Signalers can vary their vocal behavior, depending on the presence or absence of conspecific group members, and on the composition of the group. Here we asked whether Carolina chickadee (Poecile carolinensis) signalers varied their vocal behavior, depending on whether they were in the presence of familiar or unfamiliar flockmates. We sorted 32 Carolina chickadees into 4 groups with 4 familiar birds each and 4 groups with 4 unfamiliar birds each and recorded their behavior in seminatural aviary settings. We presented the familiar and unfamiliar aviary groups with a variety of stimuli ranging in level of threat and assessed birds’ calling behavior in these contexts. Birds housed with familiar flockmates called sooner in threatening stimulus presentations compared with birds housed with unfamiliar flockmates. Call rates and note compositions of calls produced did not differ for the 2 types of flock. Upon release from the captive aviaries, birds from familiar flocks were also more likely to call in flight than were birds from unfamiliar aviaries. These findings suggest that chickadees vary their calling behavior as a function of experience with their surrounding social audience and provide insight into mechanisms of flock cohesion and relationship formation and maintenance within flocks.