Rappole J.H. & Jones P. (2002) Evolution of old and new world migration systems. ARDEA 90 (3): 525-537

The apparent nomadism of Palearctic migrant birds during the wintering period in Afrotropical savannah resulted in their characterisation as wandering interlopers, subordinate to Tropical residents, forced to subsist on temporary resource concentrations in marginal habitats. These ideas persist, and, indeed, provide the theoretical basis for many modern studies of migrant evolution. However, recent studies have established that migrant ecology is more complex, with many migrants using stable resources and demonstrating site fidelity to a broad range of Tropical habitats, both within and between wintering seasons. Furthermore, intra-Tropical movements are now known to occur in a number of Tropical resident species as well as migrants, and appear related to resource and habitat seasonality. These findings, plus extensive taxonomic evidence, indicate that most long-distance migrants to temperate and boreal breeding sites are derived from Tropical-breeding resident birds, emphasising the critical importance of taking into account selective factors and evolution occurring during the non-breeding portion of the life cycle. The three principal Holarctic migration systems are similar in these and many other aspects, but differ in that, while roughly one third of the Nearctic/Neotropical and eastern Palearetic/Asian Tropical migrants winter in forest, almost none of European/Afrotropical migrants do so. We propose that habitats connecting Holarctic and Tropical regions serve as a filter for potential migrants to the Holarctic, and that absence of forest from northern Africa has limited exploitation of European forests by Afrotropical species