Sex-Related Differences in Diet and Feeding Method in the Oystercatcher Haematopus ostralegus

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Abstract
1. Previous papers have related individual differences in diet and feeding method in oystercatchers on the Exe estuary to age. In this paper they are also related to sex. 2. Oystercatchers caught and ringed on the Exe estuary were sexed by discriminant analysis. 3. Bill tip shape was measured and related to diet and feeding method. Birds with pointed bill tips were mudflat feeders, birds with chisel-shaped bill tips were mussel stabbers and birds with blunt bill tips were mussel hammerers. 4. Whilst in birds with chisel-shaped bill tips the sex ratio was 50:50, over 70% of birds with pointed bill tips were females and nearly 90% of birds with blunt bill tips were males. 5. Differences between the sexes in diet and feeding method can be explained to a large extent by differences in overall bill measurements; in particular, the ratio of bill depth to bill length. Only those birds that had relatively thin bills were worm feeders and only those birds with relatively thick bills were mussel hammerers. 6. As different diets and feeding methods have different costs and benefits the possibility of sex differences in overwinter survival is discussed.

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