Why do bowerbirds build bowers?

Male Bowerbirds of Australia and New Guinea clear and decorate courts and build bowers at display sites where they mate. Bowerbird species, however, differ in several characteristics, including the type and color of court decorations and the form of a bower, if one is even built. Moreover, some male bowerbirds possess bright crest and body plumages, and others do not. Charles Darwin's observations of satin bowerbirds--in the Blue Mountains of Australia during his round-the world journey on the HMS Beagle--contributed to the then-controversial central element of his theory of sexual selection called female choice. The highly sculptured structure of a bower and a male's use of brightly colored decorations suggested to Darwin that female bowerbirds might shop for the most attractive bower, thereby directing the evolution of these display traits.

Nevertheless, several other mechanisms could have driven the evolution of bowers. The so-called good-genes model, for instance, suggests that male-display traits, including bowers, might indicate a male's vigor and, ultimately, his quality as a sire. That is, more vigorous males might have better bowers. A bower could even directly benefit a female, perhaps protecting her from threats, including predators that might attack her during mating or males that might try to force her to copulate. Bower building could even arise from an arbitrary or pre-existing female preference, such as an attraction to nest-like structures.

I have used Darwin's method of comparisons of related species to reconstruct the evolution of bower building. My work on several species of bowerbirds confirms the existence of female preferences for males with well-built and highly decorated bowers. The origins of bower building, however, can be best explained as a trait that attracts females because of the protection it provides them from forced copulation by bower owners.

Evaluating Bower -Building Hypotheses

Picking one model of bower-building evolution over another proves difficult because of several problems. One cannot always reconstruct what happened long ago, especially for display behavior that leaves no fossil record. Moreover, bower building may have evolved over a period of time, and different stages of its evolution may have served different functions. Although experiments can show the plausibility of a particular evolutionary process, understanding the origins of traits can best be accomplished by careful comparisons between species whose relationships...