Protandry and sexual dimorphism in trans-Saharan migratory birds

Earlier arrival to reproductive sites of males relative to females (protandry) is widespread among migratory organisms. Diverse mechanisms have been proposed that may select for protandry, including competition for limiting resources (e.g., territories) or mates. In species with large variation in male reproductive success, such as polygamous species and those with intense sperm competition, early arriving males may accrue a fitness advantage because they acquire more mates or have larger chances of paternity. Comparative studies of birds have shown that sexual size dimorphism (SSD) is positively associated with the level of polygyny, whereas intense sperm competition is associated with sexual dichromatism (SD). Positive correlations between protandry and SSD or SD can therefore be expected to exist across avian species. Because large males are predicted to be better able to cope with adverse ecological conditions early in the breeding season, selection for protandry, in turn, may have a correlated response on SSD among migratory species breeding in boreal latitudes. Although previous studies of birds have analyzed the association between SSD and protandry, none has analyzed SD in relation to protandry. Here we analyze the association between protandry during spring migration, SSD, and SD in 21 trans-Saharan monogamous migratory bird species. The difference in median migration dates between females and males, reflecting protandry, was positively associated with SD but not with SSD. Because dichromatism is positively related to sperm competition across species, present results are consistent with predictions derived from sexual selection hypotheses for the evolution of protandry mediated by sperm competition.