Does the migration programme constrain dispersal and range sizes of migratory birds?

Aim It is generally believed that the migration programme constrains the dispersal and hence range sizes of migratory bird species. This conclusion is based on analyses of breeding ranges of migratory versus non-migratory (resident) terrestrial bird species, and rests on the assumption that there are no ecological or evolutionary constraints on extending the non-breeding range. To investigate this assumption, the abilities of migrant and resident terrestrial species to colonize new wintering areas were compared. Location Three major wintering regions of long-distance migrants: South America, sub-Saharan Africa and the Indian Subcontinent. Methods It was determined whether the relative numbers of residents and shortand long-distance migrants were the same in those species that have dispersed to a novel wintering region as in the source species pools. Results At the species level, long-distance migratory species are more likely to have non-breeding ranges that include more than one of the above regions than resident species. This indicates that the dispersal of migratory species is less constrained than that of resident species. The pattern holds irrespective of the inclusion or exclusion of species associated with coastal, freshwater and wetland habitats, and also holds for ecological groups such as aerial feeders. The pattern is most pronounced between the regions separated by the strongest dispersal barriers (South America and sub-Saharan Africa). Main conclusions It is unlikely that the migration programme per se constrains dispersal, but rather that difficulties in establishing new non-breeding areas prevent range expansions in migrant species.