Foraging Behavior of a Central-Place Forager: Field Tests of Theoretical Predictions

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Abstract
This study reports on field tests of the Schoener (1979) and Orians-Pearson (1979) models of central-place foraging for single-prey loaders. Solitary pine squirrels defend territories and harvest the cones of lodgepole pine for storage at a central midden site. Squirrels forage differentially among the trees in their respective territories because the cones of different trees differ significantly in size, shape, and number of seeds per cone. Cone choice was investigated as a function of distance from the central cache and as a function of whether cones were harvested for immediate or later use. The results are as follows. 1. The positive relationship between prey size (i.e., prey quality) and distance predicted by both the Schoener and Orians-Pearson models is strongly supported. Two measures of prey quality (the percentage of total cone weight attributable to seeds and the number of seeds per cone) correlate positively with distance from the central place, not only for the cones harvested by squirrels but also for the cones remaining on trees. 2. The Orians-Pearson prediction that prey selectivity should be minimal at intermediate distances receives qualified support. Not all data sets reveal the predicted pattern, however, and it is not certain that some of the assumptions underpinning the theoretical development of the model are valid in this system. All results indicate that variation in prey taken declines as the distance from the central place increases. 3. The Orians-Pearson prediction that prey superiority hinges on relative handling time rather than on energy content when traveling time is small appears to be supported by the data. Cone width, a variable positively correlated with prey handling time, is positively correlated with distance from the central place. Squirrels appear to minimize handling time for cones taken near the central place. 4. Pine squirrels forage nonrandomly in distinguishing between cones harvested for immediate consumption and those harvested for caching relative to numbers of seeds per cone, distance from the cache, and serotiny of cones.

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