DENVER--For the club-winged manakin, love knows few bounds. In its quest to secure an attractive partner, this small, stocky bird has evolved bigger wing muscles and heavier bones that likely compromise its flying efficiency, biologist Kim Bostwick reported here last week at the annual meeting of the Society of Integrative and Comparative Biology.
One of about 40 manakin species found in the tropics and subtropics of the New World, Machaeropterus deliciosus gets its common name from the male's peculiar wing feathers, which play a key role in the bird's unusual mating strategy. Like about two dozen other manakins, the male uses its wings in a way that biologists don't yet understand to "sing" for its mate--producing clicking, rattling, and humming noises as it prances around during group courtship displays.
The arrangement and proportions of the wing's muscles and bones are quite similar from one species to the next; the laws of aviation don't allow much variation. But manakins are an exception, says Bostwick, a graduate student at the University of Kansas in Lawrence, who measured the muscles, feathers, and bones of museum specimens of several species of wing-snapping manakins and some of their quieter cousins.
The shapes and density of some feathers are altered in the group of wing-clicking species, Bostwick noted, especially the inner flight feathers. Also, their wing bones are thicker than normal, with knobs to support bulked up muscles connected to the feathers. And the birds have modified a joint and enlarged a muscle to enable them to flip their wings back and up into the proper clicking posture. In this bird, even the outline of the vane (the flat, broad part of the feather) is quite irregular and in some feathers, the vane is twisted toward the tip--a luxury that most likely impairs the bird's ability to fly.
With these peculiarities, males can make just the right noises to tempt prospective mates, says Bostwick. Manakins court in leks, gatherings wherein all the males strut their stuff at once. "The sexual selection has really translated into extensive remodeling of the wing," Bostwick concludes. Her colleagues agree. "The fact that there is so much change must reflect something about function," says George Goslow Jr., an evolutionary biologist at Brown University in Providence, Rhode Island. "It's certainly a distinct change from the norm."