Re-assessing niche partitioning in MacArthur's Warblers: foraging behavior, morphology, and diet metabarcoding in a phylogenetic context

Due in large part to MacArthurs classic 1958 paper, wood-warblers (Parulidae) are ecological icons, textbook protagonists of a story of competition and niche partitioning. As the story goes, subtle differences in foraging behavior are the principal means by which these nearly morphologically indistinguishable species are able to co-occur and avoid extinction. Yet, MacArthurs study was in fact quite limited in scale, and he said little about the relevance of evolution to the study system. Here, we reassess MacArthurs conclusions across an expanded set of syntopic warbler species in a forest in northern New York. We combine morphometrics, quantitative foraging data, and fecal metabarcoding--a direct measure of warbler diet--to study competition and niche partitioning in an evolutionary framework. We find close and kinematically realistic relationships between morphology and foraging behavior, but little connection between warbler ecomorphology and the 2,882 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 that competition has shaped their behaviors over evolutionary time. MacArthur (1958) may have drawn a few incorrect inferences, but his overall conclusion that evolved differences in foraging position, driven by competition among close relatives, does indeed appear to be a key reason these warblers can occur in such close sympatry.