Stealing from the Neighbors

by Irby Lovette

In their native eastern Australia habitat, male Satin Bowerbirds use grass and twigs to construct elaborate structures that could be mistaken for nests, but which are actually display courts for attracting females. Male bowerbirds decorate their display bowers with colorful objects, employing both natural materials such as feathers and man-made objects like clothespins. Bright blue objects are their favorite decorations, and a ring of blue detritus surrounds the best-tended bowers.

An extensive series of studies has shown that female bowerbirds are attracted to the males who maintain the most elaborately decorated bowers — the more blue objects, the sexier the females find the male. Further investigation has shown that the extent of a male’s bower decorating might provide females with a forthright gauge of his overall quality: by choosing to mate with males that have highly decorated bowers, females obtain the best-quality genes for their offspring. Since male bowerbirds do not contribute at all to raising those offspring, it makes sense for female bowerbirds to choose among mates based on differences in overall genetic quality, as measured in this case by the male’s ability to gather and retain bower decorations.

One problem faced by male bowerbirds is that the best decorations are usually in short supply. For example, researchers working at a study site near Brisbane have found that in their bowerbird population, bright blue plastic bottle-tops and the azure tail feathers of Crimson and Eastern Rosella parakeets are the most highly coveted types of decorations. Male bowerbirds frequently build their bowers in loose clusters, with each bower only a hundred meters or so from its nearest neighbor. Occasionally a male bowerbird might happen upon a newly dropped parrot feather or freshly discarded bottle-top. Most of the time, however, the easiest way for him to acquire a new decoration is to steal it from another male.

By individually numbering all of the bower decorations at their study site and tracking them among different males’ bowers, these researchers were able to determine that the bowerbirds most prone to poaching decorations from their rivals were those that already had the most decorations in the first place. These bowerbirds generally did not travel far when raiding, as most of the stealing happened among adjacent bowers and was reciprocal, with neighbors stealing from neighbors. The bowerbirds who stole the most objects also had the most stolen away from them, perhaps because their frequent raiding forays often left their own home bowers unguarded.

To explore this stealing behavior further, the researchers conducted an experiment by first removing all of the original decorations from a cluster of ten bowers, and then substituting new decorations. Some of the bowers had originally been embellished with many decorations and some had had only a few, but now the researchers stocked each male’s bower with exactly the same number and type of decorations.

Tracking these new objects over subsequent months, they found that the general patterns of stealing did not change dramatically, but over time the birds that had originally possessed more decorations once again accumulated a disproportionate share of the substituted decorations, further evidence that having a highly decorated bower is a symbol of a bowerbird male’s mating appeal. For these male bowerbirds, the best males are the best thieves, and stealing is one route to success.

— Irby Lovette is director of the Cornell Lab of Ornithology’s Fuller Evolutionary Biology Program

Wojcieszek, J. M., J. A. Nicholls, and A. W. Goldizen. 2007. Stealing behavior and the maintenance of a visual display in the satin bowerbird. Behavioral Ecology  18: 689-695.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu