For many years past, I have observed that, towards Christmas, vast flocks of Chaffinches have appeared in the fields. . . . when I came to observe them all narrowly, I was amazed to find that they seemed to me to be almost all hens. (Gilbert White 1789.)
In many bird populations, sex and age groups differ in aspects of their migrations. Such differences include the proportions of each sex and age group undertaking migration, the timing of outward and return journeys, and the distances travelled. The latter produce geographical gradients in the sex and age ratios of species in the non-breeding season, with one group predominating nearest to the breeding areas and the others furthest away. One or more such sex and age differences have been recorded in a wide range of bird species, from at least nine different Orders, including passerines, shorebirds, ducks, raptors, herons, various seabirds and others. In some species the differences are great, with little or no overlap between sexes or age groups, but in other species the differences are manifest chiefly in mean values, with extensive overlap between sex or age groups. Establishing statistical significance then depends largely on sample sizes or consistency in patterns between years. In this chapter, I have cited as examples only those studies in which statistical significance has been established, but similar trends occur in many other species.
Sex differences in migration have been known for a long time. The Swedish taxonomist Linnaeus in 1758 named the Chaffinch Fringilla coelebs (meaning 'bachelor finch') because it was chiefly the males that stayed to winter in Sweden where he lived, while females moved to lower latitudes. Subsequent work, based on ringing returns, showed that the males which did migrate moved, on average, less far than females (Payevsky 1998). The same was true for the closely related Brambling Fringilla montifringilla, and for many other passerine species (Table 15.1).
In a review of so-called differential migration, Cristol et al. (1999) listed 146 bird species in which sex or age differences were known or suspected to occur, against 16 for which no evidence was found (though often on small samples). Our knowledge of differential migration is of course restricted to species in which the sex and age classes can be readily distinguished by plumage or size differences. In some species, including many passerines, shorebirds and seabirds, it is practically impossible to separate the sexes in this way, so without special techniques, any sex differences that might occur are effectively undetectable (for use of DNA methods see Catry et al. 2004, Remisiewicz & Wennerberg 2006).
Sex and age differences in migration are associated with at least three aspects of species biology, based upon: (1) the different roles of the sexes in breeding, which can be linked to their migration timing; (2) the timing of other events in the annual cycle, especially moult, which can again influence the timing of departure from breeding areas; and (3) body size and dominance which often also differ between sex and age groups, and can be linked to migratory timing and distances. Some of these features thus relate to only one aspect of migration, while others relate to more than one. Moreover, they are not mutually exclusive, and more than one may apply to the same species. These different aspects are explored below, followed by discussion of the movements of immature non-breeders, and then of sex- and age-related differences in local distribution patterns.
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