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Foraging strategy of the black oystercatcher Hartwick, Earl Brian

Abstract

The study was concerned with the foraging strategies of animals and, in particular, with a recent model of optimal foraging proposed by Royama in 1970. The black oyster-catcher Haematopus bachmani Audubon was chosen as a study animal because it foraged in an open habitat where it was possible to observe both the hunting of the adult and the feeding of the young. Low survival rates resulting from predation and drowning indicated that efficiency in feeding would have survival value. A natural history study revealed that the birds moved their chicks to the feeding area. Thus the model was examined under three different feeding contexts, (1) adult feeding, (2) feeding of chicks at the nest, (3) feeding of chicks at the feeding area. Interactions with gulls prevented the movement of some chicks to the feeding area and the effects of this on the foraging of the parents were recorded. The feeding area was studied in some detail and a natural zonation typical of rocky shore habitats was found to exist. Shore-level size gradients were found in many of the species. The birds exhibited a number of different hunting modes on the feeding area and a tendency to select larger than average sized prey items. Values were obtained for the parameters of Royama's model, including handling time, average weight of prey, rate of successful search and prey density. Profitabilities were calculated for some prey in the three feeding situations. The most profitable food items were mussels when they were vulnerable. A study of the diet indicated that mussels formed the greatest part of the diet in terms of weight. This more profitable food item occurred even more frequently in the diet of chicks at the nest. The adults carried large items to the chicks while feeding on smaller items themselves. When chicks were moved to the feeding area, small items previously lacking in their diet suddenly appeared quite frequently. Risks not considered in the model are thought to account for this. Crabs which appeared in the diet only when chicks were present may play a special role in the diet, although an explanation in terms of profitabilities may also be reasonable. In the situation where chicks remained isolated from the feeding area, mussels formed an increasing percentage of the diet of the chicks. A number of different responses to potential prey were recorded during the study. The effects of prey size and encounter rate were examined experimentally. The birds showed a rapid conditioning to large prey with specific searches being carried out after only one contact. The specific search continued in spite of a high encounter rate with smaller prey. The need for a joint interest in strategies and tactics was discussed. The study illustrated the advantage of carefully choosing a study animal in order to test and ultimately improve models advanced by theoreticians.

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