An Ecologic Analysis of Three California Avifaunas

Abstract

One facet of ecology is the study of the adaptations and adjustments which exist between animals and the environmental conditions surrounding them. Studies of such adjustments are made in the laboratory by placing an animal under a particular set of conditions and measuring its response when a condition is changed. Similar studies of adaptation can be made in the field on either a single species or an entire fauna. Here, variation in environmental conditions may be studied in two ways: one may remain in a single area centering attention on adjustments made by animals to the seasonal changes in the environment, or one may study several areas at the same season each having different environmental conditions and compare the responses of the animals. To make such field studies of adaptation in birds, one requires a method of analyzing the composition of an avifauna which applies at all seasons of the year (this rules out analyses based on some aspect of breeding behavior) and which will permit comparison between avifaunas having differing species composition. The feeding habits and the foods taken provide a basis for such an analysis. The method of analysis described in this paper, while technically original, is derived from similar systems and concepts used by many workers. Particular acknowledgement should be made to the writings of Colquhoun and Morley (1943), Moreau (1948), and Smith (1951), and to the teachings of Loye Miller. The analysis is performed by sorting the members of an avifauna into four major groups based on the feeding station within the habitat where they feed. In order of increasing height off the earth, these are: "ground"; "timber," a term used to include the trunks and larger branches of the trees and shrubs; "foliage," used to include both the leaves and the twigs supporting the leaves; and "air." For purposes of simplicity no distinction is made between feeding stations in shrubs and in trees, nor is any effort made to distinguish between stratd of different height within a forest. Additions of this sort are possible where the community structure is complex enough to make them desirable.