Morphological and Vocal Variation Across a Contact Zone between the Chickadees Parus Atricapillus and P. Carolinensis

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Abstract
A contact zone between Black-capped and Carolina chickadees (Parus atricapillus and P. carolinensis) exists in southwestern Missouri. It was less than 15 km wide and paralleled the interface between the relatively treeless Great Plains and the forested Ozark Plateau. Many birds in this zone were intermediate in morphology or vocalizations or both. Moreover, both morphological and vocal discriminant analysis scores of contact zone birds were unimodally distributed and there was no correlation between morphological discriminant scores of mated males and females in the contact zone, indicating little or no assortative mating. Playback experiments demonstrated that birds to the north or south of the contact zone responded aggressively only to their own song type, while contact zone birds responded to either song type. We believe that southwestern Missouri contact zone populations are derived from extensive hybridization between atricapillus and carolinensis.