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Sex allocation theory, when combined with 'genetic benefit' models of female choice, predicts that it would be adaptive for female birds to bias the sex ratio of extra-pair offspring in favour of males. We tested this prediction for a population of collared flycatchers Ficedula albicollis by combining data on parentage obtained by microsatellite typing, with data on the sex of nestlings, obtained by polymerase chain reaction single-stranded conformational polymorphism typing of a sex-linked gene. Although there is evidence from this and other species that females bias the sex ratio of their offspring in favour of sons when mated to attractive males, we found no evidence to suggest that the sex of extra-pair offspring was other than randomly determined. We discuss two possible explanations for this: either females do not possess sufficient physiological control to influence which male's sperm are fertilizing which egg, or the mechanisms by which they manipulate offspring sex ratio are not sufficiently finely tuned.

Original publication




Journal article


Proceedings of the Royal Society B: Biological Sciences

Publication Date





1017 - 1021