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

Abstract

Switching among four alternative 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. INTRODUCTION Generalist predators have been shown to switch among alternative prey as the relative densities vary (Hassell, 1978; Murdoch, 1969; Murdoch and Oaten, 1975). Switching involves a frequency-dependent functional response where predators change from selecting the predominant prey type to another as its relative density increases. The dynamics of switching are contained in the functional response of the individual predator. Functional responses relate the predator attack rate to prey density (Hassell, 1978). Mechanisms proposed to explain switching behavior include: foraging for alternative prey in different patches, the formation of search images by the predator, decreases in handling time with experience, or increases in searching efficiency within patches (reviewed in Murdoch and Oaten, 1975). The first two mechanisms have been studied in most detail. When predators forage for alternative prey living in different patches and transit time among the patches is not negligible, increased foraging activity in the patch with higher prey availability will lead to the sigmoidal switching response described by Oaten and Murdoch (1975). Predators may form search images where the psychological processes involved with locating and recognizing prey against the background are altered depending on the relative abundances of the various prey types (Dawkins, 1971; Gendron and Staddon, 1983; Pietrewicz and Kamil, 1979, 1981). It is likely that all of these mechanisms operate in natural foraging situations. Switching by predators has important implications for many other ecological and evolutionary processes. Frequency-dependent predation within a population is affected by the degree of morphological (or behavioral) polymorphism among individuals in a population and is expected when foraging on alternative prey species as well (Clarke, 1962). Visually orienting predators may select for morphological divergence of coexist-