Stable isotope analysis and DNA metabarcoding reveals elevational shifts in diet of a montane breeding bird

Breeding insectivorous songbirds require access to high quality invertebrate prey to produce successful young. A lack of suitable prey (e.g., low nutritional quality or low invertebrate quantity) to fulfill energetic demands can negatively affect key fitness parameters such as survival and reproduction. In high elevation ecosystems (> 900 m), cooler and wetter climates can have a negative impact on invertebrate availability, which in turn can affect bird diets. Yet we lack empirical comparisons of how diet composition changes over elevation gradients. Here, we assessed the diet of Swainson’s Thrush (<em>Catharus ustulatus</em>) within the White Mountains, New Hampshire using stable isotope analysis and DNA metabarcoding to test for dietary shifts with elevation. We found that the proportion of detritivore arthropods in thrush diets increased with elevation, whereas the proportion of predatory arthropods and overall niche-width declined. Further, we show that high-elevation thrushes had diets that were different in composition, but similar in diversity to thrushes at low-elevation sites. Lepidoptera, Araneae, and Coleoptera were important diet items across all elevations, but increases in woodlice (Isopoda) and ghost spiders (Anyphaenidae) contributed to familial-level differences in diet composition at high-elevation sites. This research suggests most passerine birds may be consuming lower quality prey (e.g., millipedes) at high elevation sites, though these results warrant further research for other montane birds. Because of climate change, environmental contamination, and cascading impacts on the diet and nutrient availability for breeding montane birds, understanding diet composition changes along environmental gradients can provide information on nutrient availability for species that breed in harsh climatic conditions. Future work on invertebrate availability and nutritional composition, daily energy expenditure, dietary niche, and fitness consequences of diet shifts, would help contribute to conserving montane birds within these sensitive, high elevation ecosystems.