Experiments on the Control of Deceptive Signals of Status in White-Crowned Sparrows

Abstract

We showed previously that crown plumage brightness signals relative dominance ability in White-crowned Sparrows (Zonotrichia leucophrys gambelii). We now test the social-control hypothesis for the evolutionary stability of status signaling against invasion by individuals with brighter crowns than justified by their intrinsic dominance ability. "Cheating" immature females, with crowns painted to resemble bright adult males, and "control" immature females, with crowns painted a typical immature appearance or left unpainted, were released into two separate wild sparrow flocks and into two different captive groups that contained birds of all age and sex classes. In the field experiments, cheaters were more successful than controls in dominance interactions with other immatures; both groups were equally unsuccessful against adults. In the captivity experiments, cheaters generally dominated controls and were more successful than they in interactions with immature males and adult females but not with adult males. Cheaters did not receive more aggression from dominant individuals than did controls in any experiment, and in two experiments they received significantly less aggression. Thus, we found no socially mediated costs for sparrows with deceptive signals. Instead, we showed that immature females that cheat can benefit. The stability of the White-crowned Sparrow signaling system remains unexplained. We discuss the potential influence of predators on individual plumage differences and offer additional hypotheses for the control of deceptive signals.