Intra- and intersexual functions in the call of a non-passerine bird

摘要

The thin-billed prion Pachyptila belcheri is a long-lived, monogamous and nocturnal seabird breeding in burrows in dense colonies in the southern hemisphere. Like all other Procellariiformes, this species shows delayed sexual maturity, and pair formation takes several years of active courtship. Its mating behaviour nearly exclusively relies on vocal cues. Its call is strongly sexually dimorphic. In this paper, we examine intra- and intersexual functions of call, and test whether these functions vary according to (1) the status of the recipient birds (breeding versus non breeding), and (2) the stage of pair formation. We used specifically designed playback experiments as three successive stages of the pair formation are recognized (stage 1: male and female in flight, stage 2: male on ground and female in flight and stage 3. male and female an ground). The first set of playback experiments was performed outside breeding colonies (stage 1). We found that wandering non-breeders (i.e. young birds which have still to find a partner) were strongly attracted and caught in mist nets when playback calls of both sexes were used. Conversely, breeders significantly avoided playback. Playback of male calls caught slightly more non-breeding females than did female calls, the reverse also being true. However, capture rates (i.e. number of birds caught per time unit) did not differ according to the sex of the call played-back. Thus, at the earliest stage of pair formation, wandering non-breeders of both sexes are attracted by their species-specific call, regardless of the sex of the call. The second set of experiments was performed on the breeding colonies and was aimed at testing the response of birds in Right (stage 2). Playback of male calls resulted in a significant increase of the vocal activity of flying females as well as a decrease of flying males calling activity. Playback of female calls had no detectable effects. Thus, at this stage of the pair formation process, birds do take into account the sexual information that is encoded within the call, and react according to their sex. The third set of tests was addressed to birds within their burrows (stage 3. We found that breeding birds responded far more to the calls of their own sex, remaining silent to those of the apposite sex: birds took into account the sexual information, but in a reverse sense compared to stage 2. Conversely, non-breeders occupying burrows responded more often to birds of the opposite sex. Our results therefore showed that the same signal can be used in intra- and intersexual communication, depending on the status of the signaller (its sex) and of the recipient (its sex and breeding status).