BULLETIN FLORIDA STATE MUSEUM


After a count was made of all individual birds recorded during each
census, relative scores were derived as follows:

 U (uncommon)-5-20 individuals/20 hr.
 FC (fairly common)-20-100 individuals/20 hr.
 C (common)-100-300 individuals/20 hr.
 VC (very common)-300 individuals/20 hr.

Excluded from the following summary tables are the rare or accidental
species, that is, those observed less than five times in the total counts.
 As a rule, bird species diversity is a positive function of vegetational
complexity (MacArthur, Recher, and Cody 1966). Foliage height diver-
sity is believed to be a good and reasonably accurate indicator of bird
species diversity, particularly for tropical avifaunas. In the Cayman Is-
lands, however, use of vegetation as an indicator was impractical for two
major reasons: (1) with the exception of some limestone and inland man-
grove forests, a given habitat studied for its avifaunal composition usually
did not exceed 5 m in height; and (2) the honey-combed limestone
terrain with its exceedingly dense vegetation (often impenetrable and
thorny) precluded attempts to accurately measure vegetational strata
and the organisms occurring in presumed strata. Rather, as will be in-
dicated later, attention was given to average relative feeding heights of
only potential competitors, especially congeners.
 BIRDS OF FRINGING REEFS, LAGOONS, AND SAND-CORAL BEACH.-Table
2 summarizes the relative abundances of birds in this formation. It is
important to note that few species are ever common here. Thalasseus
maximus is the most common year-around inhabitant of the reef-lagoon-
beach area where fish are plentiful in the shallow waters. Actually, no
birds breed in this formation in the Cayman Islands; the summer occur-
rences of Phaethon lepturus, Sula leucogaster, S. sula, and Fregata mag-
nificens represent birds feeding or flying over from nest-sites on nearby
bluffs or lagoons farther inland. Otherwise, most of the 18 species in
Table 2 are either migrants or winter residents that feed on the marine
fauna (chiefly invertebrates and small fishes) found in this formation.
Forms such as Florida caerulea and Hydranassa tricolor breed elsewhere
on the islands and make only occasional visits to the beach area.
 Of all the ecological formations studied here, this one has the lowest
bird species diversity (10 in summer, 13 in winter) and the smallest
populations.
 BIRDS OF SEA GRAPE-ALMOND WOODLAND.-As stated earlier, this
formation forms a belt around most of the islands and is an important
woody, terrestrial community for birds because it provides (a) suitable
nesting sites for 5-7 species, (b) a dependable food supply, and (c) at


Vol. 19, No. 5