THE ROLE OF COMPETITION IN THE EVOLUTION OF MIGRATION

One of the major objectives of modern ecology is analysis of the role of ecological processes in the evolutionary origin of adaptations. The problem of the origin of migration, however, has received little attention from this standpoint. Even for birds, in which the phenomenon of migration has been a major focus of research, most attention has been directed to the study of proximate factors important in initiating migration and the physiological processes and orientation mechanisms involved in the actual movements. Few studies have concentrated on the ultimate factors responsible for the origin of these complex mechanisms. General theories of the evolutionary origin of bird migration have been in existence for a long time. Thomson (1926) and Lincoln (1939) have summarized early theories, which' tend to fall into two main groups: (1) northern ancestral home theories, and (2) southern ancestral home theories. Northern ancestral home theories assume that most presently migratory forms were permanent residents in preglacial northern areas corresponding approximately to their present breeding ranges. With advance of continental glaciers and attendant climatic changes these birds were forced southward, from where they either attempted to return yearly to breed as close to their ancestral home as possible, or in some way retained a racial memory of the ancestral home, to which they were able to return seasonally following glacial retreat. It is often suggested that the stimulus for these return movements is the crowded conditions created in tropical and subtropical areas due to the northern birds being forced south. Southern ancentral home theories assume that most present migrants were originally permanent residents in southern areas, possibly corresponding to their present winter ranges. Following glacial retreat these species began to invade seasonally favorable northern areas for breeding as a result of competition for food or breeding sites in southern areas. Some authors (Berlioz, 1950; Dorst, 1961) have suggested the operation of both of the above patterns. Although these ideas have received extensive criticism (Mayr and Meise, 1930; Wolfson, 1948; Moreau, 1951) they have found their way in almost unmodified form into some recent textbooks of ornithology (Wallace, 1955; Welty, 1963). Mayr and Meise (1930) have suggested that the existence of areas with seasonally favorable and unfavorable conditions, rather than the occurrence of glaciation, is the condition required for the evolution of migration. They suggest that the normal reproductive excess, primarily through competition for food, is the factor favoring development of mechanisms allowing seasonal occupation of such areas. Although recognizing that migration must thus have existed in preglacial times, they conclude that, due to the great restriction of migration routes which must have occurred during glacial maxima, modern migration route patterns are almost entirely post-glacial in origin. They show that extreme crowding of birds in tropical and subtropical areas during glacial maxima is unlikely, and hypothesize that changes in distribution patterns and migration routes during glacial advances and retreats are the results of extinction of local populations and dispersal resulting from normal reproductive excess. Intraspecific competition and intensification of migratory drive are suggested as factors important in the elongation of migration routes and the origin of "leapfrog" patterns in which races breeding farthest north winter