THE EVOLUTION OF PALEARCTIC MIGRATION - THE CASE FOR SOUTHERN ANCESTRY

The thesis developed in this paper is that the phenomenon of bird migr ation is founded on (1) pre-adaptations for flight that reduce the cos t of long-distance directional movement, and (2) intensive reproductiv e effort that makes the use of the northern short season of high food abundance very beneficial for breeding. It is proposed that the first step in the evolution of migration is long-distance pre-breeding dispe rsal of a fraction of the juveniles of southern species to northern la titudes, where they breed and then disperse south. Further adaptations for timing and navigation mechanisms turn the dispersal into directio nal seasonal migration. Finally, reproductive isolation, drift, and se lection result in speciation of the northern-breeding migratory fracti on from the nonmigratory southern-breeding ancestor species. The evide nce supporting this hypothesis is scanty and circumstantial, and inclu des the high incidence of migration among northern species, several ex amples of species that represent steps in speciation of Palearctic mig rants from resident African relatives, and the high incidence in many avifaunas of ''vagrants'' and ''occasional breeders'' that may represe nt the initial stages of an ongoing evolution towards migration. Resea rch attention should be paid to the phenomenon of post-fledging juveni le dispersal as a potential precursor of the evolution of migration.