In 2 experiments, captive female cowbirds, both isolation reared and wild caught, were exposed during the breeding season to experimentally manipulated versions of isolate male song. Data show that a single song element, a brief note between the song phrases, played a pivotal role in eliciting the female's copulatory response to song. Data also demonstrate that relative amplitude variation and the fine structure within the interphrase unit affected the potency of a given song. Results suggest that the acoustic properties of this unit may account for the enhanced effectiveness of isolate song over normal cowbird song as a sexual releaser. (14 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1979 American Psychological Association.