Sexual size dimorphism in relation to resource partitioning in North American dabbling ducks

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Abstract
Data from museum specimens were used to examine patterns of sexual size dimorphism in bill morphology in relation to resource partitioning in 7 spp. of North American dabbling ducks Anas clypesis, A. platyrhynchos, A. acute, A. strepera, A. cyanoptera, A. crecca and A. americana]. All species were dimorphic with respect to bill length. Male and female bill length distributions were skewed in opposite directions in 5 of 7 spp. (P < 0.01). Of the 7 spp. 4 were dimorphic with respect to lamelar density, but male and female distributions were skewed in opposite directions in only 2 spp. The 4 dimorphic species were positioned between other species along a prey size dimension; species which were not dimorphic had only 1 neighbor on the prey size dimension. These results are consistent with the hypothesis that intersexual competion for food promotes niche divergence between sexes and contributes to sexual size dimorphism in ducks.