Bright male plumage and predation risk in passerine birds: Are males easier to detect than females?

Several theoretical models assume that bright plumage in male passerine birds makes them easier to detect, increasing the risk of predation. Female plumage, on the other hand, is assumed to be cryptic. These assumptions have not been tested in the wild, and are not necessarily true: crypsis takes many different forms, and the sexes may evolve to any of these forms. We studied the chaffinch Fringilla coelebs and the pied flycatcher Ficedula hypoleuca, placing pairs of mounted specimens (a male and a female) in several habitat types, and using human observers to quantify detectability. Male and female pied flycatchers did not differ in detectability in trees, suggesting that the black-and-white male is cryptic against contrasting backgrounds in trees (disruptive coloration). On more homogeneous backgrounds on the ground, the male flycatcher was more conspicuous than the female. In the chaffinch, males tended to be more easily detected in trees, whereas on the ground the sexes did not differ in detectability. Male chaffinches cover their white wing patch while foraging on the ground, making them less conspicuous. Birds, but not humans, perceive ultraviolet light; measurements of UV reflectance from the plumage of the species suggested that this was no serious bias. Our results indicate that predation risk has influenced colour evolution in males of the two species.