Predator–prey size relationships in North American dabbling ducks

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Abstract
Diets of dabbling ducks (Anas: Antidae) usually have been recorded only with respect to taxonomic composition and not prey size. Interspecific diet overlap thus has been termed large. Using published taxonomic diet lists for 7 spp. of ducks [A. platyrhynchos, A. acuta, A. crecca, A. americana, A. discors, A. strepera and A. clypeata] measurements of bill morphology from museum specimens, and handbook data on the sizes and caloric density of invertebrates, prey size distributions were found to differ among species (P < 0.001). Also, the sizes of prey in each species'' diet differed from that found in the environment. The number of lamellae per centimeter of bill negatively correlated with mean prey size (P < 0.001); body size and bill length did not. Based on the similarity of environmental and dietary prey size distributions, species were classified as specialist or generalist foragers. This classification corresponded well with one generated from analyses of foraging behavior alone. An explanation for the diminuitive sizes of ducks on islands was advanced as follows: if interspecific competition for food selects for niche divergence among coexisting dabbling ducks along a prey size niche dimension, then in the absence of competition from small species on islands, large solitary species have evolved to a smaller size which may be optimal for the distribution of energy among prey size classes in the environment.