The impact of urbanization on body size of Barn Swallows Hirundo rustica gutturalis

Urbanization implies a dramatic impact on ecosystems, which may lead to drastic phenotypic differences between urban and nonurban individuals. For instance, urbanization is associated with increased metabolic costs, which may constrain body size, but urbanization also leads to habitat fragmentation, which may favor increases in body mass when for instance it correlates with dispersal capacity. However, this apparent contradiction has rarely been studied. This is particularly evident in China where the urbanization process is currently occurring at an unprecedented scale. Moreover, no study has addressed this issue across large geographical areas encompassing locations in different climates. In this regard, Barn Swallows (Hirundo rustica) are a suitable model to study the impact of urbanization on wild animals because they are a widely distributed species tightly associated with humans. Here, we collected body mass and wing length data for 359 breeding individuals of Barn Swallow (H. r. gutturalis) from 128 sites showing different levels of urbanization around the whole China. Using a set of linear mixed-effects models, we assessed how urbanization and geography influenced body size measured using body mass, wing length, and their regression residuals. Interestingly, we found that the impact of urbanization was sex-dependent, negatively affecting males’ body mass, its regression residuals, and females’ wing length. We also found that northern and western individuals were larger, regarding both body mass and wing length, than southern and eastern individuals. Females were heavier than males, yet males had slightly longer wings than females. Overall, our results showed that body mass of males was particularly sensitive trait to urbanization, latitude, and longitude, while it only showed a weak response to latitude in females. Conversely, while wing length showed a similar geographical pattern, it was only affected by urbanization in the case of females. Further research is needed to determine whether these phenotypic differences are associated with negative effects of urbanization or potential selective advantages

Genomic approaches to understanding population divergence and speciation in birds

© 2016 American Ornithologists\u27 Union. The widespread application of high-throughput sequencing in studying evolutionary processes and patterns of diversification has led to many important discoveries. However, the barriers to utilizing these technologies and interpreting the resulting data can be daunting for first-time users. We provide an overview and a brief primer of relevant methods (e.g., whole-genome sequencing, reduced-representation sequencing, sequence-capture methods, and RNA sequencing), as well as important steps in the analysis pipelines (e.g., loci clustering, variant calling, whole-genome and transcriptome assembly). We also review a number of applications in which researchers have used these technologies to address questions related to avian systems. We highlight how genomic tools are advancing research by discussing their contributions to 3 important facets of avian evolutionary history. We focus on (1) general inferences about biogeography and biogeographic history, (2) patterns of gene flow and isolation upon secondary contact and hybridization, and (3) quantifying levels of genomic divergence between closely related taxa. We find that in many cases, high-throughput sequencing data confirms previous work from traditional molecular markers, although there are examples in which genome-wide genetic markers provide a different biological interpretation. We also discuss how these new data allow researchers to address entirely novel questions, and conclude by outlining a number of intellectual and methodological challenges as the genomics era moves forward

Data from: Male competition drives song divergence along an ecological gradient in an avian ring species

Sexual selection operates via female choice and male competition, which can act independently, in concert, or in opposition. Female choice is typically considered the stronger selective force, but how these two processes interact to shape phenotypic divergence is poorly understood. I tested the hypothesis that variation in sexual selection in different habitats drives song divergence in the greenish warbler ring species. I evaluated the strength, direction, and targets of female choice and male competition in three populations spanning 2400km of latitude. Average song length increased with latitude, concomitant with a decline in population density. Within populations, males sang longer songs when females were fertile and shorter songs during territory establishment. Females consistently preferred males with longer songs and larger song repertoires. By contrast, playback experiments showed that males used short songs in territory defense. Songs were shortest at high densities, and in the highest density population only, song traits preferred by females correlated with male territoriality. Stronger male competition at high population densities likely constrains maximum song length, whereas weaker competition at low densities allows expression of female choice for long songs. Interactions between male competition and ecology may be a crucial but oft-overlooked component of phenotypic divergence and speciation

Data from: Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry

Hybrid zones are geographic regions where isolating barriers between divergent populations are challenged by admixture. Identifying factors that facilitate or inhibit hybridization in sympatry can illuminate the processes that maintain those reproductive barriers. We analyzed patterns of hybridization and phenotypic variation across two newly-discovered hybrid zones between three subspecies of barn swallow (Hirundo rustica). These subspecies differ in ventral coloration and wing length, traits that are targets of sexual and natural selection, respectively, and are associated with genome-wide differentiation in allopatry. We tested the hypothesis that the degree of divergence in these traits is associated with the extent of hybridization in secondary contact. We applied measures of population structure based on > 23,000 SNPs to confirm that named subspecies correspond to distinct genomic clusters, and assessed coincidence between geographic clines for ancestry and phenotype. Although gene flow was ongoing across both hybrid zones and pairwise FST between subspecies was extremely low, we found striking differences in the extent of hybridization. In the more phenotypically differentiated subspecies pair, clines for ancestry, wing length, and ventral coloration were steep and coincident, suggestive of strong isolation and, potentially, selection associated with phenotype. In the less phenotypically differentiated pair, gene flow and phenotypic variation occured over a wide geographic span, indicative of weaker isolation. Traits associated with genome-wide differentiation in allopatry may thus also contribute to isolation in sympatry. We discuss potentially important additional roles for evolutionary history and ecology in shaping variation in the extent hybridization between closely related pairs of subspecies

Data from: Effects of assortative mate choice on the genomic and morphological structure of a hybrid zone between two bird subspecies

Phenotypic differentiation plays an important role in the formation and maintenance of reproductive barriers. In some cases, variation in a few key aspects of phenotype can promote and maintain divergence; hence the identification of these traits and their associations with patterns of genomic divergence are crucial for understanding the patterns and processes of population differentiation. We studied hybridization between the alba and personata subspecies of the white wagtail (Motacilla alba), and quantified divergence and introgression of multiple morphological traits and 19,437 SNP loci on a 3000 km transect. Our goal was to identify traits that may contribute to reproductive barriers and to assess how variation in these traits corresponds to patterns of genome-wide divergence. Variation in only one trait – head plumage patterning – was consistent with reproductive isolation. Transitions in head plumage were steep and occurred over otherwise morphologically and genetically homogeneous populations, whereas cline centers for other traits and genomic ancestry were displaced over one hundred kilometers from the head cline. Field observational data show that social pairs mated assortatively by head plumage, suggesting that these phenotypes are maintained by divergent mating preferences. In contrast, variation in all other traits and genetic markers could be explained by neutral diffusion, although weak ecological selection cannot be ruled out. Our results emphasize that assortative mating may maintain phenotypic differences independent of other processes shaping genome-wide variation, consistent with other recent findings that raise questions about the relative importance of mate choice, ecological selection and selectively neutral processes for divergent evolution

A migratory divide spanning two continents is associated with genomic and ecological divergence

Migratory divides are contact zones between breeding populations with divergent migratory strategies during the nonbreeding season. These locations provide an opportunity to evaluate the role of seasonal migration in the maintenance of reproductive isolation, particularly the relationship between population structure and features associated with distinct migratory strategies. We combine light-level geolocators, genomic sequencing, and stable isotopes to investigate the timing of migration and migratory routes of individuals breeding on either side of a migratory divide coinciding with genomic differentiation across a hybrid zone between barn swallow (Hirundo rustica) subspecies in China. Individuals west of the hybrid zone, with H. r. rustica ancestry, had comparatively enriched stable-carbon and hydrogen isotope values and overwintered in eastern Africa, whereas birds east of the hybrid zone, with H. r. gutturalis ancestry, had depleted isotope values and migrated to southern India. The two subspecies took divergent migratory routes around the high-altitude Karakoram Range and arrived on the breeding grounds over 3 weeks apart. These results indicate that assortative mating by timing of arrival and/or selection against hybrids with intermediate migratory traits may maintain reproductive isolation between the subspecies, and that inhospitable geographic features may have contributed to the diversification of Asian avifauna by influencing migratory patterns