DNA metabarcoding illuminates nestling diet of the at-risk Western Purple Martin ( Progne subis arboricola )

The Western Purple Martin (Progne subis arboricola) is an aerial insectivore of conservation concern throughout the Pacific Northwest in the continental USA. In contrast to the widely studied eastern subspecies (Progne subis subis), there is a paucity of information on Western Purple Martin life history and ecology. Aside from anecdotal observations, there is no information on prey use or diet of the Western Purple Martin. The Purple Martin belongs to a dietary guild—aerial insectivores—in which most member species are experiencing population declines. Knowledge of diet and environmental factors influencing prey is likely critical for informing conservation actions. Our objectives were to identify prey taxa and compare dietary composition between three ecosystems (coastal, freshwater, upland forest) used by breeding Western Purple Martins in western Oregon and southwest Washington. We collected and analyzed 164 fecal samples (ncoastal = 41, nfreshwater = 83, nuplandforest = 40) from Purple Martin nestlings in 2017. Fecal samples were collected upon removal from the nest. DNA metabarcoding, a minimally invasive method of identifying prey taxa, revealed that Diptera was the most frequently detected order in nestling diets in all ecosystem types. Larger flying insects like dragonflies (Odonata), bees, and wasps (Hymenoptera) were detected at higher frequency as nestlings grew in age and size. Western Purple Martins used a more taxonomically diverse array of arthropod prey items in freshwater ecosystems than in coastal and upland forest ecosystems. This may confer greater resistance to changes in prey availability for colonies in freshwater ecosystems compared with those in coastal and upland forest ecosystems. However, future studies on prey availability, prey use, and corresponding nestling survival rates are needed to identify the advantages and limitations of each of the three ecosystems as breeding habitat. Western Purple Martin population resiliency to environmental change will likely depend on conservation efforts to maintain food resources in all ecosystems.