Metabarcoding reveals selective dietary responses to environmental availability in the diet of a nocturnal, aerial insectivore, the European Nightjar (Caprimulgus europaeus)

Many bird species are vulnerable to environmental change, so knowledge of their diet and its variation can help understand population status and flexibility to respond to change. Insectivorous species are predicted to have a flexible diet within and between individuals, which can respond to naturally fluctuating prey abundance, thus allowing opportunistic exploitation of available resources. We analysed the diet of a nocturnal, aerial insectivore, the European Nightjar Caprimulgus europaeus, using high-throughput metabarcoding. We quantified diet diversity and composition of 130 faecal samples from nests and roosts on a northern breeding site in the UK from 2015 to 2018, and compared differences among individuals and years. Although dominated by moths, diet varied significantly between individual faecal samples and between years and months. Prey species composition varied between years, and was more variable between samples in 2017 than in other years. Faecal samples were significantly more likely to contain moth species with a wingspan of > 60 mm and less likely to contain moth species of < 25 mm wingspan. This indicated size-selective foraging, which also varied between months and years. Diet was driven by interindividual variation, indicating population-level flexibility in prey choice. Metabarcoding provided a valuable tool in the exploration of insectivorous diets, but efforts are needed to build comprehensive reference libraries in order to compile full prey species lists.