ENVIRONMENTAL HETEROGENEITY AND AVIAN COMMUNITY STRUCTURE IN SOUTHEASTERN ARIZONA SEMIDESERT SHRUB-GRASSLANDS.

The environments in which avian species exist fluctuate widely in space and time. In the grasslands of southeastern Arizona, there are annual cycles of rainfall which distribute rainfall unevenly throughout the year. Two peaks of rainfall usually occur, one in December and January, and a second in July and August. The temporal pattern and magnitude of rainfall can vary from year to year. There are also significant patterns of rainfall variation across the landscape, both long and short term. The end result of this variation is that the environment in which birds in southestern Arizona breed varies on a number of spatial and temporal scales. Several sites on the Santa Rita Experimental Range and the Research Ranch were censused during the summers of 1982 and 1983 to study the response of the avian community to heterogeneity in their environment. In the Santa Rita Experimental Range, two habitat types were studied: mesquite savannah and grassland. Assuming no temporal dynamics in community structure, mesquite habitats appeared to have higher total densities than grasslands in 1982 and 1983. However, this was true only during April-June. In July and August densities appeared to be higher in the grassland habitats. Patterns of species richness and eveness also appeared to be different when temporal dynamics were considered than when they were ignored. The timing of individual species appears to be responsible for the differences in community structure obtained by the two methods, and this suggests that the assumption of no temporal dynamics in community structure during the season is invalid. The densities of eight species of emberizids were correlated to characteristics of the vegetation on the Research Ranch and Santa Rita sites. One group of species appeared to be associated with open grassland habitats, while a second group appeared to be associated with mesquite habitats. There were a large number of nonlinear relationships of species with certain habitat measurements. While the associations of birds with habitat features were statistically significant, the causal relationships between habitat variables and avian densities were not elucidated by the correlations