How birds use optical illusion to woo females

To attract females, male great bowerbirds of Australia build huge, elaborate structures called bowers. They're impressive structures, complete with courtyard and triumphal arch. But the males also use forced perspective to make themselves look more impressive to prospective mates.

You can see an example of one of these bowers in the photo on the left, which was taken as part of a recent study by Dr. Laura Kelley and Dr. John Endler of Australia's Deakin University. The courtyard in front of the thatched arch is decorated with bones, shells, and stones - basically, anything gray the birds can find. And there's a consistent pattern to how these objects are arranged, as the males place the largest objects furthest away from the arch, while smaller objects are placed closer in.

This is an example of forced perspective, and the effect actually makes the bower look smaller than it really is. But as Dr. Kelley told BBC News, there's some logic to that:

"One possible explanation is that the visual illusion makes the display court look smaller, so that any objects displayed over the court appear larger than in reality, and therefore that male is interpreted as a better mate. Males that produce the more even patterns required for a high-quality visual illusion gained more mates than males that had less even patterns. Either the pattern required to create forced perspective is an indicator of male quality [that attracts females]...or the illusion holds the female's attention for longer, making mating more likely."

I appreciate the fact that the researchers don't seem to think the females are actually fooled by this rather basic optical illusion. Instead, the reaction is probably some combination of, "Yes, yes, nice effort there," or "Wait, what the hell am I looking at here?" In either case, the effect is the same - the females spend more time to the birds who have the most convincing forced perspective. And so ends the story of what really must be the nerdiest courting strategy in the entire animal kingdom.

Via BBC News. Top image by tatters:) on Flickr. Image of bower by Laura Kelley.