Bower decorations attract females but provoke other male spotted bowerbirds: bower owners resolve this trade-off.

Elaborate secondary sexual traits offset the costs that they impose on their bearer by facilitating reproductive benefits, through increased success in intrasexual contests or increased attractiveness to choosy mates. Some traits enhance both strategies. Conversely, I show that spotted bowerbirds Chlamydera maculata may face a trade-off. The trait that best predicts their mating success, numbers of Solanum berries exhibited on a bower, also provokes increased intrasexual aggression in the form of bower destructions by neighbouring bower owners, which reduce the quality of the male's bower. At natural berry numbers, levels of mating success in the population are skewed, but levels of destruction do not vary with berry number. When berry numbers are artificially exaggerated, increased levels of destructions occur, but mating success does not increase. When offered excess berries, either to add to the bower or artificially placed on the bower, bower owners preferred to use numbers of berries related to the number that they displayed naturally. This decision is made without direct experience of the attendant changes in destruction or mating success. This indicates that bower owners may assess their own social standing in relation to their neighbours and modulate their display accordingly.