JOHNSTON: CAYMAN ISLAND AVIFAUNA


resident breeding complement) separately, it is apparent that the climax
limestone forest does not support the densest population in this sere. An
explanation for the relative decrease in winter in this climax formation
may be found in part in the "irregularity principle" cited earlier (Willis
1966). Perhaps, in turn primary productivity is higher annually in the
subclimax logwood-thatch palm-red birch formation than in the limestone
forest.
 Note should also be made of the changing proportions between seral
stages of uncommon and fairly common species to the common and very
common ones. In early seral stages common and very common species
represent 66 percent of the birds, but in the subclimax and climax stages
these classes comprise only 25 percent of the breeding avifauna. Con-
versely, in the climax seral stage the proportion of uncommon and fairly
common species increases. A completely satifactory explanation of this
phenomenon cannot be provided at the present time, but several factors
are influential. First, in both the subclimax and climax stages, there is
an obvious increase in niches available to the avifauna, the niches mark-
edly influenced by increasing tree species diversity and attendant strati-
fication of the foliage. Second, among the species found only in the
limestone forest and yet ranked only as uncommon or fairly common are
Amazona leucocephala, Columba leucocephala, and Leptotila jamaicensis.
All these birds are large in body size and from numerous field observa-
tions appear to have large home ranges. Thus, on a unit area basis each
of these species would maintain lower densities than smaller birds with
smaller territories. Finally, a factor undoubtedly influencing the habitat
distribution of these three species is food. All are frugivores and on
Grand Cayman their preferred fruiting trees during the breeding season
are most abundant in the limestone forests (see Appendix III).

 BREEDING SEASONS

 Breeding seasons of tropical and equatorial birds have received the
attention of biologists especially interested in elucidating causative factors
underlying the seasonality. There is a dearth of published information
for insular avifaunas, so the present data from the Cayman Islands
should be of value despite their geographic location north of the equator.
 Few concerted efforts have been made by any single investigator to
obtain quantitative breeding data for any species on these islands, al-
though such long-term efforts would certainly be rewarding for the anal-
ysis and interpretation of annual population fluctuations, natality, mor-
tality, and population controlling mechanisms. Indeed, it is a sad com-
mentary that no one ever recorded breeding data for the extinct Grand


1975