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Bird attraction based on illusion

Great bowerbird bower.
[Image courtesy of L.A. Kelley]

Bowerbirds, which live in Australia and New Guinea, have an elaborate mating ritual in which the males build large structures or "bowers" that females stop by to inspect. In a new study, scientists report that certain male bowerbirds attract mates by decorating their bowers in a way that creates an optical illusion.

To woo females, the male great bowerbird assembles a collection of bones, shells, stones and other gray objects, collectively called "gesso." He spends hours painstakingly arranging the gesso in front of an avenue made of two walls and a floor of densely thatched sticks. As the female stands in this avenue, the male picks up brightly colored objects, such as a piece of fruit, and shows them, one by one, to the females.

The male places larger gesso objects farther away from the avenue entrance and smaller objects closer. Because objects normally appear smaller with distance, this arrangement of the gesso creates the illusion that the objects are all roughly the same size and that the gesso area is smaller than it actually is. This is illusion is called "forced perspective."

In their study, Laura Kelley and John Endler of Deakin University investigated whether these elaborate design efforts actually paid off for the males in their search for a mate.

After studying a population of great bowerbirds and measuring their bowers, Kelley and Endler report that female bower birds do indeed tend to choose males whose bowers produce the best versions of this illusion.

It's not clear yet why the illusion is tied to mating success, but one possibility is that when the gesso appears more uniform, the brightly colored objects better hold the female's attention. The authors propose that other species may also take advantage of visual illusions in their courtship behaviors.

The study appears in the 20 January issue of the journal Science.