Song versus plumage in some North American Oscines: Testing Darwin's hypothesis

Sexual selection is thought to have favored the development of both song an d plumage, but other selective pressures may impose constraints. Darwin sug gested that there is a negative correlation between song and plumage comple xity, but this hypothesis, which we dubbed the trade-off hypothesis, has se ldom been tested. We examined the trade-off hypothesis on 123 species of No rth American Oscines. Canonical correlation analyses were used to assess th e relationship between a set of variables measuring plumage conspicuousness and another measuring song complexity. Data were corrected to take phyloge netic autocorrelation into account. A weak positive correlation was found b etween plumage conspicuousness and song complexity. The positive correlatio n can be explained by the evolution of traits that may help a male to mate rapidly in regions where the breeding season is short. Factors such as the influence of habitat structure on the evolution of songs and plumages, pred ation rates, differences in mating systems and in targets of sexual selecti on, and experimental errors may explain the weakness of this correlation.