Acoustic effects complement visual displays of Great Bowerbird bowers

Abstract

Sexual selection can result in extreme development of multimodal mate-attracting traits, including complex constructions. Male great bowerbirds build bowers for attracting females. Bowers contain a thatched twig tunnel (avenue) opening onto two courts covered with decorations. Males displaying on a court are seen by a female from within the avenue. She sees and hears displays through the avenue entrance but can only see the male's head and objects in his bill as it passes repeatedly across the entrance. Because the bower may affect the auditory as well as the visual parts of the multimodal male display we investigated bower acoustic properties by playing standard sounds from multiple court positions, recording the resulting sounds at the female's head position within the avenue. Bower geometry results in a limited zone at the avenue entrance where his vocalisations can be heard with maximum intensity; this corresponds to his typical display position. Experiments show that court decorations increase the intensity of some frequencies and reduce the intensity of others. Bower structure simultaneously affects both visual and auditory male display components and could be important in sexual selection. It is important to consider more than one sensory mode, especially in the context of built signalling structures.