Birds produce a variety of vocalizations ranging in function from attracting a mate to alerting others of danger. One vocalization of Tachycineta bicolor (Tree Swallow), the chatter call, is reported to serve as both a mate-attraction call and a nest-defense call. We used playback techniques to investigate the nest-defense behavior of Tree Swallows to calls of Toxostoma rufum (Brown Thrasher; control) and Sialia sialis (Eastern Bluebird; nest competitor), and Tree Swallow shriek (alarm call) and chatter calls. We measured time spent foraging, remaining in the nest box, and on nest defense. Tree Swallows spent 80% of their time on nest defense in response to the chatter call. The chatter-call and shriek-call nest-defense responses were nearly identical. Our data show that the chatter call functions as an alarm call and elicits nest-defense behavior.