THE STRUCTURE OF WINTER BIRD COMMUNITIES IN THE NORTHERN NEGEV SHRUB-STEPPES: RELATING SPECIES COMPOSITION TO HABITAT STRUCTURE

We describe the structural patterns of granivorous bird communities in a semiarid shrub-steppe during winter and examine whether and how habitat structure and productivity play roles in determining the structure of these bird communities. We used line transects and measurements of microhabitat use to describe the species diversity, species composition, and microhabitat use of the birds in semiarid shrub-steppe habitats across the northern Negev Desert, Israel. We then related those patterns to the structural characteristics of the habitats along the bird transects at two spatial scales. The bird communities in our study were not random samples of the species pool. Bird abundance and species diversity were correlated with the amount and distribution of perennial and annual vegetation; surrogates of seed abundance and habitat structure, respectively. Species assemblies across the landscape were organized in a nested-subsets structure possibly related to habitat structure. Indeed, species composition was related to the structural characteristics of the habitat. Species-specific microhabitat use was also related to habitat structure, where different species appeared to differ in their foraging location with respect to the distance from dense vegetation patches. We suggest that species first segregate at the landscape scale based on differences in habitat use. Co-occurring species are then further segregated at the microhabitat scale, probably through the partition of the space around vegetation patches. The principal effect of food appears to be limited to control of bird abundance.