Published online 9 September 2010 | Nature | doi:10.1038/news.2010.458

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Bowers may make males look bigger than they are.

Male bowerbirds may decorate their bowers to trick their mates. Greg Miles

If elaborate decor fails to please her, perhaps she will fall for a trick used in châteaux gardens, Las Vegas casinos, and the Parthenon in Athens. At least, that is the approach taken by male bowerbirds, who weave optical illusions into their bowers that may fool potential mates into thinking that the males are larger than they are, according to research published today in Current Biology1.

Free from child-rearing duties, male bowerbirds devote 80% of their day to bower maintenance. Unlike the festooned nests of stickleback fish or the garnished shells of decorator crabs, bowers — grand exhibits constructed of treasures such as arranged sticks, stones, berries and bottle tops — serve only to impress.

Some bowers feature a dome. Others, such as that built by the great bowerbird (Chlamydera nuchalis) studied by lead author John Endler, an evolutionary ecologist at Deakin University in Geelong, Australia, and his colleagues, include a twig-lined, 0.6-metre-long avenue ending at a 'court' (see video). When seeking a mate, the female stands within the avenue to view the court where the male entices her by squawking, hopping and waving a colourful ornament. To determine what she sees, the researchers took photos of the court from within the avenue and realized that the stones, bones and shells lining the court floor were arranged by size — from small to large — creating an optical illusion known as forced perspective.

"This is a fascinating example of the complexity of animal behaviors," comments James Ha, a cognitive behavior researcher at the University of Washington in Seattle. "I can't think of any other animal that clearly uses optical illusions."

If bowerbirds see as humans do, the small-to-large gradient tricks the female into perceiving a court lined with an extremely even pattern or one that is smaller than it is. By contrast, a male on display may appear more striking or larger than he is.

"In either case," says Endler, "the gradient is obviously very important to him." When Endler and his team disturbed the patterns, male bowerbirds recovered size gradients within three days, and had restored the original optical illusion within a couple of weeks.

Renaissance birds

The authors ponder whether bowerbirds might have used perspective sooner than human civilization learned to depict it. Bowerbirds are closely related to crows, which display their famous ingenuity through behaviour such as tool making — bending straight wires into hooks to grab food, for example. By putting their bowers together, bowerbirds could also be displaying intelligent behaviour, but the jury is still out on this. Ha says that there is no proof that their use of forced perspective requires complex thought. "Wasps make elegantly symmetrical nests, but they have very simple brains," he explains. "Likewise, this could be a hard-wired behaviour requiring no cognition."

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If it is a cognitive behaviour, females who fall for the illusion might be inadvertently choosing the brainiest male. Alternatively, a well-executed illusion could mean that the designer is skilled at collecting objects, stealing objects from other bowers or defending his bower from thieves. Then again, dramatic size gradients might simply reflect an object-rich habitat.

Suggesting that the birds make aesthetic decisions when meticulously arranging their bounty ruffles the feathers of many researchers. "I'd hate to see people read this and run off saying bowerbirds are smarter than Michelangelo," says Gerald Borgia, a biologist at the University of Maryland in College Park.

Video courtesy of Charles Delwiche.