JOHNSTON: CAYMAN ISLAND AVIFAUNA


male, previously unrecorded from this island, even constructed unsuccess-
ful nests in at least two successive years. The point is that with the pos-
sible exception of the congeneric vireos in which competitive exclusion
may be an important limiting factor (see discussion beyond), most if not
all of the terrestrial birds currently restricted to a single island could
probably successfully colonize any of the other islands (contra Bond
1934: 345-346). Perhaps the sedentary habits of species now restricted
to Grand Cayman and the distance (97 km) to the other islands have
served as barriers to their dispersal to the smaller islands. This latter
point, emphasizing a "poor" immigration rate of birds such as wood-
peckers, is in substantial agreement with the views of Ricklefs and Cox
(1972: 215).
 Among the most curious distributional features of the Cayman Island
avifauna is the absence of resident hummingbirds. This fact is especially
anomalous when it is recalled that (1) every other major island and land
mass east, west, and north of the Caymans all have several resident hum-
mingbird species (Moynihan 1968; Bond 1971), (2) hurricane paths in
the past argue for the possibility of widespread dissemination of birds in
the Caribbean region, and (3) occasional hummingbirds are known to
migrate through Grand Cayman (Johnston et al. 1971). Collectively,
these points tend to negate an hypothesis that hummingbirds could not
or have not reached the islands in the past. The problem is, I believe,
whether or not conditions are currently favorable for their existence in
the islands. The absence of breeding hummingbirds is related to two
factors, a dependable food supply and the possible role of potential
competitors.
 As pointed out elsewhere in this paper, the small size, low relief,
 desiccating winds, sparse soils, and porous limestone base of the Caymans
 preclude the support of luxuriant tropical vegetation, despite the mod-
 erately heavy, though seasonal, rainfall. The extensive expanses of lime-
 stone and mangrove forests of the Caymans (Figs. 1, 2, 3) are not
 characterized by the diversity, distribution, or abundance of flowering
 trees and shrubs that one finds elsewhere in the West Indies. Although
 quantitative support cannot be marshalled for this statement on plant
 diversity and distribution for the Cayman Islands, it is obvious that most
 of the favorite hummingbird nectar-bearing plants of Jamaica (Hibiscus,
 Bougainvillea, Nerium, Tamarindus, Psidium and many others) are at
 best limited chiefly to scattered and relatively sparse house sites on the
 Caymans. Thus, I believe that even if potentially immigrant humming-
 birds did currently find their way to the Caymans, a dependable food
 supply would not be available.
 Another consideration in the lack of hummingbirds is the possibility of