on populations compared to other sources of fragmentation where ridges are not disturbed. Models based on the information-theoretic approach indicated that Cerulean Warblers were more likely to be present in productive sites on northwest to southeast facing slopes, upper slope positions (midslope to ridgetop), and forests with low sapling density. Cerulean Warbler abundance was positively associated with more productive sites, higher snag density, large blocks of mature deciduous forest, and low amounts of edge in the landscape. In addition to outright loss of forested habitat, mountaintop mining-valley fill alters the spatial configuration of forested habitats, creating edge and area effects that negatively affect Cerulean Warbler abundance and occurrence in the reclaimed mine landscape. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We examined use of 2 silvicultural treatments (clearcut and two-age harvests), 15-18 years post-harvest by cerulean warblers (Dendroica cerulea) in mixed mesophytic and northern hardwood forests of the Allegheny Mountain region in West Virginia. Cerulean warbler abundance and occurrence were greater in 70-80-year-old mature forests than in 15-18-year-old clearcuts. Although abundance did not differ statistically between clearcut and two-age treatments, it was almost 5 times greater in the two-age treatments, likely because they provided a more complex canopy structure. Abundance of cerulean warblers in unharvested periphery stands adjacent to clearcut and two-age harvests was similar to that in unharvested control stands, suggesting that small harvests within mature forest do not negatively impact cerulean warbler abundance in the remaining forest, only within the clearcut harvests themselves. [ABSTRACT FROM AUTHOR]

Predator 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. [ABSTRACT FROM AUTHOR]

Individuals within a population vary in important fitness components, such as reproductive success. In general, females can maximize the number of young they produce by altering either the number of young per breeding attempt or the number of breeding attempts per season. In short-lived species, and especially in small passerire birds, number of breeding attempts per season varies markedly among individuals. Mere, we evaluated factors influencing whether female Black-throated Blue Warblers (Dendroica caerulesceus) initiated additional nests after a successful breeding attempt (i.e. double-brooded). The percentage of females that laid a second clutch lifter successfully fledging a first brood ranged from 0 to 87% and averaged 53% (n = 7 years). Multiple logistic regression and AIGc model selection indicated that double-brooded females bred in territories with greater food avail-ability and produced heavier nestlings than single-brooded females. Female age, male age, date of first breeding attempt, and number of young in the first clutch were not included in the best-fit model. Older females, however, produced heavier fledglings, and females mated to older males occurred on territories with greater food availability, indicating that age contributed to individual variation in reproductive output. Because the proportion of females that produce multiple broods within a season can have a substantial effect on the annual fecundity of a population, variation among fema es and among the territories they occupy (i.e. habitat qualify) are key factors influencing population dynamics in this and other multibrooded, short-lived species. [ABSTRACT FROM AUTHOR]

Accurate assessment of color is essential in testing the adaptive significance of color variation in avian plumage. Over the past decade, use of objective methods for assessing color has increased, with particular emphasis on ultraviolet (UV) wavelengths. Researchers have used various source materials, most notably museum specimens, to extend or represent color measurements of individuals in natural populations. Here, we address whether the colors seen in museum specimens accurately represent the colors seen in natural populations. We focus on UV wavelengths and carotenoid-derived colors across 10 species of wood-warblers (Parulidae). Our results indicate an uneven decrease in brightness across the color spectrum, with greater relative decrease in shorter wavelengths in museum specimens. That decrease leads to differences in both hue and chroma between living and museum specimens. The difference from live specimens appears to increase with the museum specimen's age. Our results suggest that caution is needed when using data from museum specimens to test hypotheses on plumage coloration, particularly those involving communication. [ABSTRACT FROM AUTHOR]

Although host–parasite interactions can affect the ecology and distribution of species, virtually nothing is known about range-wide variation in haematozoan levels in North American birds. We investigated haematozoan prevalence and parasitemia in 112 adult male cerulean warblers (Dendroica cerulea (Wilson, 1810)) in six populations throughout this species' breeding range. Thirty-six birds were infected with Haemoproteusparuli at low parasitemia (mean ± 1 SE, 6.18 ± 1.27 parasites/2000 erythrocytes). Prevalence did not differ between age classes, and there was no significant relationship between parasitism and body condition. However, prevalence and parasitemia were significantly higher in southern populations and further study is required to elucidate factors underlying regional differences in cerulean warbler haematozoan levels. [ABSTRACT FROM AUTHOR]

In short-lived species, fecundity strongly influences population size. For those species with multiple breeding attempts per breeding season, variance in fecundity is best explained by the number of breeding attempts. For birds, multiple brooding may he influenced by food availability. Here, we report results of a food supplementation experiment that tests the role of food as a mechanism driving variation among individuals in the frequency of multiple brooding in a Neotropical migrant songbird. the Black-throated Blue Warbler (Dendroica caerulescens). Supplementally fed females produced more second broods, spent less time foraging and more time loafing, and stayed closer to their nests than did control females. Fed and control females did not differ in the number or mass of young fledged from the first nesting attempt. Supplemental food increased the probability that females would initiate second broods in both a low and an average food year, suggesting that this population is food limited during the breeding season in most years. Our results thus demonstrate that food availability can strongly influence annual fecundity in migratory bird species breeding in temperate forests, which, in turn, affects annual recruitment rates arid population size. [ABSTRACT FROM AUTHOR]

Male North American wood-warblers (family Parulidae) subdivide their song repertoires into two different categories, or modes, of singing (first and second category songs). These two modes are thought to be specialized for interacting with females and males, although the data are inconclusive. I conducted an acoustic analysis of the song types used by yellow warblers ( Dendroica petechia) for type I (first category) and type II (second category) singing to ask whether there are consistent structural differences between them which could provide insight into how they might function as separate signals. I found that type I songs are performed closer to the upper boundary of a song performance limit, measured in terms of the difficulty of production, compared with type II songs. By contrast, the performance of specific song types did not depend on whether they were used for type I singing vs. type II singing by different males. In addition, type I songs had a greater amplitude increase across the first two syllables compared with type II songs. There was no relationship between the performance of type I or type II songs and male condition. These results suggest that wood-warblers might subdivide their song repertoire into distinct categories to highlight the relative vocal performance of their songs. [ABSTRACT FROM AUTHOR]

A monogamous mating system that includes extrapair fertilization can potentially generate higher variability in male reproductive success than monogamy without extrapair fertilization. That increased variability could provide a correspondingly higher opportunity for sexual selection and, thus, for the origin and persistence of sexual dimorphism in monogamous species. To determine whether extrapair fertilization enhanced the opportunity for sexual selection in a sexually dimorphic, monogamous bird species, we, used microsatellite DNA typing to assess the prevalence of extrapair fertilization and its effect on variation in male reproductive success in a population of Chestnut-sided Warblers (Dendroica pensylvanica). We found that the level of extrapair fertilization in our study population was at the upper end of the range reported for bird populations (47% of nestlings had extrapair fathers; 61% of broods contained extrapair offspring). We also discovered that almost all extrapair offspring were sired by paired males resident on nearby territories, In addition, we found that variation in male reproductive success was substantially higher than variation in female reproductive success, and that extrapair fertilizations made a significant contribution to variation in male reproductive success. Together. those findings suggest that extrapair fertilization creates an opportunity for sexual selection on male traits in this population. [ABSTRACT FROM AUTHOR]