In lowland Neotropical regions, where air temperature and day length remain relatively constant year round, seasonality is determined primarily by changes in rainfall. The wet season triggers the start of breeding for many Neotropical birds but also alters the antigenic environment, likely increasing the risk of disease transmission. We explored 2 hypotheses about temporal variation in constitutive innate immunity of a Neotropical bird, the House Wren (Troglodytes aedon). The antigen response hypothesis proposes that Neotropical wrens upregulate their immune function in the wet season either in anticipation of or in response to vectors that become more prevalent. The resource constraint hypothesis proposes that during periods of putative high resource demand, such as when parents are feeding young, immune function should be compromised and downregulated. Controlling for reproductive stage, we found that microbicidal capacity of blood against Escherichia coli was higher in the wet than the dry season, consistent with the antigen response hypothesis. Phagocytosis of E. coli and Staphylococcus aureus did not differ between wet and dry seasons. Microbicidal capacity and H/L ratio of tropical House Wrens did not vary among reproductive stages, and our data offered no support for the idea that immune function is compromised during the period when parents are feeding young.

Colony fidelity and dispersal can have important consequences on the population dynamics of colonial-nesting birds. We studied survival and inter-colony movements of Double-crested Cormorants (Phalacrocorax auritus; cormorants) nesting at Spider and Pilot islands, located 9 km apart in western Lake Michigan, during 2008–2014. We used live resighting and dead recovery data from both colonies, plus dead recoveries from throughout North America, in a multistate live and dead encounter model to estimate annual survival, inter-colony movements, plus temporary and permanent emigration to unmonitored sites. Annual survival averaged 0.37 (annual process variation, = 0.07) for hatch-year, 0.78 ( = 0.08) for second-year, and 0.89 ( = 0.04) for after-second year birds. The best approximating model recognized only 2 age classes for transition probabilities, indicating little difference in fidelity and movement probabilities after the natal year. Annual fidelity to Spider and Pilot islands averaged 0.53 ( = 0.17) and 0.48 ( = 0.24) for second-year and 0.55 ( = 0.23) and 0.62 ( = 0.16) for after-second year cormorants, respectively, indicating substantial emigration for both age classes. For birds that dispersed, emigration was approximately equally divided among neighboring colonies, temporary emigration sites from which surviving birds subsequently returned, or permanent emigration sites from which birds never returned (but were still encountered through dead recoveries). Our results indicate that Double-crested Cormorants in the Great Lakes have tremendous potential to disperse, which may help to explain their rapid recolonization following historically low populations in the early 1970s.

With worldwide increases in artificial light and anthropogenic noise, understanding how these pollutants influence animals allows us to better mitigate potential negative effects. Both light and noise affect the timing of daily activities, including the onset of dawn song in birds, yet the influence of these pollutants compared with social factors that also influence song onset remains unknown. We investigated the onset of dawn song, testing hypotheses aimed at understanding the influences of light and noise pollution as well as male competition, pairing status, and breeding stage on timing of dawn singing by male House Wrens (Troglodytes aedon). Overall, models with social factors fit song onset data better than models with abiotic factors of noise and sky glow, and the highest ranking model included nesting stage, number of male neighbors, and temperature. Males began singing earlier when they were building nests and when mates were fertile than during later nesting stages. Males also sang earlier as the number of male neighbors increased. The timing of dawn song by male House Wrens appeared unaffected by day-to-day variation in light and noise pollution, with social factors having larger effects on the onset of daily behavior in this species.

Most seasonally migrating songbirds exhibit protandry, whereby males arrive to breeding sites in the spring before females. The proximate behavioral mechanisms of protandry are largely unknown for most species, but could include earlier migratory departure from wintering sites by males or overall faster migration by males. Using onset and intensity of migratory restlessness as proxies for departure timing and rate of migration, respectively, we evaluated these 2 hypothesized mechanisms in a Nearctic–Neotropical migrating songbird, the Black-throated Blue Warbler (Setophaga caerulescens). Birds were captured during fall migration, held in captivity over winter, and photostimulated in the spring to induce migratory behavior. Video analysis was used to separately quantify stereotypical nocturnal wing whirring and jumping migratory restlessness behaviors. The birds were then radio-tagged and released in mid-May to compare stopover duration between the sexes and validate migratory restlessness in captivity as a proxy for the motivation to migrate in the field. In captivity males initiated migratory restlessness earlier in the spring than females, demonstrating innate differences in the onset of spring migration in this species. Males also displayed higher-intensity wing whirring behavior, suggesting potential sex differences in flight behavior that could influence migration rate. We found no sex differences in stopover duration in the field following release. However, stopover duration was negatively correlated with total migratory restlessness intensity on the last night the birds were held in captivity, which supports migratory restlessness as a proxy for the motivation to migrate at the individual level.

The period of migration can pose significant energetic challenges as birds attempt to reach their destinations. Suitable stopover habitat is, therefore, important to the success of migrating individuals, especially as they move along major migration corridors and geographic features, like coastlines. In this study, we used metrics of individual body condition (i.e. fat score, size-corrected body mass, and refueling rate) of fall migrants as they moved across the Gulf of Maine region, a complex coastal landscape. We investigated the extent to which these body condition indices varied by stopover site geography (island vs. mainland) and how these spatial patterns varied with species-specific characteristics such as migratory distance, foraging guild, and age. Geography was an important factor explaining variation in all 3 condition indices, and age explained additional variance in 2 of the 3. In general, individuals captured on islands exhibited significantly lower energetic condition than individuals on the mainland, and this pattern held true across all migratory strategies and foraging guilds. Immature individuals had, on average, lower energy reserves, with less fat and lower size-corrected mass than adults among all stopover sites. We also found that at all sites, size-corrected body mass, on average, significantly increased over the capture day, providing evidence that both island and mainland sites provided energetically beneficial stopover habitat. Our finding that birds offshore are in lower body condition at capture than those along the coast is suggestive that birds use mainland sites for longer stopover bouts than island sites, or that poorer condition birds reorient to land during over-water movements disproportionately use off-shore islands as their initial landing area. Decreases in either island or mainland stopover site availability or quality may affect individual fitness, with population-level consequences, but through different ways.

There have been an increasing number of observations of itinerancy in migratory songbirds, where individuals move among 2 or more widely separated areas during the “stationary” nonbreeding season. Knowledge of such movements and an understanding of what drives them are important for predicting how migratory populations will respond to environmental change. In this study, we investigated nonbreeding movements of the Tree Swallow (Tachycineta bicolor), an aerial insectivore that breeds across North America and spends the nonbreeding season around the Gulf of Mexico, Florida, Mexico, Central America, and the Caribbean. With year-round tracking data obtained from 133 light-level geolocators deployed at 12 breeding sites ranging from Alaska to Nova Scotia to North Carolina, we show that 44% of individuals made at least one large-scale movement (range: 301–1,744 km) within the nonbreeding range. The frequency of itinerancy decreased with longitude, such that 75% of individuals made a movement in the western portion of the nonbreeding range compared to only 31% in the east. Using the Normalized Difference Vegetation Index (NDVI) as a proxy for resource availability, we found that when individuals did move, they were more likely to move from sites where resources were deteriorating faster (a more negative change in NDVI prior to departure) than their destination sites. There was also evidence that individuals moved to destination sites with higher NDVI and temperature in the autumn, but not in the winter. Our results suggest movements of Tree Swallows during the nonbreeding season are influenced by resource availability, but because not all individuals used multiple nonbreeding sites, the density of individuals at a site and the level of competition may have also been a factor influencing nonbreeding season movements.

Heterozygosity affects mate selection and can modulate interactions among family members and their fitness-related decisions. We studied whether nestling heterozygosity affected parent–offspring interactions and sib–sib competition in the Blue Tit (Cyanistes caeruleus) while controlling for the degree of relatedness among nestlings. Demanding environmental conditions might make the detection of heterozygosity-fitness correlations easier. Thus, we also investigated whether the decision rules of family members according to offspring heterozygosity were affected by brood size, as a proxy of the strength of sibling conflict. We found that chick individual heterozygosity was positively although weakly associated with individual body mass. Mean brood heterozygosity did not predict fledging success, but broods that fledged more chicks showed a higher number of less common alleles. Interestingly, fathers, but not mothers, favored heterozygous broods with many nestlings, that is, heterozygous broods with higher potential for sibling conflict. Moreover, the lower the mean brood heterozygosity the stronger the begging intensity when parents were absent, regardless of brood size. Finally, the degree of relatedness among nestlings was not associated with any behavioral parameter, supporting a more prominent role for heterozygosity in shaping intra-family interactions. Our findings suggest that offspring heterozygosity determines sex-specific rules of parental care and that genetic diversity is associated with lower sibling competition.

Migratory birds encounter the Great Lakes while moving through eastern North America toward breeding grounds to the North, which offers a good opportunity to study variation in migratory behavior as birds face a potentially influencing topographical feature. Using passive infrared technology, we documented the direction of relatively low-flying, nocturnal, mostly passerine migration in spring along the southern coast of Lake Erie's western basin. We examined the extent to which spring migrants flew across Lake Erie as a continuation of the inland, northeasterly broad front migratory direction, as determined by weather radar and infrared observations, or displayed a tendency to deviate to more closely follow the direction of the coastline. We found that an estimated 62% of all low-flying migrants deviated their flight directions toward the coast of Lake Erie at 2 coastal sites, Cedar Point and Ottawa, which were characterized by northwest–southeast oriented coastlines. Migrants at a third coastal location, Maumee Bay, which has a more east–west oriented coastline, did not display similarly deviated flight directions. We found that even when winds were energetically favorable for a lake crossing, many migrants still routinely displayed deviated flight directions that approached paralleling the coastline. Further, the mean flight direction at one site, Ottawa, shifted more in the direction of the coastline as the night progressed, suggesting that time of night could influence the shift to a more coastal flight direction. The data indicate that the western basin of Lake Erie acts as a salient topographical feature influencing the flight directions of nocturnal migrants. The data further suggest that birds are making active decisions while in flight, based on current environmental and physiological conditions, about whether to continue to cross Lake Erie or take a coastal detour.

Whether Ca and other micronutrients are equally distributed in an avian eggshell over its longitudinal section and what portion of these local resources are utilized by developing embryos are unanswered questions in avian reproductive physiology. Here, we measured the thickness and concentrations of Ca and 16 other chemical elements (Al, As, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, Pb, Se, and Sr) in 4 shell regions (sharp pole, equator, shoulder, blunt pole) of White Stork (Ciconia ciconia) eggs representing different stages of embryonic development, from unresorbed eggshells to almost fully resorbed ones (with near-to-hatch embryos). We found that unresorbed eggshells displayed several significant differences in the concentrations of 15 elements (Al, As, Ca, Cd, Co, Cr, Fe, K, Hg, Mg, Mn, Na, Ni, Se, and Sr) between various regions of the same shell. Only 2 metals (As and Ca) showed a cross-sectional decrease in concentrations from the sharp pole to the blunt one. In particular, we observed that unresorbed eggshells at the blunt pole were less calcified (with 2.4% less Ca) compared to the sharp pole. In contrast, the concentrations of 6 other metals (Co, Fe, Mg, Mn, Na, and Se) peaked in the relatively less calcified region of the blunt pole, where concentrations (such as Se) were up to 10 times as high as in other regions of unresorbed eggshells. Our findings highlight that eggshells over their longitudinal sections are not chemically homogeneous. Surprisingly, however, and contrary to our expectations, we found that unresorbed eggshells were thinner (2.1–5.9% less depending on the region) and at the same time more strongly calcified than resorbed eggshells. This suggests that some modification has occurred in the shell structure and raises the question of thin-shelled eggs in populations of wild birds.

Life-history theory broadly predicts a fitness tradeoff between costs of raising offspring and parental survival. Waterfowl with precocial young face particularly high costs of egg production, incubation, and brood-rearing, but not all evidence supports a corresponding decline in survival. We used multi-state mark–recapture–recovery models to estimate annual probabilities of survival, reported mortality, and transition between 2 states for female Ross's Geese (Anser rossii) and Lesser Snow Geese (A. caerulescens caerulescens) that attempted nesting near Karrak Lake, Nunavut, Canada. States were possession of a brood patch (high likelihood of successful nesting, “B”) vs. no brood patch (attempted nesting but failed, “N”). Based on over 43,000 birds marked from 2000 to 2015, we found that females of each species with a brood patch had consistently higher probabilities of survival than those without, subsequent to capture in early August. Virtually all of the state differences in survival were due to nonhunting mortality. These patterns are consistent with the concept of variable individual quality impacting vital rates across multiple seasons. Higher survival of females with brood patches may be linked to greater breeding success but also to a hypothesized dominance advantage afforded to family groups of geese during winter. Moreover, although hunting pressure can play a key role in regulating Arctic goose populations, it does not appear to affect this relationship between inferred breeding state and survival. Instead, coincident with recent declines in harvest rate in these populations, higher individual quality of breeding females appears to outweigh the higher hunting vulnerability of presumed parents with young. The potential influence of social dominance in reducing natural winter mortality among families may thus contribute to the survival advantage seen in successful, relative to failed, breeders.

Several models have been proposed to explain the evolution of leks, both in terms of direct or indirect fitness benefits, and in survival. According to kin selection theory, male skewed reproductive success leads unsuccessful males to join successful relatives to increase their inclusive fitness, because their genes would be transmitted indirectly to the next generation. Wedge-tailed Sabrewing (Campylopterus curvipennis) is a hummingbird species whose males congregate at leks, in which spatially clustered males sing a particular song with marked differences among neighboring males (song neighborhoods). The maintenance of song neighborhoods presumably depends on juvenile newcomers copying the song type of their neighbors when they establish within a lek, and their acceptance could be more likely if a relative has already settled down in a territory, which in turn could offer fitness benefits explained by kin selection theory. To investigate the potential for kin selection in this species, we genotyped 126 hummingbirds at 10 microsatellite loci and estimated pairwise relatedness among males at 6 leks and in 4 song neighborhoods within 1 focal lek. Within leks, most males were unrelated and only a few were relatives. Moreover, even though relatedness within leks was higher than between leks, it was not higher than 0, which is likely due to isolation by distance. Our results do not support the idea of kin selection as an important force acting on the formation of leks in this species. Additionally, we found no evidence for kin clustering within song neighborhoods, suggesting that juveniles attempting to settle in a lek have to learn the song of the neighborhood (regardless of their kinship) to gain access to territories.

Plumage ornamentation is often considered a signal of fitness, condition, sex, or social status. This theory holds for species with structural UV color, which is influenced by a variety of factors such as environmental pressures during molt or heritability. However, little is known about the consistency of ornamentation and signaling across time in individuals with structural color. We compared juvenile and adult feathers in free-living Florida Scrub-Jays (Aphelocoma coerulescens) to measure UV color change within individuals across molts and to assess possible sources of change. We used multiple imputation to estimate missing data and a combination of pooled estimates and model averaging to infer which parameters explain observed variation in UV color. We also tested whether adult color or relative color change from juvenile to adult plumage predicted acquisition of breeding space. UV color was not consistent across annual molts, as adult feathers reflected significantly less light but greater proportions of UV light than juvenile feathers. Juvenile color was most affected by quality of natal environment and maternal effects whereas adult color was influenced by condition and juvenile color. Adults dosed with corticosterone produced feathers with less UV ornamentation compared to control adults. Feather color did not predict acquisition of breeding space in adults, but females that experienced reductions in UV color across molts were more likely to obtain breeding space, which may reflect sex-specific differences in reproductive strategies in Florida Scrub-Jays. Our evidence suggests that structural color acts as a signal of sex, age, and condition; but ornamentation is only weakly related to acquisition of breeding space and thus unlikely to be under strong sexual selection. Ornamentation may just be one factor among many, such as personality, social dominance, or position in social networks, that determine how jays interact and compete for breeding space.

Life-history theory predicts individuals should breed as soon as they are able to reproduce, but many long-lived birds delay breeding. In the Accipitriformes, delayed breeding is the norm, and age when breeding begins is influenced by competing selective pressures. In most Accipitriformes, the reproductive roles of males and females differ; males do most of the foraging and females tend eggs and young. Thus, sexual differences in age at first breeding might be expected, but these differences, possible causes, and implications for individual fitness have received little study. We investigated sexual differences in age at first breeding in a marked population of Cooper's Hawks (Accipiter cooperii) from 2011 through 2018 in central New Mexico, USA. We hypothesized that males required more experience to pair and breed successfully than females, and we predicted: (1) a lower mean age at first breeding for females than males, and (2) that expected individual fitness of early-breeding males would be lower than for early-breeding females. We found that 79% more females than males bred in their first year (hatching year, HY), and expected individual fitness of HY-breeding females was 21% greater than for HY-breeding males. HY males that attempted to breed settled on nesting territories with exceptionally high prey abundance, nevertheless they experienced 37% lower second-year survival than males that delayed breeding. Females competed for mates based on male age. HY females that paired with relatively older males had 33% higher second-year survival and 16% higher expected individual fitness than HY females that initially paired with relatively younger males. The observed annual rate of growth (λ) of our study population was 1.08, closer to λ predicted by male (1.02) than female (1.21) demographic models. Delayed breeding by males thus had important ramifications for λ, highlighting the need to consider sexual differences in age at first breeding in demographic analyses.

The diverse Old World avian family Ploceidae (weaverbirds) presently comprises 117 species in 17 genera. Despite being a well-known bird group, the family has received incomplete attention in terms of molecular systematics; systematists have often focused on subclades, with the most extensive study to date covering <66% of recognized species. As a consequence, weaverbird taxonomy remains outdated, and phylogenetic relationships, particularly of the African Malimbus (previously Ploceus and Malimbus) clade, remains largely unresolved. Here, we sampled 109 weaver species (and numerous nominal subspecies), including 99 of the 103 recognized “typical weaverbird” taxa for an 8-gene dataset. Antique DNA techniques were used to extract DNA from study skins of 27 rare taxa not available in global tissue collections. The study included 32 species and 4 genera of ploceids previously unstudied phylogenetically. Our analyses supported monophyly of the family and identified 8 distinct clades. Our results conflict extensively with current taxonomy. We suggest that plumage traits and morphology exhibit high plasticity, such that phenotype does not always reflect phylogenetic relationships in weaverbirds. We recommend (1) uniting African-Ploceus, Malimbus, Anaplectes, and Notiospiza in Malimbus; (2) retaining the monotypic genus Pachyphantes; and (3) placing Brachycope with Euplectes. This study, the first near-species-level phylogeny for the family, lays a firm foundation for downstream studies of biogeography and character evolution.

Avian vision is fundamentally different from human vision; however, even within birds there are substantial between-species differences in visual perception in terms of visual acuity, visual coverage, and color vision. However, there are not many species that have all these visual traits described, which can constrain our ability to study the evolution of visual systems in birds. To start addressing this gap, we characterized multiple traits of the visual system (visual coverage, visual acuity, centers of acute vision, and color vision) of the Red-winged Blackbird (Agelaius phoeniceus), one of the most abundant and studied birds in North America. We found that Red-winged Blackbirds have: wide visual coverage; one center of acute vision per eye (fovea) projecting fronto-laterally with high density of single and double cones, making it the center of both chromatic and achromatic vision; a wide binocular field that does not have the input of the centers of acute vision; and an ultraviolet sensitive visual system. With this information, we parameterized a Red-winged Blackbird-specific perceptual model considering different plumage patches. We found that the male red epaulet was chromatically conspicuous but with minimal achromatic signal, but the male yellow patch had a lower chromatic but a higher achromatic signal, which may be explained by the pigment composition of the feathers. However, the female epaulet was not visually conspicuous in both the chromatic and achromatic dimensions compared with other female feather patches. We discuss the implications of this visual system configuration relative to the foraging, antipredator, mate choice, and social behaviors of Red-winged Blackbirds. Our findings can be used for comparative studies as well as for making more species-specific predictions about different visual behaviors for future empirical testing.