Fig. 3. Intake rates of the bivalve prey types in experiment 1 as a function of a prey type's shell mass (values are means ± S.E.M.). Prey types are three different size classes of Macoma (triangles) and Cerastoderma (circles). Symbols are open for the small-gizzard birds and filled for the large-gizzard birds. Solid lines are the linear regression lines with fixed shell mass processing rates for each group of birds (0.24 and 2.58 mg s^-1, respectively, for the small- and large-gizzard birds; model 4 in Table 2). Broken lines give observed handling rates (1/H) for Macoma and Cerastoderma. Squares denote measurements on red knots in the field by (1) González et al. (1996) and (2) Zwarts and Blomert (1992). Grey diagonal `Kirkwood-Kvist' bar indicates the constraint on metabolizable energy intake rate according to Kirkwood (1983) and Kvist and Lindström (in press; average of the two predictions is used). The bars on top of the graph denote the relative frequency distribution of shell masses of ingestible bivalve prey in the western Dutch Wadden Sea (N=82 964).