SELECTION OF FORAGING SITES BY DESERT GRANIVOROUS BIRDS: VEGETATION STRUCTURE, SEED AVAILABILITY, SPECIES-SPECIFIC FORAGING TACTICS, AND SPATIAL SCALE

Abstract

Seed availability in the algarrobal of the Monte desert, Argentina, as in other deserts, is highly heterogeneous at small scales and is associated with vegetation structure. Granivores are expected to use space selectively (within the capacities of their foraging techniques), resulting in a heterogeneous effect on the seed bank. First, we describe the foraging repertoire of granivorous birds in the algarrobal to develop predictions about their expected use of space. Although the granivory guild as a whole tracked the temporal availability of seeds, species within the guild differed in foraging behavior and seasonal changes. We assessed selection of space by foraging birds through a two-scale bird-centered analysis, comparing the distributions of used and available sites. Granivorous birds, considered as a guild, used the whole range of available microsites, though as a result of an aggregation of contrasting partial patterns. Microsites with more cover of shrubs, grasses, and litter were preferred for predispersal consumption, which is consistent with the frequent technique of attacking grasses from low woody perches. By contrast, use of microsites for postdispersal consumption did not differ from random, which suggests that there are no safe microsites for seeds in this habitat. At a larger scale not particularly related to heterogeneity in food availability, a selective pattern was clearer: birds avoided mesosites with low shrub and litter cover, far from trees. In conclusion, patterns are not straightforward and depend on considerations of spatiotemporal scale and species-specific characteristics. Even when predictions seemed clear-cut and were tested with proper scale and target-group, granivorous birds did not select space according to environmental clues of food abundance.