Birds are meticulous about keeping their feathers clean, but males of one species intentionally dirty their own plumage, researchers have found. The rock ptarmigan's odd behavior helps the bird camouflage itself when its priorities shift from pursuing sex to staying alive.
In their immaculate white winter plumage, these arctic birds blend in with the snow, and in their mottled brown summer plumage, they melt away into tundra backgrounds of gravel, lichen, and soil. Females molt from white to brown just as the snow melts, maximizing their camouflage at all times. But males retain their snowy white plumage for several weeks, which makes them visible to sharp-eyed predators like gyrfalcons. Researchers hypothesized that by delaying their molt, males showed off an ability to overcome such risks and advertised their quality to females or rival males.
As Robert Montgomerie of Queens University in Kingston, Ontario, and colleagues pondered such questions in the Canadian Arctic in 1981, they observed something remarkable. Male ptarmigan were soiling their white plumage with mud. "We had never seen birds so scuzzy-looking, so bedraggled," Montgomerie says. "They looked like they'd been sitting in streams or caribou urine, or something."
Further monitoring over 17 years--which amounted to field data on nesting, molting, and responses to threats of predation--showed that males dirtied themselves immediately after their mates had laid eggs--just after they no longer needed the bright plumage that was so helpful in winning that mate. And when females lost their nests, the males rapidly spruced up their plumage to court and mate again, the researchers report in the July issue of Behavioral Ecology.
The study is "fantastically good," says evolutionary biologist John Endler of the University of California, Santa Barbara. "They did a beautiful job of documenting [the behavior]." The research shows that sexual selection can cause males to delay their molt, says evolutionary biologist Geoffrey Hill of Auburn University in Alabama, while grubbing in the dirt "erases the plumage when it's no longer useful."