Dual Signaling during Mating in Brown-headed Cowbirds (Molothrus ater,Family Emberizidae/Icterinae)

Male brown-headed cowbirds (Molothrus ater) vocalize to females during pair formation, a period usually lasting several days. Males also vocalize to females in the seconds immediately prior to females' adopting copulatory postures. The two major classes of male vocalizations occurring during courtship and copulation are songs and flight whistles. Observations across the species' North American range suggest that the function of these two courtship vocalizations may differ geographically. Aviary observations of eastern and midwestern populations suggested, furthermore, that the precise timing of song and whistle used during copulation sequences differs, with flight whistles occurring most often after a copulatory posture but before the male mounts and the pair copulates. Such timing of the two signals suggested different proximate functions. Here, we report three experiments that addressed the communicative properties of the two signals in two midwestern populations. First, we tested females of the two populations in two playback experiments to determine copulatory responsiveness and discrimination of the two signals. We asked whether females of the two populations gave more copulatory responses to the playbacks of songs and flight whistles of males of their own population than to those of males of the other population, and whether females responded differently to songs than to whistles. In the third experiment, we observed courtship interactions among males and females from one population in a large aviary to assess the use of flight whistles in relation to courtship success. Females of both populations responded more frequently to playbacks of songs than to playbacks of flight whistles and showed reliably more responsiveness to local song variants. Thus, information in male song can be used by females to discriminate the local population. The aviary data revealed that the rate of flight whistling correlated strongly with male courtship success. Thus, the vocal antecedents to mating in midwestern cowbirds include close-range signaling to females followed by longer range signaling, perhaps to other males and to females other than the mate. Acoustic and behavioral differences between these two signals in diverse parts of the cowbirds' range suggest that the function of ‘speciestypical’ signals such as songs or whistles may not be fixed, a conclusion in keeping with the growing evidence of vocal and social mallcability in these brood parasitic birds.