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Behavioral Ecology Advance Access published online on January 4, 2006
Behavioral Ecology, doi:10.1093/beheco/arj029
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© The Author 2006. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org
Received October 15, 2004
Revised October 17, 2005
Accepted November 22, 2005

Article

Optimal foraging on perilous prey: risk of bill damage reduces optimal prey size in oystercatchers

Anne L. Rutten 1 *, Kees Oosterbeek 1, Bruno J. Ens 2, and Simon Verhulst 3

1 Zoological laboratory of the University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands; Alterra, P.O. Box 167, 1790 AD Den Burg, Texel, The Netherlands
2 Alterra, P.O. Box 167, 1790 AD Den Burg, Texel, The Netherlands
3 Zoological laboratory of the University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands

* To whom correspondence should be addressed.
Anne L. Rutten, E-mail: anne.rutten{at}gmail.com


  Abstract

Intake rate maximization alone is not always sufficient in explaining prey size selection in predators. For example, bivalve-feeding oystercatchers regularly select smaller prey than expected if they aimed to maximize their intake rate. It has been proposed that to these birds large prey are "risky," in the sense that birds may damage their bills when feeding on large bivalves. Large bivalves yield more energy, but according to this hypothesis this is achieved at the expense of energy yield in the long term when (1) the risk of bill damage increases with prey size and (2) foraging with a damaged bill is less effective. In accordance with this hypothesis, we show that captive oystercatchers feeding on large cockles experienced a high probability of bill tip damage, while bill damage was absent when cockles were small. Moreover, among free-living oystercatchers the prevalence of bill damage was correlated with mean cockle size near the capture site, and the data on captive birds fit in this pattern. Food intake of captive oystercatchers feeding exclusively on cockles was reduced by 23% after bill damage, and free-living birds with damaged bills had 14 g lower mass. Because lower body mass was associated with higher mortality probability, these results indicate long-term costs associated with feeding on large cockles. We conclude that the risk of bill damage can potentially explain why oystercatchers avoid large bivalves and that oystercatchers may maximize long-term intake rate by selecting prey sizes that are "suboptimal" from a short-term rate-maximizing point of view.

Keywords: prey choice; prey profitability; prey size selection; risky prey.
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