Ecological Compatibility of Bird Species on Islands

Abstract

The difficulties of coexistence on islands for ecologically similar species of birds are indicated by the paucity of homogeneric species, and by the tendency for their bill lengths, and hence their feeding characteristics, to diverge. Members of the most similar pair of homogeneric species on the Tres Marias Islands differ in bill length by 21%. This is greater than the difference between members of 12 out of a total of 14 mainland pairs which are each represented by one species only on the islands. It is suggested that in some instances both members of the mainland homogeneric pairs have reached the islands, but that one member has been eliminated as a result of interactions arising from mutual ecological incompatibility. It is further suggested that this process has been important in the adjustment of number of species to island area.