Nonbreeding Distributions In Age And Sex Groups Of The Same Population

If seasonal food reduction is the main underlying reason for migration, and if individuals compete for food, one would expect that, through social dominance, some individuals would benefit more than others from migrating. In many bird species, as explained in Chapter 15, juveniles migrate in greater proportion and further than adults, while females migrate in greater proportion and further than males. This leads to partial segregation in the winter distributions of different sex and age groups from the same breeding population.

Social interactions are held as the basis for many of the age and sex differences in migration that occur within species, and for the fact that individuals of many species migrate in the early years of their life but not later (Chapter 15). In general, the dominant individuals, taking precedence over resources, are less likely to leave their breeding range than are subordinates, or if the dominants do leave, they migrate less far, or stay away for shorter periods (Gauthreaux 1978a, 1982a). In winters of unusually poor food supply, the benefits of migration may extend further up the social hierarchy to a greater proportion of individuals than usual. This type of competition could influence the migration and distribution patterns of birds directly, and independently of genetic factors, whereas most of the differences between species are presumed to be genetically determined, as a result of competition in the past.

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