The construction above, called a bower, is something of a short-term “love nest” for the great bowerbird, a species native to the eucalyptus woodlands of northern Australia.
“You can think of bowers as a teenager’s car,” says John Endler, an evolutionary ecologist and Alfred Deakin Professor at Deakin University in Australia. Built by males, “their only function is to attract females for mating.” After the deed is done, “the female goes off and builds a nest by herself in a tree and raises the young by herself.”
The great bowerbird is one of 20 species of bowerbirds found throughout Australia and New Guinea, of which all but three (the Ailuroedus catbirds) build bowers. They’re the only group of birds known to construct such elaborate structures used solely for mating, according to Endler.
Each individual’s bower is unique to its creator and varies in size and arrangement, depending on the species that built it. For instance, the bowers of the great bowerbird include densely thatched structures about a yard tall and wide, called “avenues.”
While the thatch-work—assembled with sticks and twigs—is impressive, the bower’s ornamentation seems to be most important to the birds, says Endler. A male great bowerbird carefully lays down pale-colored objects such as stones, bones, and sun-bleached snail shells outside of the avenue on either end in areas called courts, and sometimes inside. Then he displays brightly colored objects—great bowerbirds like reds and greens, though other bowerbirds prefer blues and yellows—around the avenue. The birds are true scavengers, picking up most anything, even man-made objects like Coca-Cola bottle caps, to adorn their bowers, says Endler.
Great bowerbirds in particular seem to have a sense of scale when executing their designs: They tend to place the smaller rocks and bones closer to the avenue's entrance and bigger ones farther out. The layout creates a sort of forced perspective, according to Endler. As a result, a female looking out from the entrance sees an area adorned with objects that appear similar in size, even if they aren’t in reality.
The birds can apparently distinguish when things are out of place, Endler says. He and colleagues have experimented with moving around pieces of a great bowerbird’s bower, even changing the gradient of the materials so bigger items were closer to the opening. “And we just filmed them putting [the pieces] back, and almost immediately they restored their original gradient,” he says.
The amount of time and detail spent on creating these bowers is excruciating, Endler says. He’s witnessed great bowerbirds spending days just walking in and out of the avenues, checking on the perspective from inside, then rearranging objects and checking again until they’re satisfied with the layout.
After the bower is finally completed, it’s time for a female to step inside. (Scientists aren’t quite sure how she’s lured in, but they have a few theories—one is that she’s initially attracted to some of the bright objects.) Once she enters, the great bowerbird male makes a few strange sounds, then struts around the structure to the edge of the avenue wall where the female can’t see him. While hidden, he picks up various objects and either waves or tosses them in her visual field for two to three minutes, says Endler. If the female sticks it out for all the shenanigans, the two mate in the avenue, and then she flies off to handle the rest.
After mating season, bowerbirds typically leave their bowers to decay until the next year. Most bowerbirds will build their new love nest close to their last, and even re-use some of the ornaments if they’re still around—if it worked once, it’s worth trying again.
This article was updated on August 26, 2014, to reflect the location of the second photo. It was taken in Townsville, not Dreghorn.