The geometry of the courtship site creates an illusion of uniformity: When a female bowerbird views the court from within the avenue, all of the court objects appear to be the same size.
Credit: L.A. Kelly
Everyone likes a good optical illusion, and that includes at least one animal. Male bowerbirds woo females by constructing a bachelor pad that creates an illusion of uniform décor (and the illusion that their owners are much more robust lads than they really are).
And a new study suggests the females tend to choose mates from those who produce the best illusion.
Male great bowerbirds —pigeon-size birds native to Australia — spend the majority of their time building and maintaining their courtship sites, called bowers. A bower consists of a tunnel-like avenue made of densely woven sticks that leads to a court of gray stones, shells and bones. Previous research suggested the birds arrange items in such a way that the court appears uniform and small to a female viewing it from within the avenue, which makes the male appear much larger and more impressive than he really is.
Bowerbirds are the only animals so far that have been shown to use illusions for mating.
By analyzing the geometry of various bowers and studying the mating success of the bowers' creators, researchers have now determined that the male bowerbirds creating the best bower illusion get all the females.
The purpose of the illusion may be to make displayed objects more attention-grabbing to a female, giving her more time to decide if she wants to mate, the researchers said.
The idea that illusion strength can predict mating success is "absolutely brilliant and novel," said Fabrizio Sergio, a conservation biologist at the Spanish National Research Council (Consejo Superior de Investigaciones Científicas) in Madrid, who was not involved in the current study. "It opens a new perspective on [mating] signal design and makes more complete our view of the fascinating subject of animal communication."
The power of a bower
Male bowerbirds, which live about 30 years, begin collecting objects at around age 5 for their bower courts. After building his avenue and court, which can have several thousand objects, the bird will vocally advertise his bower from the top of a nearby tree.
If a female is interested, she will inspect the bower from both outside and inside of the avenue. While the female is inside, the male will stand in the court just outside her view and display brightly colored objects, such as plastic clothespins or pieces or fruit, or the crest on the back of his neck. Then he will enter the avenue, come up to the gal from behind and try to mate.
John Endler, an evolutionary ecologist at Deakin University in Australia, and his colleagues first observed a peculiar aspect of the bower in 2010. Rather than randomly placing items while constructing a court, the birds were putting smaller objects closer to the avenue and larger objects farther away. This size gradient, when viewed from within the avenue, created an optical illusion called forced perspective: All the court items appeared roughly the same size, and the court itself looked smaller than it was.
The researchers experimentally rearranged the court items and found that the original design was no accident. "They fix it within three days," Endler told LiveScience. "The objects' placements were really important to them."
While court design was critical to the males, it was unclear whether it really mattered to the females. To find out, Endler and study co-author Laura Kelly, an ecologist at Deakin University, first monitored 20 male bowerbirds to see which bowers successfully attracted females, and then placed motion-sensing cameras around the eight bowers that drew female visitors.
Males that created the best illusions were more likely to mate with interested females, the results showed. Some males crafted perfectly reasonable size gradients of the objects, but those gradients, when viewed from within the avenue, didn't produce a suitable illusion of uniformity — the only birds that successfully mated were those that got the illusion just right.
The researchers also found that females chose to mate only if they had spent at least 55 percent of their visit checking out the court from within the avenue. The researchers suspect the bright objects, when waved by the male over the court illusion, stand out more, helping to hold the female's attention. "This might give her more time to decide if she wants to mate with him," Endler said.
Trial and error?
Sergio found the new study "catchy and interesting," though he said that the conclusions would have been stronger if the team had studied more than eight bowers. "But the authors did start with 20 bowers (an adequate sample) and had it reduced by absence of visitation by females, something out of their control," he wrote in an email to LiveScience.
Given that some birds, such as pigeons and gray parrots, are sensitive to visual tricks, the study proposed that other animals also might use illusions during courtship. Sergio agreed: "If the study is successful in stimulating further research, many examples of similar dynamics from other species will soon accompany it." [Strange Courtship Rituals]
At this point, the researchers aren't sure whether the bowerbirds' ability to use illusions says anything about their cognitive abilities. Endler said the birds simply could be good at recognizing patterns and create the forced perspective by trial and error.
"But it's amusing to think that forced perspective was invented by bowerbirds millions of years before humans," Endler said. "Bird art has a bigger history than human art."