Bowerbirds use optical illusion to attract mates
MALE BOWERBIRDS ARE well-known for the elaborate constructions they create to attract a mate. Now it seems that these constructions do more than look good – they could be designed to actually make the male appear bigger and more attractive to the female.
Great bowerbirds create a long tunnel-like ‘avenue’, usually about 60 cm long, using two walls and a floor of tightly packed sticks. Just outside each entrance to the avenue is a bower ‘court’, an area covering about 1 m x 0.5 m.
Much like a queen watching a court jester, a female sits inside the avenue observing the male display in the court. He parades his turquoise-blue crest and presents coloured objects by waving them in the direction of the female.
The males line the courts with various items – usually grey to white objects, such as shells, bones, and even wasps nests – on which they put the coloured objects, like fruit, glass and plastic. They’ll happily use man-made materials alongside natural ones.
While studying great bowerbird constructions, John Endler at Deakin University noticed that the objects lining the court get progressively bigger the further they are from the entrance and the female viewer. If female bowerbirds see things the same way we do, this could affect their sense of perspective, so that they would see the male as being larger than he in fact is.
“This is speculation, based on the fact that birds are known to see at least three optical illusions that humans see,” John cautions. But the evidence seems to suggest that this is what’s going on – and it’s fascinating. “No other animal, besides us, creates scenes with perspective,” he says.
It’s clear that the ordering of the objects isn’t accidental, John stresses. When his team reversed the size order of the objects in the courts of 15 males, the males put them back within three days.
“We were amazed at how fast the gradients were restored to their original levels. This implies that the gradients are very important to the male bowerbird.” The team is now planning more work to find out whether the quality of this ‘forced perspective’ really does influence a male’s mating success.
“This is certainly an interesting paper, and may demonstrate a previously unknown dimension of bird cognition,” says Ron Johnstone, curator of ornithology at the Western Australian Museum.
It could also be art, says John. If visual art is defined as the creation of an external visual pattern by one individual in order to influence the behaviour of others, and an artistic sense in the ability to create art, then this is true of male bowerbirds, he says.
The research will be published in the next issue of the journal Current Biology.
VIDEO: bowerbird mating display