JOHNSTON: CAYMAN ISLAND AVIFAUNA


 A GENERAL ASSESSMENT OF FEEDING ECOLOGIES AND
 HABITAT DISTRIBUTIONS
 It is instructive now to generalize and summarize data on feeding
ecologies or feeding niches of the resident terrestrial avifauna. As shown
in Table 17, most of the bird species (57% ) are arboreal foragers, either
on arthropods or fruits. Furthermore, with the expected exception of
pastures and cultivated areas, these same species are each widely dis-
tributed in the seven upland formations and have comparable frequency
of occurrence and relative abundance therein. Conversely, ground-feed-
ing forms (21'i ) are relatively less common as a whole in the Caymans
and are virtually restricted to the non-forested formations. The latter
distribution strongly suggests the scarcity of insects and seeds on the
ground in the several forest formations. Indeed, ground cover is notice-
ably scarce in the several forest formations due to a combination of
factors (edaphic karst topography, light penetration). The relatively
high proportion of arboreal insectivores and frugivores, augmented by
the timber-probing and -gleaning woodpeckers would be expected on
islands, such as the Caymans, where forest formations of several types
predominate in areal extent (see Figs. 1-3).
 Bird species diversity is a known function of habitat or plant species
diversity. Although plant species diversity was not determined quantita-
tively in the present study, it is nonetheless empirically true that in the
comparisons of avifaunal compositions of the several ecological forma-
tions, bird species diversity does attain its peak in those forest formations
with the greatest plant diversity. Complexity of floristic composition
(presence of many codominant trees vs. one or two dominants, height of
vegetation, some semblance of stratification, etc.) may be sequentially
arranged in descending order (number of bird species in parentheses)
as follows: towns and house sites (20), limestone forests (17), logwood-
thatch palm-red birch forest (15), sea grape-almond woodland (16),
mangrove swamps (11), pure logwood forests (11), and pastures and
cultivated areas (11). A relationship between number of terrestrial bird
species and habitat complexity is implied by these data.

 TAXON CYCLES
 A subject of recent interest to some island biogeographers is that of
taxon cycles, and for the West Indian avifauna these have been explored
generally and in detail by Ricklefs and Cox (1972). Whether an anal-
ysis of taxon cycles for individual small islands or archipelagos is mean-
ingful (see Ricklefs 1970, for Jamaica) is a moot point because of great
areal and relief differences in large island groupings, but an attempt at