Visual trickery key to luring lover
A new study claims male bowerbirds deliberately decorate their bowers to make themselves look larger to females.
If correct, the discovery makes them the only animal besides humans, known to create a scene with altered visual perspective - constructed for viewing at a particular angle.
Writing in the journal Current Biology, evolutionary ecologist Professor John Endler from Deakin University in Geelong says he noticed a consistent geometric size pattern while studying a species called the great bowerbird (Chlamydera nuchalis).
"It appears the male creates a staged scene, only visible from the point of view of their female audience," says Endler. " He places white and grey pebbles, bones, and shells in the court ... putting smaller objects near the front and larger ones further back."
"If you move stuff around, he'll move it back, not to the same spot as some reports claim, but certainly to the same distance from the front of the court."
Bowerbird males are well known for making elaborate constructions, lavished with decorative objects, to impress and attract mates.
The males construct bowers with an avenue approximately 60 centimetres long with a floor and walls made of sticks opening out to a court.
As part of the mating ritual, the female stands in the avenue watching the male displaying in his court.
Endler, who conducted the research with colleagues Dr Lorna Endler and Natalie Doerr, says this staged scene only works from one viewing angle, which happens to be the avenue.
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," he says.
"It could make him look like a better catch or it might just make him easier to see against the background."
Previous research has shown colour is important to bowerbirds. Endler says great bowerbirds prefer white and grey objects, followed by stronger colours like green and red.
"Interestingly they really seem to dislike yellow. That could be because they have a yellowish tinge on their chests, so they're looking for a contrast to make them stand out," he says.
"Satin and regent bowerbirds like blue and yellow, which is why you see things like blue clothes pegs in their bowers. In the days before blue plastic they decorated their bowers with blue feathers and fruits like blue quandong."
Endler says it's too early to say whether the male has empathy for the female perspective.
"He spends 70% of time arranging the bower, so it could just be a case of decorating the bower to suit his own tastes rather than doing it to impress the females."
The researchers are now conducting experiments using motion-activated cameras to test whether the size gradients are also related to mating success.