Great bowerbirds, which live in Australia, get their name from the elaborate grass structures, or bowers, that the males build to woo prospective mates.
The bowers are impressive, featuring a long avenue of bones and stones — called a gesso — for females to inspect. But now scientists are reporting an even more impressive feature: The gesso includes a sort of optical illusion to make it appealing to females.
The males place smaller objects closer, and the larger ones farther away, creating the illusion that all the objects are similar in size, and that the gesso is smaller than it actually is. The better the illusion, the greater the male’s reproductive success.
“It’s the female’s view” that is most important, said John Endler, an evolutionary ecologist at Deakin and James Cook Universities in Australia, who was involved in the research. He and a Deakin colleague, Laura Kelley, report their findings in the current issue of the journal Science.
In a previous study, in which the scientists inverted the display, the birds restored it within three days, a sign of that the small-to-large configuration is critical. Females enter the grass structure and observe the gesso, while the male presents the female with various colored objects, usually fruits. She is able to see only the male’s head, and makes her decision based on presentation.
If she sticks around long enough, the male takes it as a positive sign and goes out to mate with her. A female might visit several males before deciding.
In a way, Dr. Endler said, the gesso display could be called artistry.
The illusion “is evidence that bowerbirds are actually creating art,” Dr. Endler said. “It can be regarded in an aesthetic sense because judgments are made.”
