Threat Reduction as a Cause of Differences in Bower Architecture, Bower Decoration and Male Display in Two Closely-Related Bowerbirds Chlamydera nuchalis and C. maculata

Abstract

Major structural differences in the bowers of different bowerbird species have long been recognised, but the evolutionary significance of these differences has never been determined. I compared the bowers and displays of two congeneric species, the Spotted Bowerbird Chlamydera maculata and the Great Bowerbird C. nuchalis, and used the Satin Bowerbird Ptilonorhynchus violaceus as an outgroup to define ancestral characters. Spotted Bowerbird bowers and display courts differ from the more typical bower of avenue-building species in having a wide bower avenue, see-through walls, dispersed decorations, an E-W (rather than N-S) orientation, walls that are nearly vertical, and straw instead of sticks in the bower walls. These differences are coordinated with changes in male courtship patterns and female orientation in the bower. In addition, male displays are extremely vigorous, with aggressive elements not observed in other species. I evaluate three hypotheses that could account for the unique aspects of Spotted Bowerbird bower structure and display. Of these, the threat reduction hypothesis best explains changes in both patterns of male display and in the bower. It proposes that increased separation between bowers has caused selection for vigorous male display as part of mate assessment. An effect of this change is that female visitors to bowers are threatened by the increased vigour in male display. Male Spotted Bowerbirds utilise structural changes in their bowers to allow females to view male display with reduced threat. A threat reduction hypothesis is consistent with a general hypothesis that bowers function to enhance female willingness to attend male display courts and observe male courtship.