Sex ratio and sexual size dimorphism in Purple Sandpiper Calidris maritima chicks

Capsule The uneven sex ratio in favour of males found in flocks of Purple Sandpipers is not evident in chicks; female chicks are lighter than males. Aims To test if there is an uneven sex ratio at hatching and to investigate whether sex ratio changes during chick growth and after fledging. Methods Sex of chicks was determined (by DNA) and each chick given a unique combination of colour-rings for later re-sightings. Biometric measurements were made for comparisons between the sexes. Results The sex ratio of 97 chicks did not deviate significantly from parity, 52.6% males (95% CI: 42.2-62.8%), and there was no evidence that the sex ratio changed as chicks grew older. However, sightings of colour-ringed birds in their first year indicated a change to 61% males. The same sex ratio (61% males) was observed among wintering birds in Iceland. Chicks already showed sexual size dimorphism in skeletal measurements as is also found in full-grown birds. Female chicks were relatively lighter than males, suggesting poorer condition. Conclusions The uneven sex ratios seen in non-breeding flocks of Purple Sandpipers are not determined at hatching but might arise owing to higher juvenile mortality among females soon after fledging. This may result from the relatively lighter mass of female chicks.