Food and nutrient limitation can have negative effects on survival, fecundity, and lifetime fitness of individuals, which can ultimately limit populations. Changes in trophic dynamics and diet patterns, affected by anthropogenic environmental and landscape change, are poorly understood yet may play an important role in population regulation. We determined diets of Wood Thrushes (Hylocichla mustelina), a Neotropical migratory songbird species sensitive to urbanization, and explored how brood parasitism by Brown-headed Cowbirds (Molothrus ater) may be related to Wood Thrush nestling diets. Effects of brood parasitism on host nestling diets is an understudied stressor that may help explain observed population declines. We measured carbon (δ13C) and nitrogen (δ15N) stable isotopes of 7 invertebrate food sources (snails, spiders, isopods, earthworms, myriapods, insects, and caterpillars), blood plasma from adult male and female Wood Thrushes and from Wood Thrush nestlings in nests with and without Brown-headed Cowbird nestlings. Wood Thrush diet compositions were largely composed of high calcium (Ca) foods (51–62%, 95% highest density intervals [HDI]), including snails, isopods, and myriapods, as well as spiders (23–33%, 95% HDI). Caterpillars were the least common food item in Wood Thrush diets (0.01–3 %, 95% HDI). Wood Thrush nestling diets in nests without Brown-headed Cowbirds contained greater proportions of Ca-rich foods and spiders compared to the diet of nestlings in parasitized nests. Our data demonstrate that Wood Thrushes preferred Ca- and protein-rich foods, which may have important implications for adult survival and fecundity as well as nestling nutrition and development. Our results suggest that brood parasitism is related to host nestling diet, which could have potentially negative effects on developing nestlings through nutritional stress that may in turn affect survival, fecundity, and ultimately limit population growth.
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