Context-dependent foraging and enemy-free space: Grasshopper sparrows (Ammodramus savannarum) searching for grasshoppers (Acrididae)

Abstract

Aspect diversity and enemy-free-space hypotheses (Ricklefs & O’Rourke, 1975; Jeffries & Lawton, 1984) predict that predation pressure on coexisting prey taxa should result in prey communities composed of dissimilar appearing species because of competition for enemy-free space (traits that reduce a species’ vulnerability to natural enemies). For cryptic prey, the aspect diversity hypothesis further specifies that overall searching capabilities of visually oriented predators are best when prey are morphologically similar in appearance. This study examines searching efficiency by the insectivorous Grasshopper Sparrow (Ammodramus savannarum) in a community context when foraging for multiple, cryptic grasshopper species (Acrididae) as prey in an outdoor aviary. I tested the prediction that these birds search more efficiently when faced with morphologically similar versus dissimilar combinations of prey species. Different prey species combinations significantly altered bird searching ability, indicating that Grasshopper Sparrows forage in a context-dependent manner. However, results did not generally demonstrate foraging behaviors expected from the aspect diversity hypothesis, in that searching efficiency was not significantly better when confronted with a group of morphologically more similar (versus dissimilar) prey taxa. Searching efficiency increases with higher prey densities at naturally occurring levels, but the impact of different prey species associations appears to be more important. No statistical interaction was observed between grasshopper species combination and overall density for any of the measures of foraging efficiency, suggesting that the impact of these two factors on the success of Grasshopper Sparrows at finding prey are independent. Additional factors that also affect prey assemblies must be factored in to understand predator-prey community dynamics.