Sex-Specific Restriction Fragments and Sex Ratios Revealed by DNA Fingerprinting in the Brown Skua

DNA fingerprinting was used to assign the sex of individuals of brown skua (Catharacta lonnbergi). A population of this sexually monomorphic sea bird is found on the Chatham Islands off the coast of New Zealand. A large proportion of the individuals within this population breed communally. Hybridization of the DNA fingerprinting probe pV47–2 to genomic DNA from brown skua revealed several high molecular weight DNA fragments present exclusively in females. This finding is consistent with the assignment of sex based on the available morphometric and behavioral data. We conclude that these restriction fragments are located on the W chromosome. Consequently, the presence of these fragments can be used to identify female adults and chicks. We recorded a biased sex ratio among adult birds in breeding territories, with almost twice as many males as females. In contrast, the sex ratio among chicks approximates 1:1. These results are discussed in relation to contemporary ideas about the evolution of communal breeding.