Dynamics of Bird Communities in the Chihuahuan Desert, New Mexico

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Abstract
A summary of monthly census data for all the avian species occurring on 3 desert and grassland sites in southern New Mexico [USA] is presented. To obtain absolute density estimates of birds, raw data were corrected using a coefficient of detection. To simplify interpretation, the data were grouped into ecologically similar groups of species. Trends within and between years are discussed. Grasslands showed considerably more birds, principally seed-eaters, in the winter than in the breeding season. These remained longer in 1972-73 than in the previous year. The Playa had more insectivores in summer, but its flooding in the autumn of 1972 caused a considerable increase in doves and other seed-eaters which persisted through winter. The Bajada had its peak numbers during breeding season, but the insectivores present were fewer than the numbers of seed-eaters present at other sites during winter. Comparison of these areas with other grassland and desert data show an increasing dominance of winter numbers in the southern sites. The number of birds was related to food supply but the relationship was complicated by 3 factors: the availability of food could be modified by the presence of competitive groups, such as rodents, for seeds; the distribution of food items could be such that it may be uneconomical to exploit these resources unless the items achieve sufficient densities (e.g., flash flooding in autumn could dramatically change the distribution of seeds); and certain items could be unexploitable because of their size (e.g., small insects) or for unknown reasons (in the case of creosotebush seeds).