The effect of building ability and object availability on the construction of bower courts in great bowerbirds

Abstract

Animal-built structures that function in mate attraction can be constrained in signal expression by the availability of building materials in the environment and male ability to find them as well as the ability to construct the structure itself. As part of their courtship display, male great bowerbirds Ptilonorhynchus maculatus use hundreds of grey-white objects to create a bower court. Males arrange these objects to create a positive size-distance gradient, the evenness of which generates a forced perspective visual illusion that is associated with mating success. However, a range of differently sized objects are required to build a high-quality gradient, and it is unclear how local object availability and male ability to place objects in the correct location (i.e. building ability) affect gradient quality. To disentangle these effects, we provided males with a standardised set of court objects after removing their original court objects. If gradient quality is associated with male building ability, we predicted that the quality of the gradient constructed with the standardised set of objects would be positively correlated with the male’s original gradient quality. Contrary to our predictions, males that built high-quality gradients in their original court did not build high-quality gradients when given a standardised set of objects. We conclude that the bower court may not be a reliable signal of male building ability, and that object availability and/or male ability to locate appropriate objects explains some of the observed variation in gradient quality.