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

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 (5'^C) and nitrogen (S'^N) 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 5'^N values and more variable ô' -^C 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 diflfered 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. Des isotopes stables identifient des changements dans le régime alimentaire associés à des malformations du bec chez Poecile atricapillusRÉSUMÉ-Un grand nombre de malformations du bec d'étiologie inconnue ont récemment été signalées chez Poecile atricapillus et d'autres espèces aviaires résidentes d'Alaska. Nous avons étudié l'association potentielle entre le régime alimentaire et les malformations du bec. Nous avons analysé des isotopes de carbone (S'^C) et d'azote (ô'^N) dans le sang total d'individus de P. atricapillus capturés à trois sites semi-urbains dans le centre-sud de l'Alaska. Pour l'analyse alimentaire, nous avons inclus des aliments naturels (arthropodes, graines et baies) et des items anthropiques généralement offerts aux mangeoires (graines de tournesol, beurre d'arachides et suif). Des échantillons sanguins d'individus avec des malformations du bec présentaient des valeurs de 5'^N plus faibles et des valeurs de ô^^C plus variables que les oiseaux ayant des becs normaux. Les valeurs isotopiques du sang ont également différé par localisation pour le carbone et l'azote mais nous n'avons pas détecté de différence dans les items alimentaires naturels des trois sites. Les contributions des items alimentaires ont différé entre les oiseaux avec et sans malformations du bec, un pattern qui reflétait vraisemblablement une fonction réduite du bec. Les oiseaux affectés ont généralement consommé moins d'arthropodes et de graines de tournesol et plus de beurre d'arachide, de graines et d...

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