Male bowerbirds are virtuoso architects. To woo females they construct an intricate structure (a bower) from twigs that they meticulously decorate with a variety of found objects. The result is the ultimate avian bachelor pad. Biologists have long marveled at the male bowerbirds’ elaborate courtship scheme. Now new findings add to a growing body of evidence that it is even more complex than previously thought.
Among great bowerbirds, which live in Australia, the bower takes the form of an “avenue” made of twigs that opens out onto a “court” assembled from bones, stones, shells and other gray items that are together referred to as gesso. When a female pays a visit, she stands in the avenue and looks out onto the court, where the male proceeds to pick up and display a variety if brightly colored objects. If she likes what she sees, she will mate with him.
Previous research revealed that the males create visual illusions in building their bowers, carefully arranging the gesso according to a size gradient in which smaller objects are placed closest to the avenue and larger ones farther away. Because objects generally appear smaller with distance, this arrangement tricks the eye into thinking the gray gesso components are uniform in size, and that the court is smaller than it actually is. This “forced perspective” may somehow make the brightly colored objects that the male displays look more appealing to the female.
In the January 20 Science, Laura A. Kelley and John A. Endler of Deakin University in Australia report that female great bowerbirds do indeed tend to choose the males that produce the best forced-perspective illusions in their courts. In fact, they write, “illusions may be widespread in other animals because males of most species display to females with characteristic orientation and distance, providing excellent conditions for illusions.”
But in a commentary accompanying the Science report, Barton L. Anderson of the University of Sydney cautions that whether the male bowerbirds are intentionally creating these illusions to attract mates, and whether females are specifically choosing this behavior or some other, as-yet-unknown behavior that is correlated with the production of forced-perspective displays remain unknown.