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Abstract In song-learning birds, vocalizations are species recognition signals and may act as premating reproductive barriers; for allopatric taxa, testing how the signals can influence the speciation processes is quite challenging. This study aims to understand genomic divergence and species recognition via songs in 2 allopatric taxa, eastern and western Nashville warblers (Leiothlypis ruficapilla ruficapilla vs. Leiothlypis ruficapilla ridgwayi). We performed playback experiments to assess their reciprocal behavioral responses, which suggests an asymmetric barrier: the eastern L. r. ruficapilla discriminates between the 2 songs, but the western L. r. ridgwayi does not. Using whole-genome sequencing, we also examined the extent of the taxa’s genomic divergence and estimated their demographic history. We identified dozens of highly differentiated genomic regions, as well as fluctuations in historical effective population sizes that indicate independent demographic trajectories during the Pleistocene. To contextualize the magnitude of divergence between L. ruficapilla subspecies, we applied the same genomic analyses to 2 additional eastern-western pairs of parulid warblers, Setophaga virens vs. Setophaga townsendi and Setophaga coronata coronata vs. Setophaga coronata auduboni, which have existing behavior studies but are not in strict allopatry. Our findings provide insights into the role of vocalizations in defining within-pair relationship and the important legacy of isolation during the Pleistocene. 

Abstract The role of diet in driving ecological differences across the radiation of parulid wood warblers has been a topic of substantial debate. The inferences made in Robert MacArthur’s original study of their niche differences relied heavily on observations from microhabitat partitioning. How these different foraging behaviors translated to distinct diets and, more specifically, individual prey items has been less clear. Here, we use fecal metabarcoding of 2 syntopic insectivorous warblers—Setophaga virens (black-throated Green Warbler) and Seiurus aurocapilla (Ovenbird)—to complement and expand previous work, and we address past limitations by densely sampling many birds at the same location over a short period of time. We found highly significant differences in diet composition using several multivariate measures of diversity. In an analysis of individual diet proportions of insect orders, S. aurocapilla consumed more beetles and flies (Coleoptera and Diptera), whereas S. virens consumed more “true bugs” (Hemiptera) and stoneflies (Plecoptera). At the arthropod species level, we found that both warblers readily consumed invasive spongy moths (Lymantria dispar), and we identified 9 other arthropod species that significantly differed between the warblers. Of those, for 3 spider taxa, we combined warbler diet information with observations from arthropod collections and showed that spiders, which were more likely to be encountered on the ground, were exclusively eaten by S. aurocapilla whereas those encountered in the canopy were more likely to be consumed by S. virens, fitting with the expected vertical foraging stratification of the warblers. We interpret these diet differences as likely due to these 2 warbler species “opportunistically” encountering different arthropod assemblages in distinct foraging strata as opposed to “preferentially” consuming different prey. Our research emphasizes the benefits of extending analyses to more distantly related taxa—beyond those considered by MacArthur—and suggests a need for similar fine-scale studies within genera to enhance our understanding of dietary dynamics. 

Vertebrates host complex microbiomes that impact their physiology. In many taxa, including colourful wood-warblers, gut microbiome similarity decreases with evolutionary distance. This may suggest that as host populations diverge, so do their microbiomes, because of either tight coevolutionary dynamics, or differential environmental influences, or both. Hybridization is common in wood-warblers, but the effects of evolutionary divergence on the microbiome during secondary contact are unclear. Here, we analyse gut microbiomes in two geographically disjunct hybrid zones between blue-winged warblers (Vermivora cyanoptera) and golden-winged warblers (Vermivora chrysoptera). We performed 16S faecal metabarcoding to identify species-specific bacteria and test the hypothesis that host admixture is associated with gut microbiome disruption. Species identity explained a small amount of variation between microbiomes in only one hybrid zone. Co-occurrence of species-specific bacteria was rare for admixed individuals, yet microbiome richness was similar among admixed and parental individuals. Unexpectedly, we found several bacteria that were more abundant among admixed individuals with a broader deposition of carotenoid-based plumage pigments. These bacteria are predicted to encode carotenoid biosynthesis genes, suggesting birds may take advantage of pigments produced by their gut microbiomes. Thus, host admixture may facilitate beneficial symbiotic interactions which contribute to plumage ornaments that function in sexual selection.