Dirty ptarmigan: behavioral modification of conspicuous male plumage

Males of many bird species acquire a conspicuous breeding plumage through molt. Male rock ptarmigan (Lagopus mutus). however, become conspicuous in a unique way-as snow melts away from the tundra, their cryptic white winter plumage suddenly becomes exceptionally conspicuous, and remains so for at least 3 weeks. While males remain white, females molt into one df the most cryptic plumages known in birds. From our 17-year field study ill arctic North America, we show that, unlike other birds, male rock ptarmigan eventually change from conspicuous to cryptic by soiling their plumage, thereby reducing their conspicuousness six fold before they molt to their cryptic summer plumage, individual males began to soil their plumage as soon as their mates began egg-laying, and were maximally dirty and relatively cryptic by the time incubation began and their mates no longer fertilizable. Thus male plumage conspicuousness appears to serve a reproductive function. Moreover. both polygynous and bachelor males delayed soiling for a few days after monogamous males, as expected because of the prolonged making opportunities available to them. We use these data to address a variety; of hypotheses to explain both the conspicuousness of breeding males and their subsequent plumage soiling. Given the high predation rate recorded for male ptarmigan during the breeding season, we argue that male conspicuousness is best explained by sexual selection and that plumage soiling is an adaptation that reduces predation risk by increasing camouflage.