Prey Selection and Handling in Two Neotropical Hover‐Gleaning Birds

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Abstract
Prey handling and prey selection were studied by releasing over 150 live insects to wild White—fronted Nunbirds (Monasa morphoeus, Bucconidae) and a caged Bright—rumped Attila (Attila spadiceus, Tyrannidae). Handling time was determined for each insect taken and the relationship between handling time and several prey characteristics was examined. Log—transformed x— and y—axes (power function) better linearized the relationship between handling time and prey size than log—transformed y—axis (exponential function) or untransformed axes did for acridid—tettigoniid Orthoptera, the insect group used to compare handling abilities of the two species. The species power function slopes were statistically indistinguishable, but y—intercepts differed such that the smaller attila (40 g) required significantly longer than nunbirds (108 g) to handle prey of a given mass or width. An energetic cost—benefit analysis using these handling time regressions showed that handling costs wer negligible compared with benefits and could not explain the upper limit to prey size (2—6 g) for nunbirds. The cost/benefit functions of the two species largely coincided, and the energetics of prey handling would not likely predispose these two species to take different mean prey sizes. The most important difference between the two species should be the range of sizes effectively utilized; the larger predator can consume much larger prey then the smaller predator can. For all insects successfully handled, prey body width was the best predictor of handling time in both species. We argue that the primary function of prey handling in these birds is to render prey swallowable. Thus gape size sets a limit to the largest noncreshable prey, and determines how much handling is required for malleable prey. A variety of insects were rejected by nunbirds because of the insect size and/or shape in combination with hardness and distastefulness. Knowledge of prey—handling behavior in these birds predicts effective insect morphologies to impede avian handling abilities.