In many bird species, sex and age differences occur in the proportions of individuals that migrate, in the timing of outward and return movements, and in the distances travelled, leading to geographical gradients in sex and age ratios in the non-breeding season. Among passerines, short-distance partial migrants migrate after moulting in breeding areas. Typically, females migrate in greater proportion, depart earlier, travel further from their nesting areas, and return later than males; and juveniles migrate in greater proportion, depart earlier, travel greater distances, and return later than adults. Many species of obligate long-distance migrants leave after breeding, and suspend or delay moult until after arrival in a staging area or winter quarters. Typically, in such species, males leave before females, and adults before juveniles. Sex differences can be interpreted in terms of the different roles of the sexes in breeding, and both sex and age differences partly in terms of dominance and competition within populations, duration and extent of moult, and perhaps also by other (as yet unknown) factors. Many exceptions to these general patterns are associated with the particular life history characteristics and circumstances of the populations concerned. Thus, in species in which only one partner looks after the young, the other leaves breeding areas earlier (males in ducks, and in some shorebirds, females in others). Typically, in these species, adults leave breeding areas earlier than young of the year.

In most bird species, males arrive in breeding areas, on average, earlier than females, the difference varying in different species from a few days to a few weeks. The reverse sequence holds in species in which females rather than males compete for territories and mates. Within each sex, young birds arrive later than older ones, on average.

Sex and age differences in migration distances may result from current competition or from past conditions and their effects on genetic control mechanisms. In species in which individuals do not breed until they are several years of age, the immatures typically spend one or more year in their wintering areas, or migrate only part way towards the breeding areas. They also migrate later in spring than older birds, and depart earlier in autumn, their timing getting more like that of adults with increasing age. Winter distributional differences between sex and age groups are apparent: (1) on a geographical scale across a wintering range; (2) on a local scale, among particular habitats or localities; and (3) on an even smaller scale, within particular feeding or roosting flocks.

Shelduck Tadorna tadorna on moult migration

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