Evolution of Old and New World Migration Systems

Many of the ideas and controversies regarding the origin, evolution, and significance of avian migration derive from investigations of Old World migrants wintering in the European subtropics and central African Tropics. One of the critically important studies in the development of these ideas was the work by Morel & Bourlière (1962) on migratory bird ecology in Senegal, from which they concluded that migrants as a class were subordinate to Tropical resident species, constituting a floating population (population flottante) of wandering consumers taking advantage of the seasonal surplus existing in their wintering areas. MacArthur (1972) elaborated on these observations, using wintering migrants as examples of ‘fugitive species,’ i.e., ‘species that are able to survive by their relatively quick and temporary occupancy of suitable new habitats as these first become available.’ (MacArthur & Wilson 1967). Several studies in both the African and New 525