DNA metabarcoding reveals dietary divergence among sympatric swallows and flycatchers

ABSTRACT Aerial insectivore (AI) populations have been in steep decline in North America since the 1970s, with swallows, swifts, and nightjars declining more rapidly than flycatchers. As AIs share a common diet of flying insects, reductions in insect abundance are likely one of the major factors driving population decline. Previous studies have shown major dietary differences between swallows and flycatchers; flycatchers have exhibited more diverse, generalist diets than swallows. However, no study has directly compared the diets of sympatric swallows and flycatchers using the same method of dietary analysis. To investigate these differences, we compared the diets of 6 AI species living in sympatry during the breeding season. We collected fecal samples from adult Riparia riparia (Bank Swallow), Hirundo rustica (Barn Swallow), Petrochelidon pyrrhonota (Cliff Swallow), Tachycineta bicolor (Tree Swallow), Empidonax alnorum (Alder Flycatcher), and E. minimus (Least Flycatcher). We used DNA metabarcoding to identify the taxonomic composition of invertebrates in the feces and compared the richness of genera by insect order, insect family, and dipteran family between all species. Through a Bray-Curtis distance-based redundancy analysis, we identified significant differences in dietary composition between bird species at all 3 levels; however, the greatest amount of dissimilarity is seen in the dipterans consumed. Empidonax alnorum, E. minimus, H. rustica, and T. bicolor had broader, more generalist diets than P. pyrrhonota and R. riparia. By comparing the diets of multiple species living in sympatry, our study improves our understanding of a possible cause of disproportionate population declines observed among AI species.