THE NEARCTIC-NEOTROPICAL BIRD MIGRATION SYSTEM

A review of the Nearctic-Neotropical migration system reveals that: (1 ) 90% of breeding land and fresh-water species in the sub-Arctic are m igrants, 80% in southern Ontario, and 50-60% in Florida and Arizona. ( 2) Insectivorous Panulinae winter from 30 degrees N to 10 degrees S; s eed-eating Emberizinae from 40 degrees to 15 degrees N; aerial feeding Tyrannidae and diurnal birds-of-prey trans-equatorially; and ducks fr om 50 degrees to 5 degrees N. (3) Go-occurrence of migrant species is facilitated by geographic and habitat allopatry, varying levels of eco logical distinctness, sparse dispersal, and, sometimes, intraspecific spatial separation of sexes and age-groups. (4) Migrant-resident co-oc currence is facilitated by high proportions of the two belonging to di fferent taxonomic and ecological groups and, within families, partial feeding zone and habitat segregation. All, however, draw on a common f ood pool. Too little is known about the winter food support base, diff erential habitat utilizations, species abundances, and biological need s, either to gauge whether interspecific competition has been a major evolutionary factor, or to differentially predict the efforts of futur e deleterious habitat change. In two appendixes, the Nearctic-Neotropi cal, Palearctic-African, and Asian migration systems are compared, and data pertaining to current population trends in Neotropical migrants are compiled.