The vocal repertoire of male Brown-headed Cowbirds (Molothrus ater) during the breeding season is rich and varied, and includes several different vocalizations. The best-known cowbird vocalization, the species-typical song, consists of several low-frequency introductory notes followed by a high-frequency whistle (Friedmann 1929). A second vocalization heard throughout the breeding season is the flight whistle (FW). Cowbird FWs consist of two or more syllables that are largely pure tones, although some FWs may contain extensive frequency sweeps (Rothstein and Fleischer 1987, Rothstein et al. 1988). For example, the FW given by male cowbirds in our study population consists of two parts (Fig. 1). The first half is a single syllable of relatively pure tone, although it has rapid frequency sweeps at its beginning and end. The second half of the FW contains two syllables that always accompany each other in the order shown in Figure 1. The first of these syllables is brief and has an overall rise in frequency, while the second begins with a gradual frequency descent that ends as a pure tone over the last half of the syllable.