BULLETIN FLORIDA STATE MUSEUM


3); breeding of this species has not been confirmed for Cayman Brac,
an island where open water and mangrove swamps are quite limited.
 Columba leucocephala and Amazona leucocephala evidently do not
 currently breed on Little Cayman, although both species are at least
 moderately common on the other two islands. Longtime human residents
 of Little Cayman report that both of these species breed on Cayman Brac
 and that small flocks of each undertake daily round-trip flights to Little
 Cayman for feeding purposes only. Leptotila jamaicensis, a large ground-
 feeding dove, is a species with one of the most restricted ranges of all
 the Cayman Island avifauna, being found only in the remote inland lime-
 stone forests of Grand Cayman. Why it does not occur on the other two
 islands is unknown. Quiscalus niger, a ubiquitous and conspicuous resi-
 dent of both Grand Cayman and Little Cayman, is virtually unknown
 on Cayman Brac. Small groups sometimes fly back and forth between
 the two smaller islands (8 km) but there is presently no rational expla-
 nation for its absence as a breeding bird on Cayman Brac.
 The eight remaining species in Table 14 present puzzling distribu-
 tional patterns among the three islands. Why are Colaptes, Centurus,
 Myiarchus, Vireo magister, Spindalis, and Melopyrrha all restricted to
 Grand Cayman? Why does not Mimocichla plumbea occur permanently
 on Grand Cayman and Little Cayman, the latter only 8 km from Cayman
 Brac? Apparently, favorable habitats for this species would be the same
 for all three islands. One is tempted to explain these patterns by sug-
 gesting that the original successfully breeding pair(s) reached only one
 island (usually the largest) from the source population. In subsequent
 times, perhaps fortuitously, propagules have failed to reach the other
 islands, and the populations now established on the single island became
 sedentary. This could be the explanation for these eight species. Is it
 probable, however, that of the four forms with definite Cuban affinities
 (Colaptes, Centurus, Melopyrrha, and Mimocichla), all but the latter one
 should colonize only Grand Cayman?
 In reviewing all these distributional patterns, I am inclined to support
a chance-colonization hypothesis for a number of reasons. In the first
place, preferred ecological formations on Cayman Brac, for example, ap-
pear to be potentially suitable to all these terrestrial species currently
restricted to Grand Cayman, and it is likely that they simply never
reached the other islands. Alternatively, populations could have become
extinct. Second, two species (Zenaida asiatica and Mimus polyglottos)
have within quite recent years spread from island to island and now
maintain breeding populations on islands where each was absent as
recently as 25 years ago. Third, Mimocichla plumbea has been recorded
as a vagrant on Grand Cayman (Johnston 1969) where an unmated fe-


Vol. 19, No. 5