JOHNSTON: CAYMAN ISLAND AVIFAUNA


small islands. Therefore, based on comparative data for the Caymans
(Grand Cayman=1.18, Little Cayman=1.25, Cayman Brac=1.27) and
those of Cuba (S/G=1.26) competition is of about equal intensity on
the Caymans and Cuba. As Simberloff indicated, small islands can sup-
port many similar species, and insular avifaunas will likely contain species
that occupy more habitats because of less competition within habitats.


 THE WINTER AVIFAUNA

 Influences of the many North American migrant birds on a resident
tropical avifauna have not been widely investigated (but see, for example,
Eaton 1953 and Willis 1966). Despite intensive studies by a few in-
vestigators, most of the major questions remain unanswered or poorly
documented. Thus the interrelationships permitting coexistence between
migrants and residents require close scrutiny, but on a protracted basis
in the field. On the Cayman Islands a detailed investigation was not
undertaken, but a number of general and interesting ecological features
have been at least partially elucidated.
 As indicated elsewhere in this paper, the resident terrestrial avifauna
of all the Caymans includes some 27 species; in winter at least an addi-
tional 20 reasonably common species also occur. Some 10 other species
are currently known from three or fewer winter records (c.f. Johnston
et al. 1971), but in time some of these forms might prove to be more
common. For the most part the wintering birds occur in loose mixed-
species flocks (especially parulid warblers), a habit also reported for
Cuba (Eaton 1953) and Puerto Rico (Recher 1970). Whether any in-
dividuals are sedentary and maintain winter territories remains unproven
for the Cayman Islands, but observations of at least the Seiurus warblers
suggest this to be the case. The 20 wintering species are found on the
islands mainly in the wooded ecological formations-logwood and lime-
stone forests, mangrove swamps, and the sea grape-almond woodland.
Some of these birds occur in smaller numbers in the town-and-house site
formation, whereas the two wintering sparrows (Passerculus sandwich-
ensis and Ammodramus savannarum) are both restricted to pastures
and grassy fields. In the wooded formations an observer is immediately
impressed by the quantitative predominance of the North American
migrants. In Table 4, for example, note that the terrestrial migrant
species outnumber the breeding residents by 13:8 in mangrove swamps.
From these data and field observations, some immediate questions arise.
What ecological features permit coexistence between migrants and resi-
dents? What role does seasonal availability of food play? How do


1975