Figure 1 in Bauer et al. (2016) incorrectly identified boxplots as representing means with standard deviations, while in actuality they represent medians within interquartile ranges.

Avian evolution has generated an impressive array of patterns and colors in the ∼10,000 bird species that exist on Earth. Recently, a number of exciting studies have utilized whole-genome sequencing to reveal new details on the genetics of avian plumage color. These findings provide compelling evidence for genes that underlie plumage variation across a wide variety of bird species (e.g., juncos, warblers, seedeaters, and estrildid finches). While much is known about large, body-wide color changes, these species exhibit discrete color differences across small plumage patches. Many genetic differences appear to be located in regulatory regions of genes rather than in protein-coding regions, suggesting gene expression is playing a large role in the control of these color patches. Taken together, these studies have the potential to broadly facilitate further research of sexual selection and evolution in these charismatic taxa.

Migratory birds generally divide the annual cycle between discrete breeding and nonbreeding ranges. Itinerant breeders, however, reproduce twice at different geographic locations, migrating between them. This unusual flexibility in movement ecology and breeding biology suggests that some species can rapidly modulate the conflicting physiological and behavioral traits required for migration and reproduction. The Phainopepla (Phainopepla nitens), a songbird of the southwestern USA, has long been suspected to breed first in desert habitats in spring, then migrate to woodland habitats to breed again in summer. However, direct evaluation of movement and gene flow among individuals breeding in different locations has previously been logistically intractable. We deployed GPS tags on free-flying Phainopeplas in southern California, all of which migrated to hypothesized woodland breeding habitats after desert breeding (an average distance of 232 km). GPS data also revealed previously unknown fall and spring stopover sites. Population genomic analyses revealed no genetic differentiation among desert and woodland breeding populations, indicating significant movement and gene flow across the region. Finally, we used random forest analyses to quantify substantial environmental differences among temporal stages. Our results provide direct evidence that individual Phainopeplas do indeed move between 2 drastically different breeding habitats in the same year, representing a rare and extreme example of life-history flexibility.

Despite advances in tracking technologies, migration strategies remain poorly studied for many small-bodied passerines. Understanding variation within a migration strategy is important as variation impacts a population's resilience to environmental change. Timing, pathway, and stopovers vary based on intrinsic and extrinsic factors that impact individual migration decisions and capacity. Here, we studied drivers of variation in migration across a linked population of Golden-winged Warbler (Vermivora chrysoptera) using data from 37 light-level geolocators. We tested if behaviors vary in response to extrinsic factors: season, year, and proximity to a large geographic barrier—the Gulf of Mexico—and intrinsic factors: age and wing chord. Spring migration was nearly twice as fast as fall migration, with tightly correlated arrival and departure dates that were consistent among years, in contrast to no correlation or consistency in fall. This aligns with predictions for selection to minimize time spent migrating in spring and a relaxation of that pressure in fall. Twenty-nine birds staged for multiple days (mean: 7.5, SE: 0.6) in stopover habitats before crossing the Gulf of Mexico in spring, but 6 individuals overwintering closer to the Gulf coast forewent the stopover and completed migration 8 days faster. These findings suggest birds capable of crossing the Gulf without a stopover may experience a selective advantage by minimizing total migration time. After crossing the Gulf, individuals reduced travel speed and stopover duration, indicating constraints on movement differ before and after the barrier. Wing chord, but not age, positively predicted the total distance and duration of migration, and neither varied with timing, suggesting migration distance impacts morphology, but strategies do not vary with age. Ultimately, we find undescribed stopover locations south of the Gulf are important for most of the population, while high variation in migration behaviors suggest potential resilience to changing environmental conditions.

It is widely assumed that colonizing species thrive because they lack natural enemies in their new range, increasing their survival and reproductive success. Barn Swallows (Hirundo rustica) started to breed in South America around 1980 and since then have dramatically increased their population size and geographic range, in stark contrast to the decline of the source population in North America. The reasons behind the growth of the South American population are unknown. However, because this species had never bred in this area in recorded times, the lack of natural predators, parasites, or pathogens could lead to higher breeding success, as predicted by the enemy release hypothesis. Here, we test whether breeding success is higher in the newly colonized range than in the native range to test the enemy release hypothesis. We studied the breeding biology of South American Barn Swallows quantifying 7 breeding parameters: clutch size, overall breeding success, offspring mortality by predation and ectoparasites, number of fledglings per breeding attempt, number of breeding attempts per breeding season, and total number of fledglings produced per pair per breeding season. Additionally, we compared these parameters with published information from North American populations using meta-analyses. We found that, while clutch size did not differ between North and South American populations, the southern population overall had lower breeding success, with higher mortality from predation and ectoparasites. An egg laid in South America was 2.6 times more likely to fail than one laid in North America, which resulted on average in 1.1 fewer fledglings per pair per breeding season for the South American breeding population. These results, demonstrating lower breeding success in a newly colonized range, do not support the enemy release hypothesis, and indicate that the growth of the South American Barn Swallow population is most likely caused by other demographic factors.

Understanding how biotic and abiotic interactions influence community assembly and composition is a fundamental goal in community ecology. Addressing this issue is particularly tractable along elevational gradients in tropical mountains that feature substantial abiotic gradients and rates of species turnover. We examined elevational patterns of avian community structure on 2 mountains in Malaysian Borneo to assess changes in the relative strength of biotic interactions and abiotic constraints. In particular, we used metrics based on (1) phylogenetic relatedness and (2) functional traits associated with both resource acquisition and tolerance of abiotic challenges to identify patterns and causes of elevational differences in community structure. High elevation communities were composed of more phylogenetically and functionally similar species than would be expected by chance. Resource acquisition traits, in particular, were clustered at high elevations, suggesting low resource and habitat diversity were important drivers of those communities. Traits typically associated with tolerance of cold temperatures and low atmospheric pressure showed no elevational patterns. All traits were neutral or overdispersed at low elevations suggesting an absence of strong abiotic filters or an increased influence of interspecific competition. However, relative bill size, which is important for thermoregulation, was larger in low elevation communities, suggesting abiotic factors were also influential there. Regardless of metric, clustered and neutral communities were more frequent than overdispersed communities overall, implying that interspecific competition among close relatives may not be a pervasive driver of elevational distribution and community structure of tropical birds. Overall, our analyses reveal that a diverse set of predominantly biotic factors underlie elevational variation in community structure on tropical mountains.

Carry-over effects from one stage of the annual cycle to subsequent stages can have profound effects on individual fitness. In migratory birds, much research has been devoted to examining such effects from the nonbreeding to the breeding period. We investigated potential carry-over effects influencing spring body condition, breeding phenology, and performance for 3 species of sympatric, declining Nearctic–Neotropical migratory swallows: Bank Swallow (Riparia riparia), Barn Swallow (Hirundo rustica), and Cliff Swallow (Petrochelidon pyrrhonota). To examine carry-over effects, we used structural equation modeling and several intrinsic markers, including stable isotope (δ²H, δ¹³C, and δ¹⁵N) and corticosterone (CORT_f) values from winter molted-feathers, and changes in telomere length between breeding seasons. We found support for carry-over effects for all 3 species, however, the specific relationships varied between species and sexes. Effects leading to lower breeding performance were only observed in male Bank, female Barn, and female and male Cliff Swallows. In most cases, carry-over effects were attributed to differences in stable isotope values (most commonly with δ²H) presumably related to differences in winter habitat use, but, for Cliff Swallows, negative carry-over effects were also linked to higher CORT_f values and greater rates of telomere shortening. This work provides further support for the potential role of nonbreeding conditions on population declines, and indicates how multiple intrinsic markers can be used to provide information on ecological conditions throughout the annual cycle.

Geographical variation of birdsong is used to study various topics from cultural evolution to mechanisms responsible for reproductive barriers or song acquisition. In species with pronounced dialects, however, patterns of variation in non-dialect parts of the song are usually overlooked. We focused on the individually variable initial phrase of the song of the Yellowhammer (Emberiza citrinella), a common Palearctic passerine which became a model species for dialect research. We used a quantitative method to compare the similarity of initial phrases from the repertoires of 237 males recorded at different spatial scales in a central European country covering all main dialect types. We hypothesized that patterns of initial phrase sharing and/or phrase similarity are affected by dialect boundaries and geographical proximity (i.e. that birds from the same dialect regions use more similar phrases or share them more often). Contrary to our expectations, initial phrase variation seems unrelated to dialects, as we did not find higher similarity either among recordings from the same dialect areas or among those from the same locality. Interestingly, despite the immense variability of phrase types detected (only 16% of 368 detected initial phrase types were shared between at least 2 males), a relatively high proportion of males (45%) was involved in sharing, and males using the same initial phrase were located anywhere from tens of meters to hundreds of kilometers apart. The patterns of variation suggest that precise copying during song learning as well as improvisation play important roles in forming individual repertoires in the Yellowhammer. Our data also confirm previous indications that the repertoires of Yellowhammer males (i.e. the composition of initial phrases) are individually unique and temporally stable. This makes the species a good candidate for individual acoustic monitoring, useful for detailed population or behavioral studies without the need for physical capture and marking of males.

Ecological speciation can proceed despite genetic interchange when selection counteracts the homogenizing effects of migration. We tested predictions of this divergence-with-gene-flow model in Coeligena helianthea and C. bonapartei, 2 parapatric Andean hummingbirds with marked plumage divergence. We sequenced putatively neutral markers (mitochondrial DNA [mtDNA] and nuclear ultraconserved elements [UCEs]) to examine genetic structure and gene flow, and a candidate gene (MC1R) to assess its role underlying divergence in coloration. We also tested the prediction of Gloger's rule that darker forms occur in more humid environments, and examined morphological variation to assess adaptive mechanisms potentially promoting divergence. Genetic differentiation between species was low in both ND2 and UCEs. Coalescent estimates of migration were consistent with divergence with gene flow, but we cannot reject incomplete lineage sorting reflecting recent speciation as an explanation for patterns of genetic variation. MC1R variation was unrelated to phenotypic differences. Species did not differ in macroclimatic niches but were distinct in morphology. Although we reject adaptation to variation in macroclimatic conditions as a cause of divergence, speciation may have occurred in the face of gene flow driven by other ecological pressures or by sexual selection. Marked phenotypic divergence with no neutral genetic differentiation is remarkable for Neotropical birds, and makes C. helianthea and C. bonapartei an appropriate system in which to search for the genetic basis of species differences employing genomics.

Allen's (Selasphorus sasin) and Rufous (S. rufus) hummingbird have long been suspected to hybridize, and potentially form a hybrid zone where their ranges overlap in southern Oregon. Migratory Allen's Hummingbird (S. s. sasin) breeds along a narrow strip of the California coast up to the Oregon border, while Rufous Hummingbird breeds from southern Oregon to Alaska. Analysis of behavioral and morphological data for 183 males and morphological data from 138 females showed that Allen's and Rufous hummingbird form a hybrid zone in southern Oregon and northern California. Linear discriminant function analysis and cline analysis of 20 phenotypic characters for males and 9 phenotypic characters for females suggested the center of the coastal transect of this north–south hybrid zone spanned from Bandon, Oregon (Coos County), to Port Orford, Oregon (Curry County). The contact zone extended north into the breeding range of Rufous (into Florence, Lane County, Oregon) and south into the range of Allen's (into Arcata, Humboldt County, California). Sporadic inland sampling suggested the hybrid zone extended at least 94 km inland from the coast. Behavioral data included courtship displays, which were composed of discrete, modular, behavioral elements. Sexual selection acted on these courtship displays, as behavioral clines related to courtship behaviors were more narrow than morphological clines. Some of the courtship behaviors analyzed included previously undescribed diagnostic behavioral characters for Allen's and Rufous hummingbird.