Stealing rates in the Great Bowerbird (Ptilonorhynchus nuchalis): effects of the spatial arrangement of males and availability of decorations

Abstract

Competitive interactions between males can affect mate-choice decisions of females, so it is important to understand the factors that underlie variation in the frequency and intensity of male-male interactions. In bowerbirds (Ptilonorhynchidae), two hypotheses have been proposed to explain within-and between-species variation in the rate at which males steal each other's decorations. Males may steal more frequently as inter-bower distances decrease because this reduces the time and energy costs of travelling between bowers, or they may steal more frequently when bowers contain relatively few decorations, compared with bowers in other populations or species, because this leads to an increase in the value of decorations to males. I compared stealing rates in two populations of Great Bowerbirds (Ptilonorhynchus nuchalis) in Queensland, Australia, in order to assess these hypotheses. Males at one site, Dreghorn, had fewer decorations, higher nearest-neighbour distances (NND), and fewer males within a 1-km radius of their bowers (termed bower density) than males at the Townsville site. Dreghorn males stole decorations at a lower rate and interacted with fewer individuals, though there was no difference between sites in the rate at which males stole decorations from their nearest neighbours. Within sites, stealing rates were not related to decoration numbers, and partial correlations revealed that stealing rates were correlated with bower density, not NND. These results suggest that differences in the spatial arrangement of males, particularly bower density, may explain variation in stealing rates both within and between populations, though alternative explanations, such as differences in resource availability, are also relevant.