THE NEARCTIC-NEOTROPICAL BIRD MIGRATION SYSTEM

ABSTRACT

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