Zoology - Theses

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    The ecology and song of the duetting Eastern whipbird (Phosphodes olivaceus)
    Rogers, Amy Catherine. (University of Melbourne, 2005)
    In this thesis I investigate the function of antiphonal duets, using as my model system the Australian eastern whipbird, Psophodes olivaceus. Fieldwork was carried out between 2000- 2003 on a colour-banded population of eastern whipbirds at Mimosa Rocks National Park, near Tathra, NSW. Eastern whipbirds form stable, long-term, socially monogamous pair bonds with a low rate of pair divorce. Males and females show sex role convergence in parental care and territory defence. While only females build the nest and incubate, both sexes feed offspring at the nest, with males feeding nestlings at a higher rate than females. On fledgling the brood of two is divided and each parent cares exclusively for one fledging during an extended period of post-fledging care. Males and females together defend an exclusive territory throughout the year. Competition for territories at this site is intense, due to a high rate of adult survival, limited dispersal by offspring and an extremely low rate of territory turnaround, despite the presence of numerous unpaired adult 'floaters'. Intra-sexual competition between females appears to be particularly strong given that sex ratios from nestling through to unpaired adult stages are female-biased. Eastern whipbird males and females coordinate their songs to form precise duets that are initiated exclusively by the male. Males and females each possess a repertoire of different song types, which are combined non-randomly to form specific duet types. Females produce two types of song: response songs, which are mainly used in the context of a duet, and 'type n' songs which are used only as solo songs. Both sexes share most of their songs with same-sex neighbours and the repertoire of song types appears to play an important role in allowing song type-matching during territorial interactions with intruders. Playback experiments suggest that the sexes differ in the extent to which they are influenced by the song types of rivals and partners during interactions. Males matched most of their songs with the song type broadcast by the speaker during simulated intrusion by male rivals. Females, in contrast, always fitted the song types of their partners rather than matching the songs of female rivals. By replying to their partner, females may be signalling their commitment to the pair bond, or demonstrating their own, or their partners, paired status to female rivals. Further playback experiments confirmed that females showed a more aggressive response to simulated intrusion by female rivals than male rivals, and answered a higher proportion of their partner's songs to form duets in the presence of a same-sex rival. Female eastern whipbirds therefore appear to reply to their partners in order to defend their exclusive position within the partnership. This behaviour appears to be the consequence of high intra-sexual competition between females for mates, coupled with high benefits associated with maintaining exclusive access to male care. The high level of intra-specific competition between females also seems to have led to the development of a novel vocal strategy in the eastern whipbird. Females not only replied to their partners but also to male intruders to form precise extra-pair duets which were similar in structure to pair duets. Simulated intrusions suggested that the rate of extra-pair duets produced was highest in the presence of a female rival. Thus, females may respond to males other than their partners as a means of type-matching and overlapping the songs of female rivals.