BULLETIN FLORIDA STATE MUSEUM


tend to minimize their predation pressure on the avifauna. On the other
hand, the resident Barn Owl (Tyto alba) has been shown recently to
be a potent predator on birds (Johnston 1972), even though its numbers
are low. The regurgitated pellets of these owls taken from five widely
scattered sites on Grand Cayman, and quite likely representative of five
different individuals, revealed a high proportion of avian remains among
their prey, ranging in size from a medium-sized egret (Egretta) to a
honeycreeper (Coereba). Furthermore, the prey items included at least
eight genera of the resident birds. These data suggest that of the actual
or potential vertebrate predators on the avifauna only Tyto alba is espe-
cially important, and I suspect this predator represents the greatest single
biological control of the resident avifauna.
 The ubiquitous hermit and land crabs are clearly potential scavengers
or predators of any terrestrial animal life. Only the Nighthawk (Chor-
deiles minor) is a consistent ground-nesting bird on the Caymans, how-
ever, and because this species is scarce and breeds only in bare fields
where crabs are also scarce, it appears highly unlikely that even this
bird, its eggs, or young would be preyed upon by the crabs. Interest-
ingly, where land crabs are most abundant on the islands (rocky barren
roadsides and limestone forests), no ground-nesting birds are known,
unless it is a very occasional dove (Columbina passerina or Zenaida
aurita).
 The widespread and abundant Coereba flaveola is conspicuous
throughout the Caymans and elsewhere in its range. In most West In-
dian island avifaunas, it is the most abundant terrestrial resident species.
Controlling mechanisms for this species were documented by Gross
(1958) and include bird predators (Quiscalus, Crotophaga) as well as
ants and lizards. In the absence of concrete evidence, we can only as-
sume that similar predators prey on Coereba in the Cayman Islands.
Coereba is an occasional victim of the uncommon Barn Owl (Appendix
III).
 The extent to which other biological and physical factors exercise
any control on population size of birds is as yet unassessed. For example,
for the Caymanian avifauna no quantitative data are available on annual
population fluctuations, clutch size, hatching success, fledging success, or,
indeed, natality rate for any bird species especially in comparison with
other insular or mainland populations. Two species are increasing in
numbers and range in the Caymans, Mimus polyglottos and Zenaida
asiatica. Conversely, the only documented recent losses there are of
Mimocichla ravida and Icterus leucopteryx bairdi; causative factors bear-
ing on their extinctions have been discussed in an earlier section.


Vol. 19, No. 5