Which food patches are worth exploring? Foraging desert birds do not follow environmental indicators of seed abundance at small scales: a field experiment

Abstract

Consumers should show strong spatial preferences when foraging in environments where food availability is highly heterogeneous and predictable from its correlation with informative environmental features. This is the case for postdispersal granivores in most arid areas, where soil seed bank abundance and composition associates persistently with vegetation structure at small scales (e.g., decimeters to meters). We analysed seasonal single-seed removal by granivorous birds from 300 experimental devices in the algarrobal of the central Monte desert. Spatial selectivity was analysed by comparing the structural characteristics of used vs. available microhabitats and evaluated against bottom-up and top-down hypotheses based on our previous knowledge on local seed bank abundance, composition and dynamics. Seed removal, which showed its expected seasonal variability, was also explored for spatial autocorrelation and environmental dependencies at bigger scales. Postdispersal granivorous birds were less selective in their use of foraging space than expected if patch appearance were providing them useful information to guide their search for profitable foraging patches. No kind of microhabitat, as defined by their vegetation and soil structure, was safe from bird exploration. The only consistent selective pattern at this scale was closer to a top-down spatial effect by birds, i.e., a cause (and not a consequence) of the seed bank dynamics. Bigger spatial scales proved more relevant to describe heterogeneity in the use of foraging patches in this habitat. Closeness to tall trees, probably related to bird territoriality and reproduction or to their perception of predation risk, seems to determine a first level of selection that defines explorable space, and then microhabitat openness exerts an influence on which patches are effectively exploited (or more frequently explored) among those accessible.