Sex and age differences

As in many other migrants, females of irruptive species often move earlier, in greater proportion or further than males, while juveniles of both sexes often move earlier, in greater proportion and further than adults. Although not all these tendencies emerged in every species examined, they suggest that competitive social interactions over food supplies influence the migration dates and distances of individuals, causing some to move earlier and further than others (Chapter 15). Females outnumbered or moved further than males in Northern Bullfinches Pyrrhula pyrrhula (Gatter 1976, M0ller 1978, Riddington & Ward 1998), Bramblings Fringilla montifringilla (Payevsky 1971, Cramp & Perrins 1994) and Evening Grosbeaks Hesperiphona vespertina (Prescott 1991). In some other species, almost all the immigrants in some irruptions were juveniles, as found in Eurasian Jays Garrulus glandarius, Great Tits Parus major, Blue Tits P. caeruleus, Coal Tits P. ater and Great-spotted Woodpeckers Dendrocopos major, as well as in crossbills and nutcrackers as discussed later (Lack 1954, Newton 1972, Hilden 1974, Van Gasteren et al. 1992, Markovets & Sokolov 2002).

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