Sexual Differences in Flight Calls and the Cue for Vocal Sex Recognition of Swinhoe's Storm-Petrels

Previous studies have shown that the Flight Calls of Swinhoe's Storm-Petrels (Oceanodroma monorhis) differ clearly between the sexes in their frequency component and that birds can discriminate the calls of either sex with few errors (Taoka et al. 1989b). But the cue for sex recognition has not yet been identified. In this study, we quantitatively analyzed the calls of both sexes for call length, number of syllables, and duration and interval of syllables. We found some sexual differences in rhythmic components, but mean ± standard deviation overlapped in all syllables. In order to make synthetic calls for playback experiments, frequency and rhythmic components were systematically manipulated. Birds recognized the synthetic calls having the same harmonic structure as female calls but those without harmonic structure were recognized as male calls regardless of whether the rhythm was that of the female or the male. Therefore, we concluded that harmonic structure is the cue for sex recognition. The mechanism of vocal sex recognition was compared with that of the closely related species, the Leach's Storm-Petrel (O. leucorhoa).