Male bowerbirds 'benefit from optical illusions'

Objects are arranged by size in a bowerbird's bower (c) L Kelley A top-down view shows the careful arrangement of objects

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Bowerbird males that create optical illusions have more mating success, according to scientists.

The house-proud males are known for the elaborate shelters or "bowers" they build to attract females.

Last year researchers in Australia discovered that the males arrange items in their bowers to create a "forced perspective".

Scientists have now confirmed that females are impressed by the trick that could make males appear larger.

The great bowerbirds' display (c) Laura Kelley Males display in the stony court while females watch from the avenue of twigs

The findings are reported in the journal Science.

Dr Laura Kelley and Dr John Endler at Deakin University in Geelong, Australia, have been studying great bowerbirds in northern Australia.

In the past, they have recorded that the males decorate the ground or "court" in front of a thatched avenue of sticks with stones, shells, bones and other grey objects they have collected.

When a female enters the avenue, the male displays over the court "showing off" brightly coloured or unusual items they have collected, as well as flashing the crest on the back of their head.

By photographing the displays, researchers found that objects on the floor of the court were arranged in a very particular manner.

Males placed smaller items near to the twig structure and larger ones further away in a pattern, even correcting the order if the pattern was disturbed.

Appear larger

"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," said Dr Kelley.

If even sized objects are placed at a distance from each other, the more distant object appears smaller when viewed "head on".

Bowerbird facts

A great bowerbird on a fence (c) L Kelley
  • The bowers are not nests; males play no part in nest building or raising chicks
  • Great bowerbird females prefer red and green decorations whereas satin bowerbirds prize blue objects
  • There are 20 species of bowerbird, eight of which are found in Australia

So by placing larger items in the distance and smaller items close to the females, the males could be distorting the perspective of their potential mates.

Now, having carefully studied mating interactions between the birds, the researchers have confirmed that a well-executed optical illusion is effective.

"Males that produce the more even patterns required for a high-quality visual illusion gained more mates than males that had less even patterns," said Dr Kelley.

The scientists suggest two possible reasons for the higher success rate.

"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," Dr Kelley explained to BBC Nature.

Although this is the first time such a display has been positively linked to mating success, Dr Kelley believes the trick could be employed across the animal kingdom.

"In many species that use visual displays in courtship the male displays from a typical distance from the female with a characteristic body orientation towards her," she said.

"This is the fundamental requirement of most geometric visual illusions. So it seems that the conditions for creating visual illusions are present, but we have never considered before that animals might be using them."

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