BULLETIN FLORIDA STATE MUSEUM


suggested that a character difference (ratio of larger to smaller bill
length) greater than 1.14 was typical of sympatric congeneric insular
birds. Congeners with such bill length differences would reduce com-
petition by selecting different-sized food particles. Of the five congeneric
pairs of birds on the Cayman Islands, two have a bill character difference
less than 1.14: Dendroica petechia-D. vitellina (1.01) and Tyrannus
dominicensis-T. caudifasciatus (1.02) (bill measurements taken chiefly
from Ridgway 1901-1950). In the two Dendroica species stomach anal-
yses revealed few qualitative differences in food choices (see discussion
beyond and Appendix III); competition between these forms is avoided
primarily by differences in habitat choice as well as feeding heights. The
Tyrannus species, although possessing similar bill lengths, (1) take dif-
ferent foods, (2) are found in different habitats, and (3) obviously feed
in different fashions. Bill size differences are, therefore, not always the
most important means of avoiding competition in all the Cayman Island
congeners. As will be discussed later in this paper, congeners and other
closely related forms clearly have evolved a spectrum of mechanisms that
in various combinations facilitates coexistence. It should be noted that
in the relatively short time that birds have been studied on these islands,
there is no assurance that competition is not causing a gradual exclusion
of one species or a slow transition into a new ecologic niche for another.
 DOVES AND PIGEONS (COLUMBIDAE).-A nearly linear relationship
exists between island size and number of species of Columbidae in the
West Indies (A. Cruz, pers. comm.). Island size is not necessarily the
prime factor, however, because certainly habitat diversity and distance
to the source population are of major importance. The largest West
Indian islands support the highest numbers of species of Columbidae
(Jamaica 10, Cuba 11, Hispaniola 10), but these islands also have greater
relief and habitat diversities than are found on more xeric, flatter West
Indian islands, such as the Caymans. Furthermore, on no island are the
several dove and pigeon populations and densities necessarily the same;
Columba inornata and Geotrygon passerina of Jamaica are both relatively
rare, as compared with the common Columbina passerina and Columba
leucocephala on that island. Relative population densities of the doves
and pigeons on the Cayman Islands seem to be, in decreasing order of
abundance: Zenaida asiatica, Columba leucocephala, Columbina pas-
serina, Z. aurita, and Leptotila jamaicensis (the latter restricted to Grand
Cayman). Thus in terms of a given island, it is ecologically misleading
merely to enumerate species without also considering relative or absolute
abundances and habitat diversity.
 The question posed here is: how can each of the Cayman Islands sup-
port its complement of four or five species of columbids? Or, how do


Vol. 19, No. 5