Bird Migration and Reproduction in Relation to Habitats for Survival and Breeding

Abstract

Birds with their great mobility often exploit habitats that, because of physical or practical (e.g. overt predation) reasons cannot be used for breeding. We propose that the relative extent of survival habitats in comparison with breeding habitats is an important factor determining migratory and reproductive tactics in different bird species. Birds with a surplus of survival habitats will, because of strong breeding competition, have a deferred age of first breeding (immatures exploit survival habitats), small clutch sizes and poor reproductive output. Furthermore, these birds should return to their survival habitats in an early migration as soon as possible after breeding. Adults postpone moult until after the post-breeding migration, and when possible, migrate in advance of their young. In contrast, a surplus of breeding habitats leaves room for an early age of first breeding, large clutch sizes, postnuptial moult on breeding grounds and a late migration away from the breeding habitat, when adults migrate simultaneously with or even, if delayed by moult, after the juveniles. On the basis of qualitative information on habitat selection in different birds, we expect sea-birds, birds diving for food to relatively great depths, littoral waders and medium-sized or large birds of open terrestrial habitats to have surplus survival habitats. In contrast, dabbling ducks, vegetation-waders, small-sized land birds (particularly those breeding in regions with a great seasonal flush of food and other resources) and medium-sized/large birds of closed habitats, have comparatively extensive breeding habitats. General differences in migration and breeding habits between these groups of birds seem to accord with predictions. Stress is laid on the importance of, besides temporal fluctuations in available resources, different patterns of habitat use, for the evolution of life-history tactics according to the r-K spectrum. Body size per se probably is of additional importance in this respect. Breeding tactics are constrained by specializations in modes of life, which in turn represent adaptations to different survival/breeding habitats and their seasonality.