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The Functional Response of a Generalist Predator 1 2 1 ´ 3 4 Sophie Smout *, Christian Asseburg , Jason Matthiopoulos , Carmen Fernandez , Stephen Redpath , Simon Thirgood5{,, John Harwood1 1 Scottish Oceans Institute Centre for Research into Environmental and Ecological Modelling, University of St Andrews, St Andrews, United Kingdom, 2 Centre for ˜ ´ Pharmaceutical Policy and Economics (CEPPE), University of Eastern Finland, Kuopio, Finland, 3 Instituto Espanol de Oceanografıa, Vigo, Spain, 4 Aberdeen Centre for Environmental Sustainability, School of Biological Sciences, University of Aberdeen and Macaulay Institute, Aberdeen, United Kingdom, 5 The Macaulay Institute, Aberdeen, United Kingdom Abstract Background: Predators can have profound impacts on the dynamics of their prey that depend on how predator consumption is affected by prey density (the predator’s functional response). Consumption by a generalist predator is expected to depend on the densities of all its major prey species (its multispecies functional response, or MSFR), but most studies of generalists have focussed on their functional response to only one prey species. Methodology and principal findings: Using Bayesian methods, we fit an MSFR to field data from an avian predator (the hen harrier Circus cyaneus) feeding on three different prey species. We use a simple graphical approach to show that ignoring the effects of alternative prey can give a misleading impression of the predat