Foraging Behavior Of Ruddy Ducks: A Test Of Optimal Foraging Theory (oxyura Jamaicensis, Marginal Value, Patch Depletion, Search Behavior).
Tome, Michael Walter
1986
Abstract
I observed the foraging of 4 male and 2 female Ruddy Ducks (Oxyura jamaicensis) in a large, concrete and glass aquarium to determine how well their foraging behavior conformed with the predictions of optimal foraging theory. The foraging sites in the aquarium consisted of 16 sand-filled trays. Prey (wheat grains) were placed beneath the sand surface of specified trays to provide food patches in the experiments. I conducted 3 experiments to determine if a patch depletion model (the marginal value theorem) accurately predicted the point when Ruddy Ducks abandoned a food patch. In 10 of the 16 comparisons in the first experiment, the number of prey consumed by the Ruddy Ducks did not differ from the number predicted by the model. In the second experiment, energy costs of travel between patches were increased by increasing the search time necessary to find a food patch. Five of 6 birds consumed numbers of prey that were consistent with the predictions of optimal foraging theory. When a low and high prey density patch were located in the foraging environment at the same time, the number of prey consumed in 8 of 12 observations did not differ from the predictions of optimal foraging theory. The hypothesis that the Ruddy Ducks foraged as predicted by the patch depletion model could not be rejected. Optimal foraging theory predicts that search paths of predators looking for a patchily distributed prey will be linear prior to encountering a food item and should become sinuous after prey are found. To test this hypothesis, I compared search path characteristics (length, duration, meandering ratio, rate of movement) prior to and after encountering food and in patches of low, medium, and high density. Search paths prior to encountering food were shorter in length, duration, had a smaller meander ratio and a higher rate of movement than search paths in patches which contained food. The duration of search paths in low quality patches was shorter than in medium and high quality patches. Rate of movement in high quality patches was slower than in low quality patches, possibly because of handling time associated with prey consumption. Ruddy Duck search behavior exhibited characteristics predicted by optimal foraging theory for predators that must search for patchily distributed prey. To test the hypothesis that Ruddy Ducks use a time or number expectation rule in deciding when leave a patch of food, I compared the time spent foraging in and number of prey consumed from 2 food patches before and after a switch in the number of prey in the patch. The number of prey consumed from and time spent foraging in the patches corresponded with the change in prey density. The Ruddy Ducks did not forage according to a time or number expectation rule.Subjects
Behavior Depletion Ducks Foraging Jamaicensis Marginal Optimal Oxyura Patch Ruddy Search Test Theory Value
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