Diet is associated with age and habitat, not foraging range, in purple martins: implications for aerial insectivores as indicators

Many aerial insects are declining yet monitoring wildlife in airspace is challenging. Aerial insectivores, which are themselves a declining guild, may be useful indicators for aerial insects. However, their use as indicators may be complicated if they differentially sample prey depending on foraging range, as predicted by central place foraging theory. We measured diet composition of purple martins (Progne subis Linnaeus, 1758) in Quebec by DNA metabarcoding fecal samples collected from adults and nestlings and used GPS biologgers to measure foraging behaviour of the chick-rearing adults. Foraging range did not predict diet or diet diversity. The proportion of individuals with Diptera and Coleoptera detected in their diets was lower in developed and water habitats. Martins fed their nestlings soft-bodied insects (Diptera and Trichoptera) more often than they ate them themselves. As nestlings aged, they were fed more chitinous arthropods and less soft-bodied prey. As the proportion of different prey orders depended on habitat use and age, but not foraging distance, we argue that purple martin diet is a potential indicator of aerial insect community variation across space, but that interpretations may be complex as birds may sample different components of the aerial insect community during different life stages.