Aggressive response of Chickadees towards Black-capped and Carolina chickadee calls in central Illinois

Aggressive responses of Black-capped (Poecile atricapillus) and Carolina ch ickadees (Poecile carolinensis) to heterospecific and conspecific vocalizat ion playbacks were measured across a historic contact zone in east-central Illinois to determine the magnitude of interspecific aggression. Within the traditional Carolina Chickadee range, chickadees responded more aggressive ly towards Carolina Chickadee calls than Black-capped Chickadee calls. With in the traditional Black-capped Chickadee range, chickadees did not respond to either vocalization significantly more than the other. The aggressive r esponse towards presumed heterospecific vocalizations for all chickadees wa s marginally more aggressive closer to the contact rone. Thus, we conclude that interspecific aggression may not act as a gap producing mechanism betw een chickadee ranges.