The focus of desire

Abstract

The females of bird species that show female choice of male traits are often presented with an impressive array. The plumage of some males show multiple colourful and showy aspects often accompanied by complex songs or dances. And amongst the bowerbirds, males further dazzle with their construction skills and a display of decorations and “painting” of the bower. So what is a discerning female to make of all this? What is that makes the difference and leads to a female mating with a male? New work by Timothy Robson and colleagues at the University of Queensland rported in the Royal Society’s Biology Letters (published online) has looked into this issue. They have looked at the mating choices of female satin bowerbirds in rainforest clearings adjoining the Bunya Mountains National Park in Queensland. The team trapped, colour-banded and measured a total of 561 bowerbirds and their strategy was then to video record a total of 21 different males’ bowers over three consecutive breeding seasons. The team monitored 14 bowers each year out of the 35-50 present in the area. They chose bowers most likely to be visited by females, based on previous correlates of male mating success because in this highly competitive business, many males’ bowers are rarely visited by females. The researchers assessed the quality of the bowers in terms of the density and thickness of the sticks used in its construction and also its symmetry. They also checked the decorations on display and grouped them into five categories: artificial and yellow, brown, blue and white natural. The behaviours recorded included: visits by females to the bowers; copulations; singing and dancing displays by the owner to females; `solitary’ displays by the owner when no other bird was in the camera’s field; bower painting by the owner, during which the male coated the inside of the avenue with a masticated mix of saliva and hoop pine leaves and stealing of decorations and/or destruction of the bower by rival males. What the researchers found was that the females appear to go through a two-stage process using different cues. Females preferred to take a peek at the bowers of larger males and those with the higher rates of solitary displays. The showy decorations appeared to influence visits to a lesser extent. The females showed some preference for visiting bowers with blue and white natural decorations and tended to avoid those displaying more natural yellow decorations. After controlling for female visitation rates, mating success of males was still related to size but the decorations now appear to play second fiddle to the male’s painting rate. The researchers noted that the females were often seen to nibble at the paint daubed on the bower sticks, which suggested it might provide a chemical signal, something highly unusual in birds. The authors believe that mate choice in satin bowerbirds is an ideal subject for the study of the evolution of multiple signals.