Foraging Behavior and Switching by the Grasshopper Sparrow Ammodramus savannarum Searching for Multiple Prey in a Heterogeneous Environment

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Abstract
Switching among four alternative grasshopper species by the grasshopper species by the grasshopper sparrow (Ammodramus savannarum) through frequency-dependent predation was examined. Grasshoppers were presented against a heterogeneous natural background in an outdoor aviary where total density was held constant but relative density of each grasshopper species varied. Switching among prey, primarily between two of the four species, and inter-bird variability in switching was observed. Species-specific preferences and interactions among the alternative grasshopper species affected the final diet choice as one grasshopper species was always preferentially selected and another disregarded independent of changes in relative density. Attributes of avian foraging behavior, including: (a) proportion of total time spent handling prey, searching for prey or engaging in nonforaging activity; (b) capture success; (c) time required to catch prey (relative to either total or search time), and (d) attack distance did not vary according to the relative abundance treatments. Results of this study are related to quantitative estimates of predation and relative densities of grasshopper species in a natural grassland setting.