THE DENSITY OF LAND BIRDS ON THE TRES MARÍAS ISLANDS IN MEXICO: II. DISTRIBUTION OF ABUNDANCES IN THE COMMUNITY

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Abstract
Census data from the Tres Marías Islands and adjacent mainland have been applied to a model which predicts the abundance of bird species in a community. The mainland data fit the model; the island data do not. The most common species on the islands is considerably more common than predicted. Departure from prediction of this type is a feature of several island bird communities, and is related to the number of bird species in the community. The considerable abundance of the most common species on islands is considered to be indicative of the opportunities afforded by the absence of potentially competitive species and by paucity of predator species.When species which are ecologically similar and in the same genus occur together on islands they differ in density by a greater amount than do their mainland counterparts. This is attributed to a greater need for exploiting different resources on the island than on the mainland because of the restricted nature of the former.