Birds prefer walls for wild flirting.

During courtship, female spotted bowerbirds like a guy to get wild--charging into walls and flinging around bleached bones--but they also prefer to watch the show from behind a see-through barrier.

Males of this robin-size Australian species weave parallel walls in the clearings where they strut their stuff for visiting females. Rearranging these bowers provides new clues to the birds' mating dynamics, report Gerald Borgia of the University of Maryland in College Park and Daven C. Presgraves of the University of Rochester (N.Y.). In the November Animal Behaviour, they argue that their first experimental redesign of bowers supports the idea that courtship features evolved to let males act wildly enough to be alluring without scaring females away.

The spotted bowerbird, Chlamydera maculata, is the only one of the bower builders to align the wails east-west instead of north-south. Also unique is the female's viewing position within the bower. She watches the male's show through a wall, which is of an unusually loose weave, instead of through an open end.

It's quite a show too, more intense than in most bowerbird species, observes Presgraves. Instead of the usual soft calling, "these guys scream their heads off," Presgraves says. Males rush at the bowers that shelter a female, often crashing into the wall. They pick up the snail shells or bleached sheep vertebrae that they have scattered by the hundreds around the clearing and toss them much farther than other bowerbirds, up to a meter away. "It looks like a temper tantrum," Presgraves notes.

The females seem extremely choosy, usually just flying away after the show. Only a small percentage of the males in a population manage to mate. To see what pleases the females, Borgia and Presgraves videotaped the action at 13 undisturbed bowers for a season. Comparing successful male displays with flops, they report, "Females preferred males with the most intense displays."

The next season, the researchers removed one wall from each of 12 bowers. In 22 of 26 courtships, the birds kept the remaining wall between them during the majority of the display, even when the investigators repositioned it. When males did move to where no barrier separated them from a female, displays were more muted, with significantly less body shuddering, long calling, and decoration flinging, say the researchers.

They argue that their findings support the idea that the species' unusual bower orientation and male shenanigans "represent a coevolved suite of changes that allow males to give higher-intensity displays."

Another bowerbird observer, Stephen Pruett-Jones of the University of Chicago, says he's fascinated by the new details of the little-known spotted species. However, he sees several interpretations. Male aggression did not have to drive evolution of the bower, he says. "It could be the reverse: Male aggression in displays evolved in response to bower structure."

Aggression is nothing unusual in bird courtship, observes Anders P. Moller of Pierre and Marie Curie University in Paris. Male mallards, for example, get so rough during aquatic sex that they drown some of their partners. He notes, "Lots of aspects of sexual behavior are actually outcomes of sexual conflict."