Size Matters: A Bird Uses Illusion to Wow a Mate

Charles Q. Choi

Date: 09 September 2010 Time: 06:01 AM ET

The court of a bowerbird as seen by a female, with its optical illusion intact (left); and the court with the rocks placed the opposite way and its optical illusion reversed (right).
CREDIT: John Endler.

To woo females, bowerbird males create optical illusions that make themselves look larger than they are, much like the ones used in the "Lord of the Rings" films to make actors look hobbit-size.

Bowerbirds are pigeon-size birds that live in Australia. The males are well-known for building elaborate shelters that they adorn with brightly colored objects to court potential mates. Now scientists find these lavish structures can also fool the eye, using an effect known as forced perspective, where objects can appear closer, farther away, larger, or smaller than they actually are.

Forced perspective plays with notions of size and distance by manipulating how objects are placed in relation to each other in front of a viewer. For instance, in the "Lord of the Rings" movies, actors who played hobbits were placed farther from the camera when they were filmed with actors who played humans — the result was the hobbits appeared unnaturally small next to the humans who seemingly stood next to them in the scene. Parts of sets were mounted on platforms that moved as the cameras did, to maintain the illusion throughout shots. The same effect is used in the "Harry Potter" movies to make the character Hagrid look like a giant.

To generate their illusions, bowerbird males construct a long avenue of sorts consisting of two hedges of tightly packed sticks flanking a stick floor that opens onto a courtyard, which serves as the stage where the male puts on displays for females. The path ensures females will look on at a specific angle, which is necessary for the optical effect to work. [Key to All Optical Illusions Discovered]

The males then place pebbles, bones and shells around the courts of their shelters, so that their real size increases with distance from the avenue entrance. 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," said researcher John Endler, an evolutionary ecologist at Deakin University in Australia. "Great bowerbirds are the first-known animals besides humans who create a scene with altered visual perspective for viewing by other individuals."

The effect is intentional. When the scientists reversed these setups by placing smaller objects farther away and larger objects closer to the avenue entrance, the males put things back to normal within three days.

Video recordings show that male birds frequently followed a pattern of repeatedly looking at the court from the female’s potential viewpoint and then moving court objects.

"Visual art can be defined as the creation of an external visual pattern by one individual in order to influence the behavior of others," Endler told LiveScience. "In this sense, bowerbirds are artists, and their viewers judge the art enough to make decisions based upon it, implying an aesthetic sense in bowerbirds."

The researchers are now conducting experiments using motion-activated video cameras to see how perceptions of size might be related to mating success. It remains uncertain how mentally challenging it is for the birds to pull off this trick. The males might place these objects correctly through trial and error, or they might actually have a sense of perspective that helps them know to put small objects close and larger objects farther away, Endler suggested.

The scientists detail their findings online Sept. 9 in the journal Current Biology.