Competition between Two West Indian Flycatchers, Elaenia

SIBING species whose ranges overlap generally exhibit some measure of ecological and morphological displacement in the area of sympatry.This presumably results from competition for common limiting factors, but as such a process is self-effacing it has seldom been observed, and its reality must be adduced from its consequences.The thesis is strengthened by the finding that differences are often less marked in regions of allopatry.Several such cases are reviewed by Lack (1944).Few of these studies have been extended to determine the effects of interspecific competition on population density.The greater density of passerine populations in Bermuda (Crowell, 1962) suggests that competition in the larger North American communities limits the populations of these species there, but relationships between particular species are not clear.A quantitative study of competition between congeneric species thus seemed desirable.Islands are ideal for such • a study because areas of sympatry and allopatry are sharply defined, while the fact that each family is represented by fewer species makes competitive relationships between these species more evident.Such a situation is reported here.The ranges of the Yellow-bellied Elaenia (Elaenia/lavogaster), a species of Central and South America, and the Caribbean Elaenia (E.martinica), a West Indian species, overlap in the Windward Islands (Figure 1).The area of sympatry includes the islands of Grenada and St. Vincent; the two species occupy different habitats on Grenada, but coexist on St. Vincent (Bond, 1948).E. /lavogaster occurs alone on Trinidad, and E. martinica alone on St. Lucia.I studied th'e distribution and ecology of the two species on their respective islands during the spring of 1961, visiting St. Lucia in late March and mid-April, St. Vincent in early April, Grenada in late April, and Trinidad in early May.