BULLETIN FLORIDA STATE MUSEUM


Brac by C. H. Blake (Bond 1958), and the species became well estab-
lished on that island around "inhabitated areas" by 1966 (Harvey, in
Bond 1967). Harvey also noted the species on Little Cayman in the same
year. In the summer of 1971, I recorded nine well-spaced individuals
on Little Cayman during two hours of observation along 8 km of that
island's newly constructed perimeter road. It is apparent that construc-
tion of this road has created open habitats preferred by this species.
The mockingbird probably immigrated to the two smaller islands from
Grand Cayman where it continues to increase in number, probably in
response to continued clearing of mangrove swamps and logwood forests,
thus increasing its preferred habitat.
 As discussed elsewhere in this paper, the distribution of the genus
Vireo on these islands is curious, because V. crassirostris occurs on all
three islands with either V. magister (Grand Cayman) or V. altiloquus
(Little Cayman and Cayman Brac). No more than two species of Vireo
are sympatric on a given island. Bond (1966b) mentioned a single old
record of two V. altiloquus on Grand Cayman that probably represented
vagrants.
 The fragmentary data available on the ecology of Mimocichla ravida
 (English 1916) and Icterus leucopteryx on Grand Cayman conform to
 Mayr's (1965) belief that small population sizes and probable genetic
 uniformity have made such populations exceptionally vulnerable to the
 smallest environmental change. This is another way of invoking genetic
 drift as a contributing factor in the extinction of these two forms on
 Grand Cayman. Both existed in quite restricted habitats (thus reducing
 gene flow), and their population densities in any one year probably never
 exceeded 100 breeding pairs. In these small populations, by genetic drift
 some alleles favored by selection could have been lost and less favored
 ones, perhaps lethals, could have increased in frequencies. These birds
 might not have been able to adapt to ensuing environmental changes.
 As stated before, the nature of that change, or changes, is unknown, but
 possibilities would include some habitat disturbance or hurricane effects.
 Furthermore, each of these small Cayman Islands appears to exemplify
 Mayr's statement (1965: 1587) ". . that the smaller the island the lower
 the percentage of endemic species . ." In fact, plotting data from these
 small islands on Mayr's figure 2, the linear relationship between double
 logarithmic plottings for island area on percent of endemic species would
 become curvilinear, the line extrapolating to zero endemic species with an
 island area of approximately 70 square miles or less. According to this
interpretation, none of the Cayman Islands should have any endemic
species, and currently they do not.
 In the first analysis of the birds of these islands, Cory (1886) de-


Vol. 19, No. 5