Using optimal foraging theory to determine the density of Mussels Mytilus edulis that can be harvested by hammering Oystercatchers Haematopus ostralegus
Meire, P. (1996). Using optimal foraging theory to determine the density of Mussels Mytilus edulis that can be harvested by hammering Oystercatchers Haematopus ostralegus. Ardea 84A(SI): 141-152
In: Ardea. Nederlandse Ornithologische Unie: Arnhem & Leiden. ISSN 0373-2266; e-ISSN 2213-1175, meer
In a previous paper (Meire & Ervynck 1986) it was shown that Oystercatchers, opening Mussels by the hammering method, selected the most profitable length classes of Mussels in terms of energy gain. It was assumed in the model that thick-shelled Mussels could not be opened. In this paper the possibility is explored that opening is not impossible, but would take a disproportionate amount of time. To this end, the numbers of blows of an artificial Oystercatcher bill, necessary to open a Mussel, was measured experimentally and was found to increase supraproportionally with shell thickness. Based on this experiment and measurements in the field the profitability of Mussels as a function of shell thickness was calculated for different length classes. Profitability decreased sharply with shell thickness and differed, for each shell thickness class, between length classes. Based on this result it was predicted that (1) per length class of Mussels, the thick-shelled Mussels should be dropped from the diet and (2) that the shell thickness of Mussels accepted should increase with mussel length. Both predictions were supported by the data. As birds selected the thin-shelled Mussels it was expected that the average shell thickness of the Mussels on the bed should increase in the course of the winter. This was not found, probably because the fraction of the mussel population that is harvestable for hammering Oystercatchers was very small. However, the consumption of the birds over the winter amounts to 70% of the production of this harvestable fraction. It is concluded that a clear description and understanding of the foraging behaviour is crucial to understand the relation between the distribution of a predator and its prey.
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