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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. 

Owing in large part to Robert MacArthur’s classic research, wood warblers in the family Parulidae are textbook exemplars of species competition and niche partitioning. Conventional wisdom suggests that subtle differences in foraging behaviour are the principal means by which these nearly morphologically indistinguishable species are able to co-occur and avoid extinction. Yet, MacArthur’s study was in fact quite limited in scale, and he said little about the relevance of evolution to the study system. Here, we reassess MacArthur’s conclusions across an expanded set of syntopic warbler species in a forest in northern New York. We combine morphometrics, quantitative foraging data and faecal metabarcoding—a direct measure of warbler diet—to study competition and niche partitioning in an evolutionary framework. We find close relationships between morphology and foraging behaviour, but little connection between warbler ecomorphology and the 2237 invertebrate taxa detected in their diets. Instead, diet remains phylogenetically conserved—closely related warblers eat similar suites of invertebrates, regardless of where they forage. Finally, we present evidence that these species not only partition niche space in the present day but also that competition has shaped their behaviours over evolutionary time. 

The dynamics of hybridization between golden-winged (Vermivora chrysoptera) and blue-winged warblers (V. cyanoptera) has been of interest for over a century. Whole genome analysis found only a small number of genomic regions that differed between the species. We previously developed a restriction enzyme-based RFLP approach to genotype large numbers of individuals at each of these loci. Here we extend this approach to an amplicon sequencing method to genotype individuals at these six plumage-associated regions. We demonstrate the efficacy using preliminary data from 4 golden-winged and 4 blue-winged warblers as well as provide the data and scripts necessary to analyze these data for other interested in replicating this approach. Our hope is that these data are useful for other researchers interested in genotyping Vermivora warblers.