JOHNSTON: CAYMAN ISLAND AVIFAUNA


 3 species, in only 5 formations
 4 species, in only 6 formations
 1 species, in all 7 formations

By extrapolation one arrives at an average figure of 3.9 habitats per spe-
cies. Data for mainland populations (Fig. 19) are in sharp contrast; for
example in Georgia the figure is 1.6 habitats per species (Johnston and
Odum 1956) and for Panama about 1.3 (MacArthur et al. 1966). On
Puerto Rico the value is 2.5 (Recher 1970) and Jamaica 2.9 (Ricklefs
1970). For Grand Cayman the relatively large number of habitats oc-
cupied by each species indicates either reduced interspecific competition
per se, the evolution of mechanisms to avoid competition, or both.
These properties are discussed at length in the previous section on com-
petition.
 To exemplify these features more generally, a number of published
breeding censuses from typical island and mainland populations have
been reviewed with respect to number of available habitats occupied by
the species. This assessment is graphically summarized in Figure 19.
In regions where relatively few habitats are available (three or four),
mainland bird populations tend to be specialized, and as more habitats
become available (five or six), the mainland populations tend to become
even more specialized. Large islands, such as Puerto Rico and Jamaica,
include bird populations that are intermediate between the extremes of
generalization and specialization. On small islands (including Grand
Cayman) the bird populations reach peaks in generalization. Grand
Cayman is unique in this feature by having the highest known value of
mean number of habitats per species, namely 3.9.
 Lack concluded (1969: 207) that ". . the small numbers of resident
bird species on islands are due . to ecological limitations, to which
the islands' birds are often specially adapted, and which enable fewer
species with broader niches to exclude a greater number of specialists."
If I interpret this sentence correctly and if "ecological limitations" are
chiefly those of habitat diversification, then the graphic presentation of
Figure 19 supports his contention only for small islands. In fact, with
respect to number of habitats occupied by each species, the larger islands
of Puerto Rico and Jamaica, with their large number of available habitats,
resemble the mainland populations. Grand Cayman, Bermuda, and St.
John each tend to have (fewer?) species with "broader niches." The
extent to which such ecological generalization might "exclude a greater
number of specialists" is certainly undemonstrable from present evidence.
 Another, yet different, approach to this subject is Simberloff's species/
genera ratios (1970). Similar S/G values for large and small islands
would indicate, for example, that competition is at least as intense on the


1975