Stable isotope analysis reveals shifts in diet of a breeding montane bird

<p>Insectivorous breeding birds require access to high quality prey to produce a successful nest. A lack of suitable prey (e.g., low nutritional quality or low invertebrate availability) that fulfill energetic demands can negatively affect nestling growth and survival. In high elevation ecosystems (>900 m), cooler and wetter climates can have negative influences on invertebrate availability which in turn can affect bird diets. Yet we lack studies of how diet composition changes over elevation gradients. Here, we assessed the diet of Swainson’s Thrush (<em>Catharus ustulaus</em>) within the White Mountains, New Hampshire using stable isotope analysis and DNA metabarcoding. We found that the proportion of detritivore arthropods in thrush diets increased with elevation, while 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 and ghost spiders contributed to familial-level differences in diet composition at high-elevation sites. This research suggests montane breeding birds may be consuming low quality prey (i.e., millipedes) at high elevation sites, due to either availability or preference. With considerations due to climate change, environmental contamination, and residual 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, and dietary niche would help contribute to important context to conserving montane birds within these sensitive, high elevation systems. </p>