JOHNSTON: CAYMAN ISLAND AVIFAUNA


these. Attention must be given to the relative population densities of
each species in each formation: where petechia is very common (sea
grape-almond woodland and mangrove swamps), D. vitellina is absent;
where D. vitellina is fairly or very common logwoodd, logwood-thatch
palm-red birch, and limestone forests), D. petechia is at best uncommon.
To some extent then, these species are segregated by habitat.
 Feeding heights of these warblers are also important, D. vitellina
choosing the lower strata up to about 3 m, and D. petechia usually oc-
curring from 3 m upwards. At present no available data support an
hypothesis that the two species either have different foraging habits or
select different foods.

 PREDATION AND POPULATION CONTROL

 Of special importance for the bird populations of the Cayman Islands
is the obvious paucity of vertebrate predators. Grant (1940) docu-
mented systematics of the herpetofauna but devoted little attention to
their biological features. He did note (p. 49), however, that the colubrid
snake Alsophis angulifer is "diurnal, active, feeds on lizards, frogs, birds
S. .Lewis took a number of specimens in the tops of trees, one over 30
feet off the ground . [where they were feeding on] Hylas." On Cay-
man Brac a specimen of this snake, reported by Grant (p. 50), had a
honeycreeper in its stomach. In my several years of observing and col-
lecting on these islands, I never encountered an Alsophis pursuing birds
or with a bird in its stomach, nor did any of the local people indicate an
ornithophagous habit for this species. Still, Grant does provide some
evidence that the snake at least occasionally preys on birds. The abun-
dant Anolis lizards, especially A. conspersus, might prey on birds' eggs
(there is no concrete evidence) because elsewhere in the West Indies
Anolis occasionally eats eggs of honeycreepers (Biaggi 1955). No mam-
malian carnivores occur on the Caymans. Both Mus musculus and Rattus
(mostly R. rattus but some R. norvegicus) are found, but they are only
locally abundant. Specimens of these mammals have been collected or
observed chiefly in areas of human habitation and not in the "wilder"
portions of the islands.
 A conspicuous hiatus in the avifaunal trophic structure is a diurnal
predator. In winter an occasional hawk (Circus cyaneus, Falco sparver-
ius and F. columbarius) does visit the islands (Johnston et al. 1971).
The food habits of these predatory birds in the Caymans are unknown,
although elsewhere birds do constitute some portion of their diets. But
even if they did prey on an occasional (or more) land bird in winter,
the small numbers and infrequent occurrences of these hawks would


1975