Bird communities of dry forests and oak woodland of western Mexico

Bird species-richness and densities were studied in spring and autumn of 19 91 and 1992 in dry forest and oak woodland in western Mexico. Classificatio n of the vegetation resulted in the identification of six dry forest types: from thorn forests, dominated by Acacia, through broad-leaved tree forest (typical dry forest of western Mexico) and small-leaved tree forest to Ipom oea forest. Oak woodland and mixed woodland, which are a transition between dry forest and the more mesic oak woodland, formed two further forest type s. In the study area as a whole, bird densities were higher in autumn, foll owing the rainy season, than in spring, which is the dry season, and the be ginning of the breeding season. Densities in the two autumns were very simi lar (mean 93 +/- 7.8 birds per 2.8 ha) but differed significantly between s pring 1991 (57 +/- 2.2 per 2.8 ha) and spring 1992 (70 +/- 3.2 per 2.8 ha), possibly due to unseasonal rain in 1992. In autumn, bird densities were hi ghest in thorn forests and lowest in broad-leaved tree forests and oak wood land. In spring 1991, bird densities were highest in Prosopis thorn forest and mixed woodland, and lowest in oak woodland. Bird species-richness was l ow in oak woodland, in both seasons, and high in mixed woodland and Ipomoea forest in spring. Bird species composition between forest types was more s imilar in the spring than autumn. The spring similarity in species composit ion is explained by the presence of generalist species in times of food sca rcity during the dry season, and by the homogeneity of the predominantly le afless vegetation. Comparison with the bird species composition of a dry fo rest in Venezuela and the Pacific coast of Mexico indicates that the sites in the present study have an intermediate number of species. Although the p resent study recorded a smaller number of bird species than on the Pacific coast, 16 were different, expanding the total for Mexican dry forest by 14% from 118 to 134.