OIKOS 93: 488 – 496. Copenhagen 2001 Shape and sources of variations of the functional response of wildfowl: an experiment with mallards, Anas platyrhynchos Herve ´ Fritz, Daphne ´ Durant and Matthieu Guillemain Fritz, H., Durant, D. and Guillemain, M. 2001. Shape and sources of variations of the functional response of wildfowl: an experiment with mallards, Anas platyrhyn - chos. – Oikos 93: 488–496. Understanding the variations of the functional response of an organism, i.e. the predation rate in relation to prey density, is necessary to understand the interactions between the animal and its food supply. This has received little attention in dabbling ducks so we investigated experimentally the shape of the functional response of mallard feeding on poultry pellets, and assessed the influence of several factors such as the size of food items, sex or individual performance on this functional response. Individual differences in intake rate are of crucial importance in group or gregarious foraging species. We used two approaches of the functional response: 1) the relation between feeding rate (pellets/s) and pellet densities (pellets/m 2 ), and 2) the relationship between instantaneous intake rate (g/s) and biomass density (g/m 2 ). For both approaches, we found that the Type II functional response gave better estimates than a Type I linear functional response but explained only a third of the variance. Our results show that pellet size has a large effect on instantaneous intake rate. The comparison of the functional response parameters suggest that handling time per prey may not reflect the real constraints on intake rate, but that handling time per gram ingested may be more appropriate to integrate the effect of item size in the functional response. We then discuss the possible mechanisms involved. We also found individual variations in the functional response for each of the experiments, with some consistency in the hierarchy regarding feeding efficiency. We did not find any differences between males and females. Our results provide an evaluation of individual variations in intake rate in interfer- ence-free conditions, which has rarely been done, and call for more controlled experiments to allow a finer understanding of the mechanisms of food acquisition in dabbling ducks. H. Fritz, D. Durant and M. Guillemain, CNRS -UPR 1934, Centre d’Etudes Bi - ologiques de Chize ´ , 79360 Beauoir -sur -Niort, France ( fritzh@cebc.cnrs.fr). Measuring and understanding the causes of variations of the functional response of an organism, i.e. instan- taneous predation rate in relation to prey density (Holling 1959), have direct consequences for the un- derstanding of ecological processes at the individual level, such as the role of dominance and interference on foraging efficiency (Goss-Custard et al. 1984, Still- man et al. 1996, Norris and Johnstone 1998), and to evaluate the role of these individual differences at the population level (Rubenstein 1981, Goss-Custard et al. 1995, Piersma et al. 1995). There are several mechanisms that can generate the various functional responses, particularly the asymptotic part: the compe- tition between handling and searching for a prey, but also the interference between harvesting food and the velocity of the animal, or the competition between cropping and processing food (see Spalinger and Hobbs 1992 for the theoretical approach on herbi- vores). The understanding of the shape and parameters of the functional response can thus allow for the test- ing of hypotheses related to processes limiting intake rate (Gross et al. 1993). Accepted 19 January 2001 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 93:3 (2001) 488