JOHNSTON: CAYMAN ISLAND AVIFAUNA


 My own field work and collections on these islands were initiated in
1965 and have continued intermittently for eight years, covering all sea-
sons and all three islands, although most of the data were taken on Grand
Cayman. Additional valuable field notes, specimens, bird stomachs, and
other experiences on the islands in the past decade have been kindly
shared with me by Albert Schwartz, Erma J. Fisk, Alexander Cruz,
Ronald Pulliam, Jon C. Barlow, and Donald W. Buden.

 ECOLOGICAL FORMATIONS

 So that the distribution of the avifauna can be analyzed generally,
major ecological formations for the islands are proposed, defined, and de-
lineated here. Fundamental to the definition of the formations are major
and minor plant associations, distinctive topographic features, and perti-
nent geologic, edaphic, and hydrologic conditions. These ecological for-
mations are similar to, if not identical with, some of the major plant
communities described by Asprey and Robbins (1953) from Jamaica.
Implied for each formation are those general and specific niche require-
ments essential to the successful occurrence of each of its avifaunal com-
ponents, including food and other trophic relationships, available nesting
and feeding sites, and protective cover. In addition to the plant com-
munity analyses by Asprey and Robbins (1953), other reports have
proven valuable in the definitions of these formations, namely those by
Doran (1954) and Richards (1955) on geology, and Swabey and Lewis
(1946) on forestry.
 In evaluating insular avifaunas it is sometimes useful and desirable to
include a "marine littoral element," essentially pelagic birds depending
upon offshore waters for food and the island for breeding sites. Although
this contingent is not considered here as a distinct, important ecologic
formation, it is emphasized that at least four species of birds breeding
on the Cayman Islands would be considered pelagic during the non-
breeding season-Phaethon lepturus, Sula leucogaster, S. sula, and Fre-
gata magnificens.
 FRINGING REEFS AND LAGOONS.-For the most part the three islands
possess fringing coral reefs that enclose shallow lagoons or sounds (Fig.
5), although on Cayman Brac the reefs are limited to the southwestern
tip. The reefs, usually 30 to 100 m offshore, typically vary from about
0.5 m below sea level to about 0.3 m above. Water in the enclosed la-
goons may be 3 m or more deep but is usually less than 1 m deep. De-
spite the incessant pounding of surf on the reefs and water depth of the
lagoons, both are visited periodically for food by some herons, egrets, and
terns, plus an occasional shorebird, especially at low tide.