BULLETIN FLORIDA STATE MUSEUM


competition specifically with the ubiquitous Bananaquit (Coereba fla-
veola). Throughout the rest of the West Indies and Central America
where hummingbirds and Coereba are sympatric, there is much evidence
that hummingbirds dominate Coereba at food sources (see, for example,
Wetmore 1927). Furthermore, hummingbirds and these bananaquits
generally feed in different fashions, though often utilizing the same food
source simultaneously. None of these features suggest that competition
does or would play a significant role in explaining the absence of hum-
mingbirds on the Caymans.
 Intriguing questions on niche breadth, occupancy, and vacancy are
 invoked by these inter-island distributional patterns. Characteristic
 timber-probing woodpeckers (Colaptes and Centurus) are absent from
 the two smaller islands. For these woodpeckers island size and habitat
 diversity are important limiting factors. Three species that are frugi-
 vorous in low trees or on or near the ground surface on Grand Cayman
 are Leptotila, Spindalis, and Melopyrrha. Are these feeding niches
 simply vacant on the smaller islands? They probably are, but not
 enough is known of comparative feeding behaviors of species with
 similar feeding patterns where a competitor is absent. For example, it
 is possible, though not documented, that Elaenia has a broader feeding
 niche on Cayman Brac (both qualitatively and quantitatively) than on
 Grand Cayman, where it undoubtedly competes with Spindalis, Melo-
 pyrrha, and other species for small fruits. Similarly, the absence of the
 foliage-gleaning insectivorous Myiarchus on the two small islands might
 release potential competitors (Vireo, Dendroica) to the extent that their
 feeding behavior patterns are broadened on Little Cayman and Cayman
 Brac. These and related points certainly merit future intensive studies.

 IMMIGRATION AND EXTINCTION

 An equilibrium model for the number of species on an island was de-
veloped by MacArthur and Wilson (1967), who predicted that the num-
ber of species on an island is determined by a balance between
immigration and extinction rates. Our current knowledge of the Cayman
Island avifauna provides some concrete examples in support of this
model, despite the fact that few bird species and a relatively short period
of time are involved. There is sufficient evidence, for example, that
two birds (Mimocichla ravida and Icterus leucopteryx bairdi) are now
extinct on Grand Cayman, where formerly (1900-1916) both were mod-
erately common and were last observed in the 1930s (Johnston et al. 1971).
Unfortunately, no precise information is available on the causes of their
extinction, whether by hurricanes, partial habitat destruction, human dis-


Vol. 19, No. 5