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 likely reflected reduced function of the beak. Affected birds generally consumed fewer arthropods and sunflower seeds and more peanut butter and natural seeds and berries. Although some individuals with beak deformities relied heavily on feeder foods, we did not find evidence of an anthropogenic food source shared by all affected birds. In addition, dietary differences were most pronounced for moderately to severely affected birds, which suggests that these differences are more likely to be a consequence than a cause of deformities.