The predator search image hypothesis predicts that predators will focus on prey that are abundant and with which they have had success. However, the predator dilution effect hypothesis predicts a decrease in the per capita chance of being depredated with an increasing amount of prey. We evaluated these hypotheses through an artificial nest survival study. We estimated nest survival during both the non-breeding and breeding seasons of 2014 at a restinga (sand-coastal-plain) area in southeastern Brazil. We installed 450 artificial nests (cup shaped) in each season and placed a Common quail (Coturnix coturnix) egg in each nest, totaling 900 nests. During the non-breeding season, our nests were the only ones present and thus the predator dilution effect should be absent. We estimated daily survival rate (DSR) using the “Nest Survival” function in the program MARK. We found a decrease in DSR at the middle and end in comparison with the onset of the non-breeding season, which supports the predator search image hypothesis. In comparison with the non-breeding season, we also found that DSR of artificial nests was lower at the onset and end of the breeding season, but higher in the middle of the breeding season. This pattern matched the higher availability of prey (i.e. natural nests) in the middle of the breeding season in the study site, supporting the dilution effect hypothesis. The survival rate of artificial nests suggests that both the search image and the dilution effect may occur during the breeding season, and that the predator dilution effect masks the predator search image in the middle of the breeding season. Our results expand our understanding regarding the effects of the search image and predator dilution effect hypotheses on nest predation, showing that both are density-dependent and may occur during the breeding season.

Vocal duets occur when 2 individuals vocalize in temporal coordination. In birds, duet participation functions to cooperatively defend shared resources, localize mates, and in some species, guard the mate. Previous work indicates that duetting tends to co-evolve with a non-migratory lifestyle, probably because the absence of migration facilitates greater cooperation between mates. We examined the evolution of duetting and migration in New World warblers (Parulidae), a group that has been largely ignored by duetting research. Of the 95 species in our analysis, we found evidence of duetting in 19 (20%) species, and evidence of migration in 45 (47.4%) species. Ancestral character reconstruction indicated that the last common ancestor of the New World warblers did not duet. Duetting evolved multiple times in this group, including 2 early origins and several more recent origins. Migration was present in the last common ancestor and was lost several times. Both duetting and migration exhibit phylogenetic signal. A phylogenetically explicit correlation analysis revealed a significant negative relationship between duetting and migration, in keeping with findings from other avian taxa. This study, the first description of the evolution of duetting in a large avian family with a temperate-zone origin, supports the hypothesis that duetting co-evolves with a sedentary natural history in birds.

Irruptive migration, or movements that are irregular in timing and extent, are common in many boreal forest species that feed on conifer seeds. Potential population consequences of such movements have not been studied, largely because the population of interest is literally a moving target. Broad-scale citizen science surveys may overcome this limitation, and Red-breasted Nuthatch (Sitta canadenis) is a good choice for investigation because it is well sampled in all seasons and is less nomadic than many other irruptive species. Fall nuthatch abundance at the Long Point Bird Observatory (LPBO, southern Ontario, Canada) is shown here to be correlated with winter abundance across the entire eastern U.S. as reported by the Christmas Bird Count, Project FeederWatch, and eBird. Irruption dynamics at LPBO can therefore serve as a surrogate for events taking place at much broader regional scales. Increased size of fall irruption recorded at LPBO corresponded with reduced Breeding Bird Survey abundance throughout eastern Canada the following summer. Despite presumed benefits of irruption as an adaptive response to food shortage when population levels are high, negative population consequences can ensue.

Woodhouse's Scrub-Jay (Aphelocoma woodhouseii) comprises 7 subspecies, ranging from the Rocky Mountains to southern Mexico. We quantified the phenotype of specimens throughout Mexico and found support for significant phenotypic differences between “Sumichrast's group” in southern Mexico (A. w. sumichrasti and A. w. remota) and the 2 subspecies in northern Mexico, or “Woodhouse's group” (A. w. grisea and A. w. cyanotis). Despite significant differentiation in body size and mantle color, we found no clear geographic boundary between the groups, suggesting either a geographic cline or hybridization upon secondary contact. We tested for selection against hybridization by fitting models to geographic clines for both body size and back color, and found support for a stable contact zone centered near Mexico City, with selection against intermediate back color. Based on these results, we infer that Sumichrast's and Woodhouse's groups diverged during a period of geographic and genetic isolation. The phenotypic introgression between Sumichrast's and Woodhouse's groups near Mexico City likely represents a case of recent secondary contact, with selection against hybridization maintaining a geographically stable contact zone.

Bird movements vary spatially and temporally, but the primary drivers that explain such variation can be difficult to identify. For example, it is well known that the availability of updraft influences soaring flight and that topography interacts with weather to produce these updrafts. However, the influences of topography on flight are not well understood. We determined how topographic characteristics influenced flight altitude above ground level (AGL) of a large soaring bird, the Golden Eagle (Aquila chrysaetos), over several regions within the State of California, USA. Primary drivers of flight AGL, those to which eagles showed the same response at all spatial scales, were topographic roughness, ground elevation and the east-west component of aspect (eastness). Each of these is related to formation of thermal updrafts. Secondary drivers, those to which eagles showed region-specific patterns, included topographic position, percent slope, and the north-south component of aspect (northness). In contrast to primary drivers, these secondary drivers were related to formation of both thermal and orographic updrafts. Overall, drivers of flight altitudes that were related to thermal updrafts showed different levels of complexity due to spatial and temporal variation of those drivers than did flight altitudes related to orographic updrafts.

Obligate brood parasitic birds have evolved a rare avian strategy for reproduction by laying eggs in the nests of other species. In doing so, their breeding ranges, but not necessarily their foraging habitats, have become intimately related to the nesting territories of their hosts. We studied home range sizes and distribution patterns in Common Cuckoos (Cuculus canorus) on their breeding grounds in central Hungary, where cuckoos parasitize only Great Reed Warblers (Acrocephalus arundinaceus) in channel-side reed-beds at a high frequency (>50%). The geographic coordinates of tagged cuckoos were monitored by high-precision, remotely downloadable non-Platform Terminal Transmitter global positioning system (GPS) loggers, attached to 9 females and 6 males. Our results revealed bimodal use of habitat patches: (1) the home ranges of male and female cuckoos were packed densely along the channels where the hosts breed, and their distribution maps had high overlaps between sexes; (2) ∼71% of cuckoos also visited nearby woodland patches, presumably for foraging, where the host species was not present. The size of cuckoo home ranges varied to an unusually great extent: 0.3–185 km² as calculated by the minimum convex polygon method (85%), or 1–17 km² when calculated by the more suitable kernel density estimation (KDE) method (Utilization Distribution 85%) for patchy habitats. Male and female cuckoos had similar home range sizes as estimated by the KDE method, consisting of 1–4 areas within the 2 habitat types of channel reed-beds and woodlands. No preference was revealed for night roosting locations between the 2 habitats or sexes. Female cuckoos were more likely to use reed-beds in the afternoons, when females parasitize host nests. Remote downloadable GPS methods offer an effective way of tracking cuckoos across large areas, but the estimation of home range sizes requires caution due to this species' patchy and disconnected habitat use.

In migratory animals, the degree to which individuals return to the same wintering sites across multiple years can affect fitness and population dynamics, and thus has important implications for conservation. Despite this, long-term evaluations of wintering-site fidelity are rare for migratory birds: many populations are intensively studied on their breeding grounds but tracking the migratory movements of small birds once they leave the breeding grounds is challenging. To evaluate patterns of overwintering location and fidelity, we collected winter-grown claw tissue from 301 Song Sparrows (Melospiza melodia; 449 samples) captured in spring at their breeding grounds over 6 consecutive yr and assessed stable hydrogen isotope (δ²H_c) values to determine within-individual repeatability and between-year variation in wintering latitudes. We also retrieved useable data from 8 geolocators over 2 consecutive winters. Geolocator-derived wintering positions correlated with origins based on δ²H_c values. Consistent with previous findings, male δ²H_c values reflected more northerly wintering areas than those of females, indicating shorter latitudinal migration distances for males, but the magnitude of the sex difference varied across years. The distribution of wintering latitudes was generally consistent among years, except for the 2015–2016 winter, which had unusually negative δ²H_c values. Values of δ²H_c were repeatable for males but not for females, suggesting that winter-site fidelity could differ between sexes. The data presented here emphasize the importance of tracking migratory populations across multiple years to uncover factors affecting population dynamics.

New study systems and tools are needed to understand how divergence and speciation occur between lineages with gene flow. Migratory birds often exhibit divergence despite seasonal migration, which brings populations into contact with one another. We studied divergence between 2 subspecies of Northern Saw-whet Owl (Aegolius acadicus), in which a sedentary population on the islands of Haida Gwaii, British Columbia (A. a. brooksi), exists in the presence of the other form (A. a. acadicus) during migration but not during the breeding season. Prior research showed fixed mtDNA divergence but left open the question of nuclear gene flow. We used 2,517 ultraconserved element loci to examine the demographic history of this young taxon pair. Although we did not observe fixed single nucleotide polymorphism differences between populations among our genotyped individuals, 100% of the birds were diagnosable and δaδi analyses suggested the demographic model best fitting the data was one of split-bidirectional-migration (i.e. speciation with gene flow). We dated the split between brooksi and acadicus to ∼278 Kya, and our analyses suggested gene flow between groups was skewed, with ∼0.7 individuals per generation coming from acadicus into brooksi and ∼4.4 going the opposite direction. Coupled with an absence of evidence of phenotypic hybrids and the birds' natural history, these data suggest brooksi may be a young biological species arising despite historic gene flow.

Evolutionary biologists have long used behavioral, ecological, and genetic data from contact zones between closely related species to study various phases of the speciation continuum. North America has several concentrations of avian contact zones, where multiple pairs of sister lineages meet, with or without hybridization. In a southern California contact zone, 2 species of woodpeckers, Nuttall's Woodpecker (Dryobates nuttallii) and the Ladder-backed Woodpecker (D. scalaris), occasionally hybridize. We sampled these 2 species in a transect across this contact zone and included samples of their closest relative, the Downy Woodpecker (D. pubescens), to obtain large single nucleotide polymorphism panels using restriction-site associated DNA sequencing (RAD-seq). Furthermore, we used whole-genome resequencing data for 2 individuals per species to identify whether patterns of diversity inferred from RAD-seq were representative of whole-genome diversity. We found that these 3 woodpecker species are genomically distinct. Although low levels of gene flow occur between D. nuttallii and D. scalaris across the contact zone, there was no evidence for widespread genomic introgression between these 2 species. Overall patterns of genomic diversity from the RAD-seq and whole-genome datasets appear to be related to distributional range size and, by extension, are likely related to effective population sizes for each species.

We use contemporary genetic diversity to characterize the within-island population structure and historical demography of Bananaquits (Aves: Thraupidae: Coerebinae: Coereba flaveola) on the West Indian island of Puerto Rico (Greater Antilles). We relate periods of population expansion, from Puerto Rico across the Lesser Antilles, to the genetic architecture of the source population, and describe differentiation of populations within Puerto Rico. Lastly, we report comparable analyses of populations of Bananaquits on Jamaica and of a related species, the Black-faced Grassquit (Coerebinae: Tiaris bicolor), on Puerto Rico. We found differentiation among contemporary populations of Bananaquits within Puerto Rico and signatures of renewed demographic expansion in eastern Puerto Rico beginning ~100 kyr ago and, in the western portion of the island, ~40 kyr ago. Populations of T. bicolor on Puerto Rico exhibit similar structure to Bananaquits, while Bananaquits on Jamaica exhibit no differentiation among locations. Both T. bicolor and Jamaican C. flaveola provide mixed evidence of demographic expansion.

Social information can spread fast and help animals adapt in fluctuating environments. Prospecting on the breeding sites of others, a widespread behavior, can help to maximize reproduction by, for instance, settling in the same area as other successful breeders. Previous studies have shown that successful broods have the highest number of prospectors and that they are visited most when offspring in nesting sites are already old, making the information more reliable. In this field study, we experimentally tested how prospectors are attracted to successful nest sites. We presented wild Zebra Finches (Taeniopygia guttata) with different visual or acoustic cues in nest boxes, simulating the presence of small or large clutches or broods. More Zebra Finches visited experimental nests that were associated with playback recordings of begging calls of large broods (7 chicks) as opposed to begging calls of small broods (3 chicks) and controls (white noise and silence). On the other hand, visual cues (nests with different numbers of eggs or rocks), representing nests at early stages, did not influence either the probability of visits, nor number or duration of visits. We present the first evidence that begging calls of chicks in the nest, a signal intended for kin communication, can also provide social information to unrelated prospecting conspecifics. This information could potentially be used for a fast initial assessment of the quality of a breeding site.

Sex-biased natal dispersal is predicted to be a consequence of differences in the costs and benefits of dispersal perceived by each sex. Although female-biased dispersal has been described for numerous birds in heterogeneous landscapes, studies documenting sex-related differences in the causes and consequences of natal dispersal in fragmented habitats are comparatively scarce. We used capture–mark–recapture data and genetic analyses in a population of Thorntailed Rayadito (Aphrastura spinicauda) from north-central Chile in order to investigate how the possible causes and consequences of dispersal are linked to sex-specific dispersal behavior in a naturally isolated and fragmented forest habitat. We analyzed 36 recaptured post-fledging birds to test whether female-biased dispersal reflected differential responses between sexes to potentially long-term acting selective pressures related to habitat fragmentation and high population densities. In this population, females disperse long distances (median distance: 780 m), whereas males are mostly philopatric (median distance: 85 m). Results suggest that female dispersal is possibly a response to the local density of breeding birds and mate availability, and comes without apparent reproductive costs. Inbreeding avoidance was not evident, but females may already be decreasing the probability of mating with relatives by moving away from their natal neighborhood where male kin usually remain. Our findings also indicate that male dispersal, while reduced in relation to females, can result from a tradeoff between the social benefits of settling near the natal territory and the associated costs of high breeding densities and kin competition. An increase of paternity loss was evident for males moving long distances. Follow-up studies should assess the differences in the costs and benefits of within- and between-fragment dispersal in this population.

Many birds use conspicuous plumage coloration to signal quality for sexual or social selection. However, it is still often unclear how intraspecific coloration and associated signaling vary spatially. Plumage coloration that is dependent on carotenoids may be ideal for studying spatial color variation because birds cannot naturally synthesize this pigment; therefore, bird coloration from carotenoids is at least partially contingent upon diet. As food availability often varies spatially, so might color and its signaling strength. While male coloration has received more research focus, less is known about female coloration and its relationships with social rank or sexual quality. To further improve our understanding of spatial variation in plumage coloration and correlations with individual quality, we compared Prothonotary Warblers breeding at 2 ecologically disparate sites separated by 1,300 km: in bottomland forests of Arkansas, USA, and the forests near the tidal freshwater James River in Virginia, USA. We assessed crown and breast plumage coloration for both sexes and compared several color metrics between sites. We then assessed surrogates of female quality (number of young fledged, number of eggs laid, provisioning rate, apparent annual survival, and nest depredation) and compared coloration-quality relationships between sites. We found that coloration of birds breeding in Arkansas was generally more elaborate than those breeding in Virginia. However, this was somewhat dependent on sex: females showed greater differences than males between sites. Additionally, color metrics of females breeding in Virginia showed stronger relationships with quality (all 5 quality metrics) than for birds breeding in Arkansas (only provisioning rate and nest depredation). Proximately, spatial variation in plumage coloration and the associated signaling may be explained by differences in diet between sites. Ultimately, spatial variation in intra- and intersexual selection pressures may explain how spatial variation in plumage signal strength evolved.