Just like humans, birds too use optical illusions to attract mates

WASHINGTON - Just like humans, birds too use optical illusions to woo a mate, says a new study.

Bowerbird males are well known for making elaborate constructions, lavished with decorative objects, to impress and attract their mates.

Now, researchers have identified a completely new dimension to these showy structures in great bowerbirds.

The birds create a staged scene, only visible from the point of view of their female audience, by placing pebbles, bones, and shells around their courts in a very special way that can make objects (or a bowerbird male) appear larger or smaller than they really are.

“Great bowerbirds are the first known animals besides humans who create a scene with altered visual perspective for viewing by other individuals,” said John Endler of Deakin University in Australia.

The effect only works from one viewing angle. Great bowerbirds ensure that females will see their courts from one particular spot by constructing an avenue-two rows of tightly packed sticks with a stick floor-that opens onto a court. That court is essentially a stage where the male displays for females.

Endler noticed something that had apparently been missed before, in part, he suspects, because no one had considered the females’ viewpoint before. The great bowerbirds line their courts with objects whose absolute size increases with distance from the avenue entrance and the female viewers.

That makes the sizes of things appear to be more regular, a feature that might be aesthetically appealing to the birds and might also help the males to stand out. But there might be another advantage.

Assuming the birds see things essentially the same way we do, that forced perspective could lead females to “perceive the court as smaller than it is and therefore perhaps perceive the male as larger than he is,” Endler suggested.

Experimental manipulation of the courts by the researchers showed how important that geometrical pattern must be to the males.

Endler said it isn’t yet completely clear why the males do this. Other aspects of bower decoration have been shown to influence mating success, and it is possible that the quality of the forced perspective may be yet another way that females pick a winner.

The researchers are now conducting experiments using motion-activated video cameras to test whether the size gradients are related to mating success.

The study has been reported online on September 9 in Current Biology, a Cell Press journal. (ANI)