Does decoration theft lead to an honest relationship between male quality and signal size in great bowerbirds?

Male–male competition can contribute to honest sexual signalling. Because male bowerbirds steal each other's decorations, decoration theft could lead to an honest relationship between male quality and signal size. To test this hypothesis, I conducted an experiment in which I standardized decoration numbers at bowers of the great bowerbird, Ptilonorhynchus nuchalis, such that all males had equal numbers. I then monitored males for 11 weeks as they stole decorations from other males, defended their decorations against theft, and collected decorations from the environment. For two commonly stolen decoration types (red wire and green plastic), there was a correlation between the numbers at bowers before and after standardization. Males that had more red wires and green plastic pieces before standardization were less likely to have these decorations stolen from their bowers after standardization, although they were not more successful at stealing decorations. These males also collected more red wires and green plastic pieces from the environment, and collecting rate was unrelated to the numbers available near the bower. Males that were more successful at guarding their decorations had longer nuchal crests and had been bower owners for longer periods, although nearest-neighbour distance explained more of the variation in guarding ability than either male characteristic. These results suggest decoration numbers are an honest signal of the male's ability to guard his bower against theft and to locate decorations in the environment, although some variation in stealing behaviours may be related to the spatial arrangement of males rather than to male quality.