The Influence of Predation Risk on Central Place Foraging Variables in the American Robin, Turdus migratorius

Abstract

A central tenet of optimal foraging theory is that organisms should maximize net energy intake. There has been a recognition lately, however, that predation risk may influence foraging decisions. Two foraging parameters are examined: bout duration and mean load of prey per bout, in American robins (Turdus migratorius) in relation to predation risk by American kestrels. Foraging adult robins nesting near an American Kestrel nest had increased mean bout duration and mean load of prey per bout compared to pairs without apparent predation risk. Central place foraging theory attempts to predict the optimal solutions to different foraging situations in cases where animals return to a central location to consume, cache, or deliver prey items (Andersson, 1981; Orians and Pearson, 1979; Schoener, 1979). For organisms that return more than one prey item to the central location (multiple-prey loaders), optimization criteria can be applied to predict load size and bout duration as functions of varying distance from the fixed point. The interpretation of data collected to test optimality models (Tinbergen, 1981) typically has relied on two assumptions: (1) the currency maximized is the rate of energy delivery to the nest; and (2) the primary constraint on energy maximization is the foraging capacity of the parents (the loading effect). Fagerstrom et al. (1983) outline constraints on energy maximization for central place foragers, and develop alternatives to the strategy of energy maximization. Organisms might lower nest visitation rates to minimally acceptable levels when predation risk is perceived (visit-minimization strategy). Under the threat of predation the rate of energy delivery should correspond to the minimum requirements of dependent offspring, resulting in 'Present address: Department of Plant Pathology and Ecology, The Connecticut Agricultural Station, P.O. Box 1106, New Haven, Connecticut 06504, USA. This content downloaded from 157.55.39.238 on Sat, 02 Jul 2016 06:02:54 UTC All use subject to http://about.jstor.org/terms 156 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE lower nest visitation rates and larger load sizes per foraging trip (bout). Since nest predation is a major source of mortality in many birds (Lack, 1954; Perrins, 1977), this visit-minimization strategy may be an important alternative to energy-maximization foraging in many birds. Variation in bout duration and load size in a multiple prey loader, the American Robin, is interpreted in terms of the visit-minimization strategy. Of three adult pairs studied, the territory of one pair was only 50 meters from a woodlot containing an American Kestrel (Falco sparverius) nest. Since kestrels are known to prey upon young robins (Bent, 1938, p. 113), it was predicted that this pair would exhibit longer bout durations and larger load sizes per bout relative to the other pairs where perceived predation risk was