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Climate change can adversely impact animals, especially those that cannot independently thermoregulate or avoid exposure to temperature variation. Altricial bird nestlings may be particularly vulnerable to temperature variation, with cold, hot, and variable temperatures leading to reduced nestling growth. At a broad scale, temperature effects on nestling growth vary across climatic zones, but how temperature effects vary with early-life developmental constraints imposed by the timing of thermoregulatory development, competition with siblings, and the amount of parental care has received less attention. We investigated whether the effects of temperature on the body mass of wild barn swallow (Hirundo rustica erythrogaster) nestlings (n = 113 nestlings, 31 nests) in Boulder County, CO depends on timing of exposure during development, relative size within the brood, or level of parental feeding. Lower minimum temperatures were associated with lower nestling mass in early but not late development (before but not after the putative development of thermoregulatory independence). Additionally, we found marginal evidence that the smallest nestling in the brood was more vulnerable to extreme and variable temperatures than other brood mates. Similarly, we found evidence that cold temperatures had stronger negative effects on nestlings in nests receiving low versus high levels of parent feeding. These findings indicate the existence of fine-scale heterogeneity in which the effects of temperature on nestling development are sensitive to metabolic constraints and early-life social environment, providing insight into the factors that may provide resilience to or exacerbate temperature effects on individual birds. 

Abstract Many socially monogamous bird species engage in multiple mating through extra-pair (EP) fertilizations, yet the role of female plumage in EP outcomes is under studied. Sexual selection involving EP fertilization is hypothesized to influence female trait evolution if heritable plumage variation informs EP mating decisions and EP mating decisions affect fitness. Here, we test for 2 key prerequisite patterns of sexual selection through EP fertilizations by asking: (1) is female reproductive success associated with EP fertilizations, and (2) are EP fertilizations associated with female plumage traits? Studying these 2 patterns is essential for better understanding how the costs and benefits of EP fertilizations should translate to sexual selection on female plumage. We provide a conceptual diagram to highlight how plumage traits of both members of a social pair can influence fertilizations within and outside their pair bond from the perspective of each sex. In our sample of 47 Hirundo rustica erythrogaster (North American Barn Swallow) social pairs, females who engaged in EP mating tended to fledge more offspring, and females with longer tails were more likely to mate with EP sires. Furthermore, female traits were more strongly associated with EP fertilizations than male traits for predicting both female and male EP outcomes. Finally, we found that female traits were not correlated with fecundity (total eggs laid) and found no association between fecundity and EP fertilizations. Thus, sexual selection on EP fertilizations may be more important than selection on fecundity for understanding female plumage variation in our study population. Taken together, we provide evidence for multiple conditions necessary for sexual selection on female plumage traits in H. r. erythrogaster that are likely also relevant for understanding female trait evolution in other socially monogamous birds. 

Abstract The food limitation hypothesis suggests that food scarcity is the main driver of direct negative effects on breeding success of urban animals. However, the mechanistic pathways by which phenological mismatches are created by the urban heat island effect, noise pollution, and artificial light at night (ALAN), which reduce food abundance and ultimately breeding success, are understudied. Additionally, these processes may have indirect positive effects on food abundance and breeding success. It is therefore critical to disentangle the relative importance of these contradictory effects of urbanization. Here, we implemented a multidimensional framework considering these multiple factors, using stable isotope analysis and structural equation modelling, to assess direct and indirect effects on food abundance, diet, and fledgling success in 53 nests of Hirundo rustica (Barn Swallow) across an urban landscape. We found that urbanization had a direct positive influence on soft-bodied insect abundance and reduced phenological mismatch, while increased ambient temperature was indirectly related to reduced phenological mismatch. We also found direct positive effects of soft-bodied insect diets on fledging success of H. rustica. The direct negative effects of laying date on fledging success were stronger compared to the indirect positive effects of laying date on fledging success. We observed no positive effects of ALAN on food abundance and direct negative effects on fledging success. Finally, our results indicate no substantial differences in fledging success across an urban gradient. This suggests that H. rustica can adapt to diverse urbanization levels by adjusting their laying dates and foraging on optimal prey types, thereby maintaining similar levels of fledging success across urban gradients. These results emphasize the importance of employing multidimensional frameworks considering multiple environmental factors to understand the complex effects of urbanization on urban trophic webs. Our study also provides insights on how to minimize the detrimental effects of urbanization on aerial insectivores. 

ABSTRACT Disentangling the drivers of genomic divergence during speciation is essential to our broader understanding of the generation of biological diversity. Genetic changes accumulate at variable rates across the genome as populations diverge, leading to heterogenous landscapes of genetic differentiation. The ‘islands of differentiation’ that characterise these landscapes harbour genetic signatures of the evolutionary processes that led to their formation, providing insight into the roles of these processes in adaptation and speciation. Here, we study swallows in the genusHirundoto investigate genomic landscapes of differentiation between species spanning a continuum of evolutionary divergence. Genomic differentiation spans a wide range of values (F_ST= 0.01–0.8) between species, with substantial heterogeneity in genome‐wide patterns. Genomic landscapes are strongly correlated among species (ρ= 0.46–0.99), both at shallow and deep evolutionary timescales, with broad evidence for the role of linked selection together with recombination rate in shaping genomic differentiation. Further dissection of genomic islands reveals patterns consistent with a model of ‘recurrent selection’, wherein differentiation increases due to selection in the same genomic regions in ancestral and descendant populations. Finally, we use measures of the site frequency spectrum to differentiate between alternative forms of selection, providing evidence that genetic hitchhiking due to positive selection has contributed substantially to genomic divergence. Our results demonstrate the pervasive role of recurrent linked selection in shaping genomic divergence despite a history of gene flow and underscore the importance of non‐neutral evolutionary processes in predictive frameworks for genomic divergence in speciation genomics studies. 

Abstract While female mate choice is well established, mutual choice may play a larger role in mate selection than currently recognized. Assortative mating is a common form of nonrandom mating in animals that can result from mutual choice. However, few studies address assortative patterns beyond the social pair, potentially overlooking assortativity in the mating pair and in the social environment that shapes reproductive decisions. We asked whether North American barn swallows (Hirundo rustica erythrogaster) breeding in a large colony form pairs, mate (through both within-pair and extra-pair fertilizations), and interact assortatively by ventral plumage color, wing length, and age. Social interactions were tracked using proximity loggers, which recorded close contact between tagged individuals when birds were mating and laying eggs. Barn swallows paired and mated assortatively by their ventral plumage color; however, the assortative patterns in mating pairs were not as strong as they were in social pairs. Barn swallows also interacted assortatively, associating more often with individuals of both sexes who had similar phenotypes relative to the other birds in the colony. Finally, older males and females with darker ventral plumage achieved the highest reproductive success. Investigation of assortative behavior beyond the level of the social pair provides a more complete understanding of mate choice and suggests a mechanism that may maintain the large variation in ventral plumage color in North American barn swallows. 

Abstract Extra-pair mating is common in avian species and can modulate the strength of sexual selection. Mate searching behavior of female birds may be an important predictor of mating opportunities and extra-pair mating, yet important knowledge is lacking as we have little data on fine-scale movement of females during the peak fertilization period. Accordingly, much is still unknown about whether and how female phenotypes contribute to extra-pair mating. Here, we examined how female space use and female plumage color are associated with extra-pair mating outcomes in wild barn swallows (Hirundo rustica erythrogaster). We tracked 10 females breeding in Colorado, USA with GPS backpack tags for two hours each morning during their fertile period following an experimental nest failure. We then used low-coverage whole-genome sequencing to determine offspring paternity and to quantify extra-pair mating in the removed clutch and the replacement clutch. Plumage and movement did not correlate with changes in paternity between successive clutches, but movement did correlate with paternity in the replacement clutch. Females that spent more time away from the nest had a higher proportion and number of extra-pair offspring in the clutch laid immediately after the tracking period. These results suggest that differences in female space use contribute to differences in extra-pair fertilizations. In contrast to the historic emphasis on male traits, our study highlights female movement behavior as an important variable associated with mating outcomes in natural populations. 

As populations diverge, genetic differences accumulate across the genome. Although genetic differentiation is well documented, evolutionary change in epigenetic modifications is less well understood. Here, we integrate genomic and methylomic data from 168 barn swallows (Hirundo rustica) to examine the relationship between genetic and epigenetic population divergence at the resolution of CpG islands and single CpG sites. Sampling across all six extant subspecies and two hybrid zones, we assess the roles of genetic ancestry and environment in shaping DNA methylation patterns. Genome-wide methylation differentiation (P_ST) closely mirrors genetic differentiation (F_ST), with secondary contributions from geographic and environmental variation. DNA methylation patterns track the genomic landscape of sequence divergence, especially in CpG islands of nonregulatory regions. Across hybrid zones, DNA methylation levels are associated with nearbycis-acting genetic variation and global genomic ancestry. Moreover, CpG sites exhibit linkage disequilibrium with nearby SNPs, extending over several thousand base pairs within a putative inversion. These results suggest that although environmental effects on genome-wide DNA methylation are present, epigenomic patterns are costructured by the evolutionary processes shaping genetic variation. 

Abstract Many animals breed colonially, often in dense clusters, representing a complex social environment with cognitive demands that could ultimately impact individual fitness. However, the effects of social breeding on the evolution of cognitive processes remain largely unknown. We tested the hypothesis that facultative colonial breeding influences attention and decision-making. Barn swallows (Hirundo rustica) breed in solitary pairs or in a range of colony sizes, up to dozens of pairs. We tested for selective attention to social information with playbacks of conspecific alarm calls and for decision-making with simulated predator intrusions, across a range of colony sizes from 1 to 33 pairs. We also evaluated the adaptive value of both processes by measuring seasonal reproductive success. Swallows breeding in larger colonies were more selective in their attention to social information. Birds breeding in larger colonies were also less risk averse, deciding to return more quickly to their nests after a predator approach paradigm. Finally, birds that showed higher selective attention hatched more eggs and birds that returned to their nests more quickly after a predator intrusion had more nestlings. Although we cannot fully attribute these fitness outcomes to the cognitive measures considered in this study, our results suggest that social breeding plays a role in adaptively shaping both the acquisition of social information and decision-making.