Habitat-linked genetic variation supports microgeographic adaptive divergence in an island-endemic bird species.

We investigated the potential mechanisms driving habitat-linked genetic divergence within a bird species endemic to a single 250-km2 island. The island scrub-jay (Aphelocoma insularis) exhibits microgeographic divergence in bill morphology across pine-oak ecotones on Santa Cruz Island, California (USA), similar to adaptive differences described in mainland congeners over much larger geographic scales. To test whether individuals exhibit genetic differentiation related to habitat type and divergence in bill length, we genotyped over 3000 single nucleotide polymorphisms in 123 adult island scrub-jay males from across Santa Cruz Island using restriction site-associated DNA sequencing. Neutral landscape genomic analyses revealed that genome-wide genetic differentiation was primarily related to geographic distance and differences in habitat composition. We also found 168 putatively adaptive loci associated with habitat type using multivariate redundancy analysis while controlling for spatial effects. Finally, two genome-wide association analyses revealed a polygenic basis to variation in bill length with multiple loci detected in or near genes known to affect bill morphology in other birds. Our findings support the hypothesis that divergent selection at microgeographic scales can cause adaptive divergence in the presence of ongoing gene flow.

Adaptive divergence in bill morphology and other thermoregulatory traits is facilitated by restricted gene flow in song sparrows on the California Channel Islands.

Disentangling the effects of neutral and adaptive processes in maintaining phenotypic variation across environmental gradients is challenging in natural populations. Song sparrows (Melospiza melodia) on the California Channel Islands occupy a pronounced east-west climate gradient within a small spatial scale, providing a unique opportunity to examine the interaction of genetic isolation (reduced gene flow) and the environment (selection) in driving variation. We used reduced representation genomic libraries to infer the role of neutral processes (drift and restricted gene flow) and divergent selection in driving variation in thermoregulatory traits with an emphasis on the mechanisms that maintain bill divergence among islands. Analyses of 22,029 neutral SNPs confirm distinct population structure by island with restricted gene flow and relatively large effective population sizes, suggesting bill differences are probably not a product of genetic drift. Instead, we found strong support for local adaptation using 3294 SNPs in differentiation-based and environmental association analyses coupled with genome-wide association tests. Specifically, we identified several putatively adaptive and candidate loci in or near genes involved in bill development pathways (e.g., BMP, CaM, Wnt), confirming the highly complex and polygenic architecture underlying bill morphology. Furthermore, we found divergence in genes associated with other thermoregulatory traits (i.e., feather structure, plumage colour, and physiology). Collectively, these results suggest strong divergent selection across an island archipelago results in genomic changes in a suite of traits associated with climate adaptation over small spatial scales. Future research should move beyond studying univariate traits to better understand multidimensional responses to complex environmental conditions.

Empirical tests of habitat selection theory reveal that conspecific density and patch quality, but not habitat amount, drive long-distance immigration in a wild bird.

Individuals that disperse long distances from their natal site must select breeding patches with no prior knowledge of patch suitability. Despite decades of theoretical studies examining which cues dispersing individuals should use to select breeding patches, few empirical studies have tested the predictions of these theories at spatial scales relevant to long-distance dispersal in wild animal populations. Here, we use a novel assignment model based on multiple intrinsic markers to quantify natal dispersal distances of Wood Thrush (Hylocichla mustelina) breeding in forest fragments. We show that long-distance natal dispersal in this species is more frequent than commonly assumed for songbirds and that habitat selection by these individuals is driven by density-dependence and patch quality but not the amount of habitat surrounding breeding patches. These results represent an important contribution to understanding habitat selection by dispersing individuals, especially with regards to long-distance dispersal.

Seasonal and population variation in male testosterone levels in breeding orange-crowned warblers (Vermivora celata).

Comparative hormone studies can reveal how physiology underlies life history variation. Here, we examined seasonal variation in plasma testosterone concentration between populations of male orange-crowned warblers (Vermivora celata) breeding in Fairbanks, Alaska (V. c. celata) and on Santa Catalina Island, California (V. c. sordida). These populations face different ecological constraints and exhibit different life histories. Alaska birds have a short breeding season, low annual adult survival, and high reproductive rates. In contrast, Catalina Island birds exhibit high adult survival and low reproductive rates despite having a long breeding season. We examined seasonal variation in male testosterone concentrations as a potential mechanism underlying differences in male reproductive strategies between populations. From 2006 to 2008, we sampled males during the pre-incubation, incubation, and nestling stages. Alaska males exhibited a seasonal testosterone pattern typical of northern passerines: testosterone levels were high during pre-incubation and declined during incubation to low levels during nestling provisioning. Testosterone concentrations in Catalina Island males, however, did not vary consistently with breeding stage, remained elevated throughout the breeding season, and were higher than in Alaska males during the nestling stage. We hypothesize that in Alaska, where short seasons and high adult mortality limit breeding opportunities, the seasonal testosterone pattern facilitates high mating effort prior to incubation, but high parental investment during the nestling stage. On Catalina Island, elevated testosterone levels may reflect the extended mating opportunities and high population density facing males in this population. Our results suggest that population variation in seasonal testosterone patterns in orange-crowned warblers may be a function of differences in life history strategy and the social environment.