Ardea
Official journal of the Netherlands Ornithologists' Union

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Pulliam H.R. (1980) Do Chipping Sparrows forage optimally? ARDEA 68 (1-4): 75-82
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In winter, a Chipping Sparrow must locate and eat one seed everyone to two seconds for 10 hours each day, or starve. Even once located, many seeds require an additional one to three seconds of 'handling time'. I estimated the abundances, weights, and handling times of all of the common seeds found in an oak woodland on The Research Ranch in south-eastern Arizona. With this information, I tested the hypothesis that Chipping Sparrows maximize their rate of energy intake while foraging. Seeds were ranked by their seed values, i.e. by the ratio of seed weight to seed handling time, and, as predicted, all seeds above some threshold value were eaten and all below the threshold were not eaten. As the availability of seeds of high rank decreased, at least one seed species of lower rank was added to the diet in approximate order of rank. These results are in agreement with the predictions of the hypothesis that Chipping Sparrows maximize their rate of energy intake while foraging; however, other findings of this study were not. Seeds do not occur in the diets in the same proportions as they are available in the soil. I suggest this discrepancy is more likely due to nutrient or toxin constraints than to differential crypticity of the seeds. Toxin constraints are also implicated by the observation that some common seeds are never eaten in the field or laboratory despite their high seed values. Chipping Sparrows do not maximize their net rate of energy intake while foraging, but they come close to doing so.


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