Because birds compete for food, and vary in dominance or feeding efficiency, some individuals could survive in conditions where others would die unless they moved out. In facultative migrants, the subordinate sex and age groups typically migrate in greater proportions, at earlier dates, or extend further from the breeding areas, than the dominants (Chapter 15). Thus, in many bird species, adult females are more migratory than adult males, juveniles more than adults, and late-hatched young more than early-hatched ones (Chapter 15; Michener & Michener 1935, Gauthreaux 1982a, Smith & Nilsson 1987). Such differences have led to the notion that competition (or its effect on body condition) is involved as a proximate mechanism stimulating migration in those individuals least able to survive in local conditions (Gauthreaux 1982a).
Depending on circumstances, the same individual might migrate in one year but remain in the breeding area another (for Song Sparrow Melospiza melodia see Nice 1937, for Mockingbird Mimus polyglottos see Brackhill 1956, for Common Starling Sturnus vulgaris see Kessel 1953). The commonest pattern is for a bird to migrate in its first year and not thereafter, but other sequences occur (e.g. for Blackbird Turdus merula see Schwabl 1983), presumably because foraging conditions in the breeding area vary from winter to winter. The sequence of behavioural events stimulating migration in facultative migrants could thus be hypothesised as: social status ^ competition ^ failure to obtain a winter territory or sufficient food ^ difficulty in maintaining body condition ^ departure. On this mechanism, the overall proportion of birds stimulated to migrate would depend on feeding conditions that year, with more birds from the dominant age
Was this article helpful?