Many animals use signals to communicate their social status to conspecifics, and the social control hypothesis suggests that social interactions maintain the evolutionary stability of status signals: low-quality individuals signal at a low level to prevent high-quality individuals from “punishing” them. I examined whether the numbers of decorations at bowers are socially controlled in the great bowerbird (Ptilonorhynchus nuchalis). In two populations, I supplemented males with decorations to determine whether they (a) rejected supplemental decorations and (b) experienced increased bower destruction from rivals. In contrast to the social control hypothesis, males in both populations accepted most supplemental decorations. Though the mean destruction rate did not increase during supplementation in either population, one of the study populations (Townsville) exhibited a negative correlation between the numbers of decorations naturally displayed at bowers and the change in destruction rate during the experiment. Townsville males that naturally had few decorations at their bowers also had more decorations stolen by other males during supplementation than males that naturally had many decorations. These results suggest that the numbers of decorations at bowers are an honest signal of the male's ability to defend his display site from rivals in at least one population of the great bowerbird (Townsville), but they do not support the social control hypothesis because males at both sites failed to limit signal expression. I discuss how the external nature of bower decorations and their availability in the environment may influence the costs and benefits of decoration theft and social control.

Many animals use extended phenotypes to attract mates, but the availability of suitable resources in the environment can affect the size and form of these signals, with unknown consequences for honest signalling. In some populations of the great bowerbird, Ptilonorhynchus nuchalis, males arrange decorations by size, with smaller decorations placed closer to the bower entrance than larger decorations. This may create a more even background pattern from the female's viewpoint within the bower than if decorations were arranged randomly. Males show consistent, individual variation in the size-distance gradient, which could reflect variation among males in the cognitive skills needed to arrange decorations. We examined whether individual consistency in gradient characteristics is related to a male's skill at decoration arrangement or the types of decorations at bowers. We paired 18 males and switched bower decorations between pairs. We measured gradient characteristics before switching and 4 and 8 days after switching. Gradient characteristics after switching were related to those of the bower from which decorations were received, not to those of the male's own bower before switching. Gradient characteristics were also related to the types of decorations received, including bones and snail shells. These results suggest that variation among males in the size-distance gradient is explained by differences in the availability of decorations at bowers, not the cognitive skills required to arrange decorations. Although variation in gradient characteristics could indicate the male's ability to locate and transport particular decorations, it could also reflect local availability of objects, with no relationship to male quality.