Unpredictable Habitats and Evolution of Bird Migration

In bird species living in areas where environmental stability and predictability is low, site fidelity is not an important factor in intraspecfic competition. If one part of a population of such a species evolves migratory behaviour (partly as a consequence of the lack of site fidelity), and if the productivity of this part of the population is higher than the productivity of the remaining part of the population, selection will favour migration. This leads to a situation where completely migratory land bird species originate from areas with a high environmental instability, like the dry woodland and savanna presently found in North and East Africa south of the Sahara, and the migration is directed towards those areas (higher latitudes) where environmental stability and production during the breeding season is higher than in the non-breeding areas (synchronization by a pulse climate). The migratory strategy is most fruitful for species with structurally simple niches, like waders, terns and swallows. Prerequisites are that there birds escape competition during the breeding season by migrating to marginal areas, and also that their productivity in the breeding areas is high enough to enable a sufficient number of young to return to the non-breeding areas and to compete successfully with the remaining part of the population.