Common Terns (Sterna hirundo) breeding at inland lakes in North America have experienced significant population declines since the 1960s. Although management actions aimed at mitigating effects of habitat loss and predation have been largely effective, numbers continue to decline, which suggests that the population may be limited during the nonbreeding season. Between 2013 and 2015, we used light-level geolocators to track Common Terns nesting at 5 inland colonies—from Lake Winnipeg in Manitoba, Canada, to the eastern Great Lakes region of the United States and Canada—to identify migratory routes and stopover and wintering sites and to determine the strength of migratory connectivity among colonies. Within 46 recovered tracks, we found evidence of a longitudinal gradient in use of migration routes and stopover sites among colonies and identified major staging areas in the lower Great Lakes and at inland and coastal locations along the Atlantic coast, Florida, and the Gulf of Mexico. Low migratory connectivity across inland colonies illustrates high intermixing within wintering sites, with many birds spending the nonbreeding season in Peru (70%) and the remainder spread throughout the Gulf of Mexico, Central America, and northwestern South America. While the large spatial spread and intermixing of individuals during the nonbreeding season may buffer local effects of climate change and human disturbance, the aggregation of individuals along the coast of Peru could make them vulnerable to events or changes within this region, such as increased frequency and intensity of storms in the Pacific, that are predicted to negatively influence breeding productivity and survival of Common Terns. Identifying sources of mortality during the nonbreeding season, quantifying winter site fidelity, and reinforcing the importance of continued management of inland breeding colonies are vital priorities for effective conservation and management of this vulnerable population.

The Trivers–Willard theory postulates that if a male in good condition at the end of the period of parental investment reproduces more successfully than a sister in similar condition, the mother should overproduce sons. Although this theory has been tested empirically in a wide variety of species, often producing equivocal results, few studies have analyzed whether the study populations conform to its assumptions. Here, we analyzed sex differences in lifetime reproductive success in a wild population of a polygynous bird, the Spotless Starling (Sturnus unicolor), and tested the Trivers–Willard prediction. We found that lifetime reproductive success was influenced by the body mass at fledging in males but not in females. However, although mothers with higher body mass produced heavier fledglings, they did not increase the proportion of sons. By contrast, we found that mothers with higher body mass produced heavier male fledglings and, after fledging, sons increased their body mass more than daughters. Our results suggest that there are sex-specific selective pressures acting on fledgling body mass. This may influence the sex-allocation tactic of the mother (i.e. biasing parental investment toward sons) or the rate of growth and the intensity of sibling competition (i.e. favoring sons to grow faster and hoard parental resources). Therefore, increasing the proportion of sons may increase the cost of parental investment and sibling competition, which may instead favor even sex ratios. Our study reinforces the idea that it is necessary to take into account the complexity of vertebrate life histories to make further predictions on sex-allocation strategies.

Efforts to understand population dynamics and identify high-quality habitat require information about spatial variation in demographic parameters. However, estimating demographic parameters typically requires labor-intensive capture–recapture methods that are difficult to implement over large spatial extents. Spatially explicit integrated population models (IPMs) provide a solution by accommodating spatial capture–recapture (SCR) data collected at a small number of sites with survey data that may be collected over a much larger extent. We extended the spatial IPM framework to include a spatio-temporal point process model for recruitment, and we applied the model to 4 yr of SCR and distance-sampling data on Canada Warblers (Cardellina canadensis) near the southern extent of the species' breeding range in North Carolina, USA, where climate change is predicted to cause population declines and distributional shifts toward higher elevations. To characterize spatial variation in demographic parameters over the climate gradient in our study area, we modeled density, survival, and per capita recruitment as functions of elevation. We used a male-only model because males comprised >90% of our point-count detections. Apparent survival was low but increased with elevation, from 0.040 (95% credible interval [CI]: 0.0032–0.12) at 900 m to 0.29 (95% CI: 0.16–0.42) at 1,500 m. Recruitment was not strongly associated with elevation, yet density varied greatly, from <0.03 males ha^–1 below 1,000 m to >0.2 males ha^–1 above 1,400 m. Point estimates of population growth rate were <1 at all elevations, but 95% CIs included 1. Additional research is needed to assess the possibility of a long-term decline and to examine the effects of abiotic variables and biotic interactions on the demographic parameters influencing the species' distribution. The modeling framework developed here provides a platform for addressing these issues and advancing knowledge about spatial demography and population dynamics.

Temporal overlap between parental care and molt occurs frequently in birds, but few studies have examined how individuals manage conflicts between these 2 demanding phases of the annual cycle. The potential for trade-offs between molt and parental care is especially high in the Hooded Warbler (Setophaga citrina) because (1) all rectrices are replaced simultaneously during primary molt, leaving birds temporarily without a functional tail; and (2) the tail plays an important role in foraging, as birds use their white tail spots and tail-flicking behavior to startle insect prey. I examined how simultaneous rectrix molt affected late-season parental care in a color-banded population of Hooded Warblers in northwest Pennsylvania, USA. Of 62 adults initiating rectrix molt before the end of parental care, 43 (69%) deserted their late-season nestlings and fledglings, leaving the mate to provide all remaining parental care. Because females initiate rectrix molt significantly later than males, most instances of uniparental desertion involved molting males abandoning fledglings or nestlings, but rare cases of postfledging desertion by females also occurred. Although most molting parents deserted, the probability of desertion decreased significantly with brood age, presumably because the costs of providing parental care during molt decline as fledglings approach independence. The probability of desertion by the male also decreased significantly with male age, suggesting that more experienced males can successfully balance the dual demands of molt and late-season parental care. In females, however, the only instances of desertion involved unusually old females ≥5 yr old, which suggests that rare cases of female desertion may occur as a mechanism to reduce reproductive effort late in life. My findings indicate that conflicts between parental care and molt, and the strategies that individuals use to manage those conflicts, merit increased attention from ornithologists seeking to understand the full annual cycle of migratory songbirds.

An organism's life history strategy is made up of a suite of physiological, behavioral, and ecological traits, which vary at both the interspecific and intraspecific levels in accordance with selective pressures operating on individuals. For birds, 2 primary ecological factors have been proposed to explain intraspecific and interspecific variation in nestling growth: nest predation and food availability. Individual nestling growth rates have important consequences for overall fitness because growth speed could influence subsequent reproductive performance and survival. We studied the relationship between ecological factors (i.e. precipitation level and predation rate) and nestling growth patterns of 2 New World flycatcher species (Tyrannidae) of the genus Tyrannus (Fork-tailed Flycatcher [T. savana] and Scissor-tailed Flycatcher [T. forficatus]) breeding at south- and north-temperate latitudes. We tested the hypothesis that nestling growth rates are driven by nest predation rates and predicted that nestling growth rates would be higher in species experiencing higher nest predation rates. We also tested the hypothesis that nestling growth rates are related to precipitation levels (a proxy for food abundance) and predicted that nestling growth rates would be higher at sites with higher precipitation levels. Growth rate was not associated with predation rate, but it varied with precipitation level, with faster nestling growth rates during wet years for the Scissor-tailed Flycatcher living at north-temperate latitudes. Among species, similar growth rates were found during wet years. These results indicate that, at least as proximate causes, precipitation explains intraspecific and interspecific growth rate variation in Tyrannus species to a larger degree than predation. Additionally, the variation in growth rate we observed between wet and dry years indicates a high level of plasticity in growth rate in this group of insectivorous birds.

Gaps in forest habitat are well documented to negatively impact the movements of forest songbirds. Much past research on avian gap-crossing decisions has utilized playback experiments. However, playbacks are limited by short observation times and often cannot be used to reveal differences in behaviors among individuals. Here, we present a novel approach utilizing radio-frequency identification (RFID) to investigate gap-crossing decisions in Black-capped Chickadees (Poecile atricapillus). Using bird feeders outfitted with RFID readers, we were able to track the movement patterns of wintering Black-capped Chickadees within forests and across gaps. We used logistic regression and an information theoretic approach to identify the factors that best predicted gap-crossing behaviors. Gaps impeded movements of wintering Black-capped Chickadees and the best predictors of gap-crossing behaviors were gap size and gap vegetation density. Birds were more likely to make crossings with decreasing gap size and when gaps contained more matrix vegetation (1–2 m height). We recommend the primary way to increase connectivity for birds in fragmented habitats is to reduce gap distances. Additionally, it may be beneficial to increase shrubby or woody vegetation within the gap to a height of over 1 m, as this also increases the likelihood of gap-crossing.

Migratory birds spend most of their journeys at stopover sites where they rest and refuel. Many migrants are in steep decline, and understanding their behavior within and among migrations is crucial for developing effective conservation strategies across the full annual cycle. One of the most rapidly declining songbirds in North America is the Rusty Blackbird (Euphagus carolinus; 85–95% decline over the past 50 yr), and stopover ecology is a major gap in our knowledge of its annual cycle. We utilized an automated telemetry array in western Lake Erie and the Motus Wildlife Tracking System to track landscape-scale movements, stopover duration, departure behavior, and between-season site fidelity in this species. We found that stopover duration during both fall and spring was nearly 1 mo (mean = 25.5 days)—exceptionally long for a passerine. During spring, birds in both poor condition and high degree of molt had longer stopovers, post-departure flights were relatively long for a songbird, and tailwinds predicted departure in both seasons. Many individuals made landscape-scale (10–35 km) relocations during stopover. Site fidelity was high for a passerine, in terms of both route and stopover site. Taken together, these behaviors describe a migration strategy that largely matches the staging behavior of shorebirds. Lastly, we found that Rusty Blackbirds migrate directly across Lake Erie and migrate primarily at night, which might expose them to mortality from offshore wind development. Collectively, our results indicate that high-quality stopover habitat may be critically important to Rusty Blackbird populations. More broadly, our results highlight the need to expand the scale of stopover studies, and to further explore all aspects of species' annual cycles to understand potential limiting factors on populations.

Crested Auklets (Aethia cristatella) and Least Auklets (A. pusilla) are crevice-nesting birds that breed in large mixed colonies at relatively few sites in the Aleutian Island archipelago, Bering Sea, Gulf of Alaska, and Sea of Okhotsk. Many of these colonies are located on active volcanic islands. The eruption of Kasatochi volcano, in the central Aleutians, on August 7, 2008, completely buried all crevice-nesting seabird habitat on the island. This provided an opportunity to examine the response of a large, mixed auklet colony to a major geological disturbance. Time-lapse imagery of nesting habitat indicated that both species returned to the largest pre-eruption colony site for several years, but subsequently abandoned it within 5 yr after the eruption. In 2010, a rockfall site in a cove north of the old colony site began to accumulate talus, and groups of auklets were observed using the site in 2011. Use of the new colony appeared to coincide with the abandonment of the old colony site by both species, though surface counts suggested that Least Auklets shifted to the new colony sooner than Crested Auklets. At-sea surveys of seabirds before and after the eruption indicated that both Crested and Least auklets shifted their at-sea distributions from the waters around Kasatochi Island to nearby Koniuji Island. In combination, at-sea counts and colony time-lapse imagery indicated that Crested and Least auklets using Kasatochi responded to the volcanic disturbance and complete loss of nesting habitat at the main colony on Kasatochi with dispersal either to newly created habitat on Kasatochi or to an alternate colony on a nearby island.

Nest survival is determined in part by a combination of large-scale environmental factors and local nest-site characteristics. Because predation is the primary cause of nest failure, those drivers likely operate by influencing predator abundance, behavior, and/or nest detectability. For example, fluctuations in landscape productivity have the potential to alter predator and prey abundance, whereas nest vegetation and patterns of nest spacing may influence predator behavior. We used 8 yr of site-specific environmental data coupled with data collected from 11,547 duck nests to evaluate the relative importance of large-scale and local factors on nest survival. We found that higher values of gross primary productivity, basins, and pond counts were associated with higher nest survival in a given year, but were associated with lower nest survival the following 2 yr. Taken in combination with the literature, our interpretation is that productive environmental conditions can result in time-lagged increases in predator abundance, leading to higher levels of nest predation in subsequent years. Local factors were generally less important than large-scale covariates in determining duck nest survival, but we found that nests laid earlier, in thicker vegetation, and with closer nearest neighbors had higher survival rates. However, as the season progressed, nests with closer nearest neighbors had lower survival rates (significant initiation date*distance interaction), suggesting predators may eventually aggregate in areas of higher nest density. Our results highlight the importance of both large-scale and local factors as they affect duck nest survival, and suggest several hypotheses about predator numerical and aggregative responses that are ripe for empirical testing.

An inserted body-feather molt has recently been documented in the Ruby-throated Hummingbird (Archilochus colubris) during its southbound migration; however, it is unknown whether other species of hummingbirds undergo similar inserted molts or molt-migrations. We examined 346 specimens of Rufous Hummingbird (Selasphorus rufus) collected throughout its range and found evidence for a previously unrecognized, inserted body-feather molt that appears to occur, at least partially, at stopover locations in California and the Mexican monsoonal region. This limited molt occurs in all young and in most (but not all) adult Rufous Hummingbirds, primarily in late June through October, before the complete overwinter molt in February–March. The location, timing, and extent of this molt in the crown, back, and underpart tracts showed similar patterns among the 4 age–sex groups; however, patterns of inserted throat-feather molt differed, occurring in all young birds and some adult females but not in adult males. In young birds, replacement of throat feathers preceded that of other body-feather tracts, a pattern that we also found in young male Anna's Hummingbirds (Calypte anna). We suspect that the unusual structure of the gorget feathers in males and the importance of these feathers to territorial defense and mate selection has resulted in these sex-specific patterns. We compare the molts of Ruby-throated and Rufous hummingbirds with those of more basal hummingbird genera and swifts. Based on the most parsimonious interpretation of presumed homologies, we propose that the summer–fall body-feather molts in these hummingbirds are inserted and include a limited preformative molt in young birds and an absent-to-limited definitive prealternate molt in older birds; we consider the first complete molt on the winter grounds to be the second prebasic molt. This terminology appears to best preserve homology during the evolution of both first-cycle and definitive-cycle molts from those of ancestral apodiform taxa.

Precise locations for the complete feather molt of North American landbirds, relative to their breeding territories, remain undocumented for most species. We analyzed >760,000 records of 140 species at 936 bird-capture stations to assess probabilities of recording both molting landbirds at their breeding sites and breeding birds at molting sites, and to investigate latitudinal, longitudinal, and elevational shifts from breeding to molting grounds. We demonstrate widespread evidence for molt-migrations among a variety of North American landbirds, including many migratory species previously thought to molt on “breeding grounds.” Geospatial differences between breeding and molting grounds were detected in all 4 compass directions as well as both upslope and downslope in elevation, while individuals of some species appeared to disperse to specific molting locations not discriminated by spatial direction or elevation from breeding territories. Although western North American species and populations are reported to undergo more molt-migration than eastern species, our molt-movement probabilities were similar in western and eastern North America and were greater in the east than in the west for several species. Combining our results with those of these previous studies, we suggest that many landbird species in western North America move longer distances to molt, whereas many landbirds in eastern North America may equally or more likely move shorter distances between breeding and molting habitats, while remaining within the overall breeding ranges of the species. Heterogeneous molt-movement responses are suggested for many species, which may relate to breeding success, resource availability on breeding territories, weather events, and other factors. Researchers using stable-isotopic, genetic, and geolocator techniques need to consider potential molt-movement strategies, and to incorporate molting habitat requirements into full-annual-cycle conservation efforts.

Male Bachman's Sparrows (Peucaea aestivalis) have large vocal repertoires containing many song types and multiple categories of songs and calls. We examined the territorial defense function of the vocal repertoire of male Bachman's Sparrows by quantifying singing behaviors and aggressiveness in response to a simulated territorial intrusion. We compared vocal and other behaviors between the intrusion and post-intrusion periods, compared more aggressive males (attackers) with less aggressive males (non-attackers), and tested for signals that predict attack. During intrusion, subjects switched among their song types at higher rates, however, song switching did not differ between more and less aggressive males. More aggressive males sang more low-amplitude “whisper songs.” In line with previous studies of aggressive signaling in songbirds, Bachman's Sparrows appear to use low-amplitude song to threaten rivals. The eavesdropping avoidance hypothesis predicts that low-amplitude songs should have acoustic traits besides amplitude to minimize signal propagation and transmission range. The whisper songs of Bachman's Sparrows are quieter versions of primary song types. Thus, Bachman's Sparrow may be a useful species for testing other predictions of the eavesdropping avoidance hypothesis, and for testing whether reduced amplitude is sufficient to reduce the costs of eavesdroppers.

Lipids stored subcutaneously serve as the main energy source for long-distance bird migration. The 2 major inputs of such lipids are dietary fats and de novo synthesis of fatty acids from acetyl-CoA derived from non-lipid sources, including ingested carbohydrates. Remarkably, relatively little is known about the specific roles of dietary fats and carbohydrates in fat-deposition processes of migratory passerines. We report the results of a large-scale field experiment complemented by a captivity experiment, aimed at testing the effects of the availability of simple carbohydrates and water on fuel deposition rates of autumn-migrating passerines in the northern Negev of Israel. We found that in the main study site, Ein Rimon, where birds mostly consume fat-rich Atlantic pistachio (Pistacia atlantica) fruits, access to both water and sucrose solution resulted in faster body mass gain. Similar patterns were evident among captive Eurasian Blackcaps (Sylvia atricapilla) subjected to the same nutritional conditions. Furthermore, supplementation with sucrose solution caused a significant reduction in the consumption of fruits by Eurasian Blackcaps, indicating that their preference for the fat-rich Atlantic pistachio fruits decreased when simple carbohydrates were available. Our results show that availability of simple carbohydrates can significantly accelerate fuel deposition rates in autumn-migrating passerines. We suggest that simple carbohydrates, dissolved in a solution (e.g., nectar), can be assimilated and absorbed faster than other food types and are therefore beneficial during the first days of stopover, when digestive capacity is reduced as a result of migration flight.

Geolocator technology has recently demonstrated that many songbird species exhibit prolonged stopovers during migration that may be analogous to the staging behavior typically attributed to shorebirds. Although staging areas can act as ecological bottlenecks, there is little information on how or why songbirds engage in prolonged stopover and whether this behavior differs among individuals. We used automated radio telemetry along the eastern coastline of North America to compare stopover and migration behavior of Blackpoll Warblers (Setophaga striata) and Red-eyed Vireos (Vireo olivaceus), species that differ markedly in migratory strategy during fall migration. We also tested whether migratory behaviors differed within each species by capture date or breeding origin. Prolonged stopover was more common and longer for Blackpoll Warblers, which suggests that it may be a particularly adaptive behavior for supporting the transatlantic endurance flights they make from the region. Both species made prolonged stopovers that consisted of extended stops at one site, as well as multiple shorter stops within a geographically broad stopover landscape. Later migrants of both species and Blackpoll Warblers from more northwestern origins exhibited migratory traits consistent with a time-minimization strategy.

Darwin's finches are considered a classic example of an adaptive radiation, and have been the focus of numerous studies from ecological and evolutionary perspectives. Few studies, however, have attempted to investigate the biogeographic origins of Darwin's finches. In this paper, we reconstruct the ancestral biogeography of Coerebinae, the tanager subfamily that contains Darwin's finches and their 14 closest relatives. We use this reconstruction to examine the origin of Darwin's finches, and the diversification of this clade of tanagers. We test multiple biogeographic models using the R package BioGeoBEARS utilizing a recent multilocus phylogeny. We used these models to examine 2 different hypotheses regarding the biogeographic origin of Darwin's finches. The majority of ancestral ranges within this subfamily were estimated as Caribbean restricted. Biogeographic models run using 8 regions suggest Darwin's finches arose from a long-distance dispersal event from the Caribbean Islands as opposed to the geographically closer mainland South America. However, models run using only 5 areas suggest equal probability between a Caribbean and a mainland South America origin to Darwin's finches. This study suggests equal probability for a Caribbean origin to Darwin's finches as a South American mainland origin. Conflict between models run using different biogeographic regimes highlights the sensitivity of these reconstructions to biogeographic region delineation. Overall, the Caribbean Islands appear especially important for the initial diversification of this clade, with many small-island restricted species diversifying early in the radiation. Colonization success was likely coupled with high dispersal ability and highly variable bill morphology to exploit vacant niche space.

Leapfrog migration systems, whereby more-northern breeding populations overwinter the farthest south, provide unique opportunities to further our understanding of how environmental variation shapes migratory behavior and the seasonal distributions of birds. Leapfrog migration in a western Fox Sparrow subspecies complex (Passerella iliaca unalaschcensis, Sooty Fox Sparrow) was described as early as 1920, and has served as an exemplar of leapfrog systems in subsequent theoretical work. However, migration behavior within P. i. unalaschcensis has never been studied directly, nor has the proposed leapfrog pattern been confirmed through the tracking of individuals. Using light-level geolocators and GPS tags, we tested the long-standing pattern of leapfrog migration in Sooty Fox Sparrows by determining spatiotemporal movement patterns for individuals originating from a northern (Vancouver Island, British Columbia) and a more southern (Point Reyes, California) overwintering region, where migratory timing, routes, and breeding locations were predicted to differ. Our results did not support the proposed leapfrog migration pattern in several ways. Individuals overwintering on Vancouver Island were predicted to be sedentary and/or breed locally, but we found they traveled more than 3,000 km to breeding sites in coastal northwestern British Columbia and southern Alaska. Birds overwintering in California had breeding locations that overlapped those of birds from British Columbia, as well as the predicted breeding regions of 4 other subspecies. Lastly, spring and fall migration routes were largely coastal for both groups, and we found no evidence of a proposed transoceanic fall migration route between Alaskan breeding sites and Californian overwintering sites. Thus, our results do not support the long-held pattern of leapfrog migration in Sooty Fox Sparrows and further highlight that bio-logging tools can reveal important new insights into patterns of migratory behavior, even in relatively well-studied systems.

Diets during critical brooding and winter periods likely influence the growth of Lesser Prairie-Chicken (Tympanuchus pallidicinctus) populations. During the brooding period, rapidly growing Lesser Prairie-Chicken chicks have high calorie demands and are restricted to foods within immediate surroundings. For adults and juveniles during cold winters, meeting thermoregulatory demands with available food items of limited nutrient content may be challenging. Our objective was to determine the primary animal and plant components of Lesser Prairie-Chicken diets among native prairie, cropland, and Conservation Reserve Program (CRP) fields in Kansas and Colorado, USA, during brooding and winter using a DNA metabarcoding approach. Lesser Prairie-Chicken fecal samples (n = 314) were collected during summer 2014 and winter 2014–2015, DNA was extracted, amplified, and sequenced. A region of the cytochrome oxidase I (COI) gene was sequenced to determine the arthropod component of the diet, and a portion of the trnL intron region was used to determine the plant component. Relying on fecal DNA to quantify dietary composition, as opposed to traditional visual identification of gut contents, revealed a greater proportion of soft-bodied arthropods than previously recorded. Among 80 fecal samples for which threshold arthropod DNA reads were obtained, 35% of the sequences were most likely from Lepidoptera, 26% from Orthoptera, 14% from Araneae, 13% from Hemiptera, and 12% from other orders. Plant sequences from 137 fecal samples were composed of species similar to Ambrosia (27%), followed by species similar to Lactuca or Taraxacum (10%), Medicago (6%), and Triticum (5%). Forbs were the predominant (>50% of reads) plant food consumed during both brood rearing and winter. The importance both of native forbs and of a broad array of arthropods that rely on forbs suggests that disturbance regimes that promote forbs may be crucial in providing food for Lesser Prairie-Chickens in the northern portion of their distribution.

Identifying the causes of individual variation in fitness should improve predictions about population dynamics and responses of populations to environmental change. Precise predictions may require long-term studies to parameterize models when the fitness of individual phenotypes depends on environmental conditions. We used a 37-yr study of a resident Song Sparrow (Melospiza melodia) population to identify traits that predicted individual variation in female lifetime reproductive success and to test for context dependence in trait–fitness relationships. Specifically, we asked how individual inbreeding coefficient, maternal age, and a suite of natal morphological traits influenced 2 components of lifetime reproductive success: (1) the probability of surviving to breed, and (2) the lifetime number of offspring produced, given that a female bred locally. We then tested whether population density influenced trait–fitness relationships. We found that differences in natal traits had life-long impacts on female fitness. Lower maternal age, a higher inbreeding coefficient, later laying date, lower nestling body condition, and longer tarsi were all negatively related to lifetime reproductive success. Maternal age and the inbreeding coefficient influenced both components of lifetime reproductive success, whereas other factors only influenced one. Therefore, traits that predict the probability of surviving to breed locally may differ from those that predict the number of offspring produced. We also observed larger effects of the inbreeding coefficient on fitness in years of low population density, which were often preceded by cool winters. Our findings demonstrate that natal traits and the environment experienced early in a bird's life can have life-long effects on individual fitness, primarily independent of population density.

Individual capacity to invest in a reproductive attempt may change over the breeding season, and such changes may be evaluated by potential partners through signaling mechanisms. Coloration of bare parts, such as exposed skin on the throat (gular skin) or feet, is common among avian species and has the potential to be a dynamic, honest signal of current condition, especially when such coloration is carotenoid-dependent. However, few studies have evaluated the repeatability of dynamic coloration of bare parts in the wild. We determined the presence of carotenoid pigments (lutein, zeaxanthin, 13-cis-beta-carotene, and trans-gamma-carotene) in gular skin samples of male and female Brown Boobies (Sula leucogaster) using high performance liquid chromatography (HPLC). We then studied color and perceived color variation in the gular skin of both sexes over the reproductive season, from courtship through parental care. We estimated color using Endler's segment method and perceived color using a tetrahedral color space model. Male and female green chroma was highest during courtship, and, in males, green chroma declined with capture date. Males' perceived greenness (theta) was higher during courtship than during the subsequent reproductive stages, and spectral purity (r achieved) was higher during courtship than afterward in both sexes. Perceived greenness and spectral purity also decreased with capture date in males. Male and female gular skin color and perceived color across the reproductive cycle showed a repeatability of 17–49%. Hence, the presence of carotenoid pigments in gular skin, the pattern of variation among reproductive stages, and the decline with capture date suggest that, in the Brown Booby, skin color may be costly to maintain, its variation can be perceived by other members of the species, and its intraindividual consistency over time may provide reliable information about individual's quality.

Life history theory assumes a trade-off between reproduction and survival. Investigations of this trade-off in birds have focused primarily on costs of rearing altricial young, because such costs are assumed to be low in species with precocial young. We experimentally manipulated nest success to test for a trade-off between (1) raising offspring for up to 1 yr after successfully nesting and (2) survival in female Ross's Geese (Chen rossii), a species with self-feeding, precocial young. We used multistate capture–resighting analysis that also incorporated recoveries of dead birds for inference about survival. We detected a general negative effect of successful nesting on survival, whereby point estimates of annual survival for successfully nesting females were consistently lower than those for failed nesters. Failed nesters had a greater proportion of mortality attributable to hunting, judging from their higher rates of reported mortality by hunters compared to successful nesters; thus, a cost of breeding associated with successful nesting likely resulted from natural mortality factors during incubation and brood rearing, and not from exploitation by humans.

Changes in habitat quality, including those caused by extreme events like droughts and floods, could alter costs and benefits of territoriality and thereby the prevalence and reproductive consequences for individuals capable of breeding that do not do so (floaters). We studied floating behavior in a population of Southwestern Willow Flycatchers (Empidonax traillii extimus) in central Arizona during one year of extreme drought, one year of lake inundation, and three years of near average precipitation. In all years, most floaters were second year (SY) males, and most subsequently settled outside of the patch where they were detected in the floating year, suggesting that floaters did not “queue” at high-quality territories in order to achieve higher reproductive success in subsequent years. Instead, cohorts that floated in non-drought years had lower apparent survival and lower reproductive success compared to territorial birds. In the extreme drought year, however, the number of floaters was 1.5 times greater than in all other years combined, more females floated, and apparent survival and mean annual productivity in subsequent years was higher for males that floated in that year than for those that were territorial. Inundation of habitat due to rising reservoir levels did not result in an increase in floaters because many birds nested in inundated areas where trees projected above the water so that the relative amount of available habitat was not reduced to the extent habitat models predicted. Overall, our results indicate that the prevalence and reproductive and demographic consequences of floating can change under extreme climatic events like severe drought.

Parasite population structure can be used to infer fine-scale movement in host species. Many penguin species form large social colonies, and are highly philopatric, returning to the same nest or burrow, along the same route, after each trip to sea. Within a colony, however, the local abundance, physical similarity, and nocturnal habits of penguins hinder the observation of fine-scale movements. To determine the extent of movement and interaction of penguins within colonies, a genotyping by sequencing (GBS) approach was used to study the fine-scale structure of ticks—which depend on host movements for dispersal—exploiting the largest Little Penguin (Eudyptula novaehollandiae) colony in Australia (Phillip Island, Victoria). No barriers to tick gene flow were identified, and we infer that extensive penguin movement occurs throughout the colony. Our findings support the hypothesis that some penguin species are highly gregarious, socializing widely within colonies despite strong nest-site philopatry.

In many cooperatively breeding species, young (“helpers”) from one year help other adults raise offspring the following year. In contrast to helper effects during the nestling or postfledging stages of the avian breeding cycle, potential benefits from helpers during incubation are poorly studied. We analyzed 39 clutches and recorded 6,027 off-bouts to document incubation behavior in the White-breasted Thrasher (Ramphocinclus brachyurus), an endangered Neotropical facultative cooperative breeder. Our goal was to test the prediction that cooperation confers benefits during incubation in terms of increased nest attendance and hatching success. We found that 65% of active hours (0500–2100) are spent on the nest, values somewhat lower than average for a tropical passerine with uniparental incubation. Highest incubation constancy was observed at a rare 4-egg clutch, which was attended by putative joint-nesters. Excluding this clutch, differences in incubation behavior between pair and cooperative groups were subtle and context dependent. We found temporal variation in incubation behavior, whereby off-bout frequency declined as the breeding season progressed, but more quickly for cooperative groups. Maximum ambient temperature was also an influential abiotic predictor. Across both social group types, incubation constancy declined as temperature increased to ∼30°C, above which constancy remained high. As expected, we found that the behavior of birds with failed and successful clutches differed. Specifically, failed clutches experienced high early-season constancy, despite temperature data suggesting insufficient warming during that time, and successful cooperative clutches had higher late-season constancy than pairs. Other factors important in avian systems were not predictive of incubation behavior here, and in general, high individual variation for all incubation behaviors swamped most other sources of variation. Results from this work highlight the individuality in incubation behavior and suggest that breeding females in cooperative groups have more flexibility in shifting between incubation demands and maintenance behaviors than do lone pairs.

Understanding inter- and intra-specific variation in mate compatibility and reproductive success can offer insight into the factors driving sexual selection, behavioral dynamics, and isolating mechanisms across natural populations. This information is particularly relevant when trying to understand the patterns that shape the causes and outcomes of hybridization in natural systems. We evaluated mating patterns and male reproductive success in a hybrid zone between the Saltmarsh (Ammodramus caudacutus) and Nelson's sparrow (A. nelsoni). Specifically, we investigated variation in male reproductive success between pure Saltmarsh and pure Nelson's sparrows and between pure and hybrid males, while testing for assortative vs. random mating. DNA samples were collected from adults (n = 342) and nestlings (n = 348), and paternity analyses and assignment of individuals to pure and hybrid classes were conducted using 11 microsatellite loci. We documented high promiscuity and reproductive skew in Saltmarsh and Nelson's sparrow males, with greater skew in Saltmarsh Sparrows. Across Nelson's Sparrow and hybrid individuals, we documented a significant correlation between reproductive success and genetic heterozygosity. F1–F2 hybrid males exhibited significantly lower reproductive success (number of offspring sired) compared with pure individuals, and Saltmarsh and Nelson's sparrows exhibited strong patterns of assortative mating. We conclude that differences in reproductive success among pure and hybrid individuals, coupled with strong assortative mating, are shaping hybrid zone dynamics in this system and may be important for maintaining species boundaries.

Most seabirds are constrained to forage near breeding sites when incubating and provisioning offspring but at other times are free to migrate to more favorable foraging habitat. Auklets (Aethia and Ptychoramphus spp., family Alcidae) are considered to be highly mobile during the nonbreeding season, except for one species, the Whiskered Auklet (Aethia pygmaea), which anecdotal evidence indicates remains close to breeding colonies year round. To clarify Whiskered Auklet year-round whereabouts, we deployed light-based archival geolocation “tags” onto breeding adults in 2013 and 2014. To quantify activity on land we used wet–dry logs from the tags and automated digital sound recording of vocalizations at Buldir Island, Aleutian Islands, Alaska, during 2014–2015. Tagged Whiskered Auklets (n = 17) breeding at Buldir remained close to the island all year (mean distance from Buldir 199 km in latitude, 49 km in longitude, which is comparable to measurement error of bird-borne tags recorded in other studies). Audio recordings confirmed presence of vocalizing birds on land from March to October and wet–dry data indicated roosting on land between sunset and sunrise year-round (including more than 14 hr of each 24 hr period in December), except for nocturnal trips to sea during full moons. Our results quantified the extraordinary year-round residency and land roosting of Whiskered Auklets at Buldir. Although it is impossible to generalize from our single study, if typical across this species' range, this places Whiskered Auklets well outside the behavior of all other auks and most seabirds. Year-round residence near breeding sites and land roosting may be behavioral adaptations that interact with the use of near-shore tide rip foraging habitat in a tradeoff of metabolic costs against foraging success.

Studies in evolutionary biology have commonly been focused on insular systems because of their natural geographic isolation and relatively simpler biotas. Using mitochondrial DNA sequences of 4 passerine bird species distributed in the Tres Marías Archipelago (TMA) and the nearby mainland of western Mexico—Cardinalis cardinalis, Turdus rufopalliatus, Vireo hypochryseus, and Icterus pustulatus—we determined interspecific and intraspecific phylogenetic relationships between insular and mainland populations, conducted insular age-based time calibration for the estimation of divergence times, and used Bayesian analyses to examine the colonization history of islands. Specifically, we tested whether the study species from the TMA share the same colonization history since the emergence of the islands ∼120 kya, taking advantage of the reduced isolation due to sea-level fluctuations during the Pleistocene, or whether there were independent colonization events. We also looked for evidence in the genetic structure of the island populations that would support the idea of colonization by a small number of individuals. Phylogenetic relationships consistently recovered lineage divergence between the TMA and mainland populations, with strong support in 3 of the 4 species. Our estimates for the sea level and coastline of the west coast of Mexico during the Pleistocene showed that the distance between the TMA and the mainland was ∼25 km. We tested several island colonization scenarios according to the phylogenetic relationships, haplotype networks, divergence time estimates, historical demography, and different glaciation dates. The most supported scenario of colonization of the TMA suggests that a single event occurred ∼120 kya when the islands emerged, which is highly concordant with geological evidence, and simultaneously affected the 4 species.

Thermal variation poses a problem for nesting birds and can result in reduced offspring growth rates and survival. To increase the thermal stability of the nest, females can adjust nest characteristics and nest attendance in response to changes in environmental conditions. However, it is unclear how and to what extent females modify parental behaviors during various stages of offspring development. We tested the hypothesis that females adjust nest characteristics and brooding patterns in response to thermal variation during the nest-building and nestling stages, respectively. We examined elevational variation in nest location, nest construction, and brooding patterns in the migratory Black-throated Blue Warbler (Setophaga caerulescens) across a 2°C gradient at the Hubbard Brook Experimental Forest, New Hampshire, USA. Density of woody stems at nest sites and nest wall thickness increased from low to high elevation, corresponding to decreasing temperatures, but we found no relationship between weather during nest building and nest characteristics. However, weather during the nestling stage was associated with female brooding patterns: at lower temperatures and with higher rainfall, females spent more time off the nest, which was associated with lower nestling mass near fledging. These results suggest that thermal cues during nest building may be unreliable as predictors of future conditions for developing nestlings and also that females might favor their own self-maintenance and compromise nestling growth under adverse thermal conditions.

Next-generation sequencing technologies are increasingly being employed to explore patterns of genomic variation in avian taxa previously characterized using morphology and/or traditional genetic markers. The hybridization dynamics of the Northern Flicker complex have received considerable attention, primarily due to the conspicuous plumage differences among these birds and the geographically extensive hybrid zone between the Red-shafted (Colaptes auratus cafer) and Yellow-shafted (Colaptes auratus auratus) flickers in the Great Plains region of North America. However, no traditional molecular techniques have been able to differentiate these 2 morphologically well-defined taxa from one another, or conclusively from the closely related Gilded Flicker (Colaptes chrysoides). Here, we use a next-generation sequencing approach to assess the genetic diversity and evolutionary history of these 3 taxa. We confirm the overall low levels of differentiation found using traditional molecular markers, but are able to distinguish between the 3 taxa for the first time, using a dataset of thousands of SNP loci distributed across the genome. Through demographic modeling and phylogenetic reconstructions, we find that Red-shafted and Yellow-shafted flickers are likely sister taxa, and that their divergence from the Gilded Flicker was comparatively older. The low level of divergence and lack of fixed differences in our dataset between Red-shafted and Yellow-shafted flickers, in particular, suggests whole-genome re-sequencing may be necessary to assess the dynamics of their hybridization and identify the genetic basis of their striking differences in plumage.

Increased exposure to parasites and other pathogens is one of the principal costs of group living, and animals have evolved diverse behavioral adaptations to detect and avoid parasitized conspecifics. In mammals, it is well established that parasitic infection can affect host body odor and that individuals use this chemical information in mate choice and other social contexts. In birds, despite intense interest in condition-dependent sexually selected displays as indicators of health status, chemical signaling of infection status remains largely unexplored. However, compounds in feather preen oil are increasingly recognized as candidate substances that may mediate chemical signaling in birds. We inoculated Song Sparrows (Melospiza melodia) with hemosporidian parasites (Plasmodium sp.) to determine whether parasitic infection alters the chemical composition of preen oil. We used gas chromatography with flame ionization detection (GC-FID) to separate and quantify the wax esters in preen oil and compared the chemical profiles of preen oil before inoculation to chemical profiles 13 days later (corresponding to peak infection intensity among the subset of birds that became acutely infected). Inoculation with Plasmodium sp. altered the wax ester profiles of preen oil, albeit with a modest effect size, regardless of whether individuals became acutely infected or resisted the infection. By contrast, wax ester profiles of preen oil were not significantly altered in individuals that received a sham inoculation with uninfected blood. Provided that birds can perceive these alterations in wax ester profiles of preen oil (e.g., after their degradation and release as volatiles), our findings suggest that chemical cues inherent in the wax esters may signal malarial infection status and/or exposure history.

Penguins exhibit an array of derived feather features and color-producing mechanisms, including distinct melanosome morphologies and beta-keratin nanofibers that produce blue structural color. Several morphologies have been proposed to have hydrodynamic or insulatory functions. An understanding of the distribution of these morphologies over the body of a single penguin species may elucidate their evolution and function. We investigated feather microstructure and color production mechanisms in blue, black, and white barbs of feathers of the Gentoo Penguin (Pygoscelis papua). Light and scanning electron microscopy revealed that all feathers, regardless of color, are semi-lunate in cross section and lack, or show greatly reduced, air-filled compartments (central vacuoles) in feather barbs. Keratin nanofibers are present in all sampled feathers (including those of 8 other penguin species) but are surrounded by melanin in black barbs and disorganized in white barbs. Reduction of the air-filled central vacuole and gain of keratin nanofibers appear to have single origins in penguins. Given that they occur in all feathers regardless of color, we hypothesize that nanofibers have been co-opted for color production but initially arose for another function.

Nesting birds must often cope with harassment from biting insects, but it is difficult to ascertain what effect such pests might have on breeding success and population dynamics. We tested the hypothesis that a black fly (Simulium annulus) that feeds on the blood of nesting Common Loons (Gavia immer) causes nest abandonment in this charismatic diving bird. In addition, we measured effects of fly-induced abandonment on a loon population, and examined potential predictors of fly abundance and nest abandonment. We also tested a second hypothesis, which holds that loon pairs that abandon a nest owing to flies should often remain at the site for their subsequent nesting attempt, since fly outbreaks last only 1–2 weeks. All predictions of the fly-induced abandonment hypothesis were supported, including strong correlations between fly counts and rate of abandonment, reduced incubation during severe fly years, and increased abandonment during cool springs, which promote longevity of the flies. The correlation between nest abandonment and population breeding success suggests that S. annulus reduced the chick fledging rate by as much as 23% in a year of severe infestation. Fly numbers on loons and their nests were highest when temperatures were high and winds were light. Surprisingly, however, exposure to the prevailing wind increased, not decreased, nest abandonment, perhaps because of wave action. Lake size was inversely and female age directly correlated with abandonment rate, possibly due to food limitation in small lakes and senescence of females, respectively. Finally, pairs that abandoned a first nest renested at the same site with much greater frequency than did pairs that lost eggs to a predator, indicating that loons are capable of responding adaptively to a cause of nest failure that is time- but not space-dependent.