Bowerbirds

The 18 species of bowerbirds are unique in that the males build and decorate a bower, a structure of sticks or grass on the ground, for the purpose of attracting and courting females. Members of the bowerbird family (Ptilonorhynchidae) are found in Australia and New Guinea, and are related to lyrebirds and birds of paradise. Most bowerbirds are about the size of a blue jay or grackle, and as a group they show a wide variety of plumage characteristics, vocal behavior, and bowerbuilding styles. The bowers of some species are quite large (3.3-6.6 ft/1-2 m) in length, and are decorated with a variety of objects, making them some of the most remarkable examples of animal architecture.

Naturalists have been fascinated by bowerbirds for decades. Early observers believed that the bower was a nest; however, in 1865, the ornithologist John Gould suggested that the bower was used for sexual display and mating. Not all bowerbirds build a bower; some, such as the toothbilled bowerbird of eastern Australia, simply clear a display court on the ground, and decorate it with leaves.

Bowers occur in several forms, each built by species that appear to share closer genealogical relationships with each other than with those species that build a different type of bower. Avenue bowers (constructed by the satin bowerbird) have two vertical walls running parallel to each other, with one end opening onto a display area where most of the decorations are arranged. Maypole bowers consist of sticks woven around a central pole, formed by a sapling or fern, surrounded by a circular, raised court. Two species, the striped gardener bowerbird and Vogelkop's bowerbird, build massive hutlike structures (up to 6.6 ft/2 m across) around the central maypole, opening onto a cleared exhibition area. The golden bowerbird places sticks against adjacent saplings which are joined by a crossbranch; this he uses as a display perch. A variety of objects are used to decorate the bowers of different species of bowerbird, including fruits, A female satin bowerbird (Ptilonorhynchus violaceus) visiting a bower. © Tom McHugh, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. flowers, feathers, moss, snail shells, colored stones and bark; the recent presence of humans has added coins, bottle tops, pieces of glass, teaspoons, nails, and screws to the bowers.

Female bowerbirds visit the bowers of numerous males in the process of selecting a mate. When a female arrives, she will take up a position within the bower, while the owner launches into a stereotyped display that is unique for each species. The male emits varied chirps, whistles, buzzes, and mechanical sounds, while performing a series of dance-like movements that have been described as "rooster walks" and "penguin walks." Males of some species pick up and hold decorations in their beak during their display, bobbing their heads or tossing the items with considerable vigor. One especially vigorous species, the spotted bowerbird, actually rushes at the visitor watching from within the bower, crashing bodily into the wall that separates the two birds. When the visiting female is ready to mate, she crouches low in the bower, lifting her tail, and the male approaches from the rear of the bower to mount her and copulate for just one or two seconds.

It is clear that a bower can be extremely valuable to its owner, for bowerbirds spend hours constructing, decorating, and maintaining their bowers, and no male has ever been reported to successfully court a female without one. However, success is not guaranteed. Males in some species are highly competitive, and are observed to steal decorations from the bowers of other males, and to destroy rival bowers. Many bowerbird males simply fail to mate with even one visiting female during a breeding season, even though the males have been tending bowers.

Researchers have sought to understand why some individuals enjoy high mating success, while most others do not, and the functional role of the bower in female mate choice. Elaborate display traits, such as bowers, suggest the influence of sexual selection—the process of evolutionary change due to competition between members of one sex (usually males), and selective mate choice by the other sex (usually females). In species where males give little or no parental care to their offspring (as is the case for most bowerbirds), researchers have attempted to address the question: What is the female choosing when she selects a mate? Bowerbird observers have attempted to ascertain whether certain features of bowers or of male display are reliable predictors of a male's success in obtaining mates.

Gerald Borgia and his collaborators have used an ingenious method to investigate this question in the field. A video camera, positioned a short distance in front of the bower, was outfitted with a motion-sensitive infrared detector, which turned on the camera whenever there was movement at the bower. Such continuous monitoring compiled a comprehensive record of activity at the bower, and allowed Borgia to measure and compare the courtship of many individuals. The results showed that female bowerbirds differentiated among males, at least in part on the basis of the quality of the bowers and the display. In satin bowerbirds, for instance, the number of decorations, especially snail shells and blue feathers, as well as the degree of bower symmetry and the density of the sticks used to construct it, were excellent predictors of male mating success. The more decorations, and the more symmetrical and densely constructed the bower, the more matings were achieved by the bower owner. The importance of decorations was underscored when the researchers experimentally removed decorations from some bowers. The owners of the manipulated bowers had far less success in attracting females. Clearly, female choice could exert a potent effect on the evolution of male display in bowerbirds, helping shape the elaborate courtship structures and behaviors observed in these animals.