Stable isotopes identify dietary changes associated with beak deformities in Black-capped Chickadees (Poecile atricapillus)

Abstract

ABSTRACT. A large number of beak deformities of unknown etiology have recently been reported in Black-capped Chickadees (Poecile atricapillus) and other resident avian species in Alaska. We investigated the potential association between diet and beak deformities. We analyzed carbon (δ13C) and nitrogen (δ15N) isotopes in whole blood of Black-capped Chickadees captured at three semiurban sites in south-central Alaska. For dietary analysis, we included natural foods (arthropods, seeds, and berries) and anthropogenic items commonly provided in bird feeders (sunflower seeds, peanut butter, and suet). Blood samples from individuals with beak deformities exhibited lower δ15N values and more variable δ13C values than birds with normal beaks. Isotopic values of blood also differed by location for both carbon and nitrogen, but we did not detect a difference in natural dietary items across the three sites. Contributions of individual diet items differed between birds with and without beak deformities, a pattern that...