Should females of migratory dabbling ducks switch mates between wintering and breeding sites?

Pairing of northern hemisphere dabbling ducks normally occurs in wintering sites. Mate choice occurs when some individuals congregate in courtship parties, in which females mainly choose mates according to male behavioural dominance. The sex ratio of some species is more male-biased at northern than at southern wintering sites, and the age ratio is adult-biased in northern areas. A possible mechanism responsible for this spatial segregation of sex and age classes is behavioural dominance, with males usually dominating females and adults dominating yearlings. Due to this latitudinal segregation of sexes during winter, presumably more males would remain unpaired in northern sites utilized by the more dominant birds than in southern sites utilized by the less-dominant birds, and this is paradoxical. I argue that some females having wintered and paired in southern sites may switch mates at stopover sites during northward migration. This hypothesis provides an explanation as to why, in spite of females being paired when initiating northward migration, some species show an increase in courtship activity during spring at stopover sites, as in common teals (Anas crecca) in southern Spain. In contrast, populations that do not migrate, e.g. mallards (A. platyrhynchos) in southern Spain, do not exhibit an increase in courtship activity during spring.