OFFSPRING SEX AND PATERNITY IN THE COLLARED FLYCATCHER

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 test ed this prediction for a population of collared flycatchers Ficedula a lbicollis by combining data on parentage obtained by microsatellite ty ping, 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 th at females bias the sex ratio of their offspring in favour of sons whe n mated to attractive males, we found no evidence to suggest that the sex of extra-pair offspring was other than randomly determined. We dis cuss 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 offs pring sex ratio are not sufficiently finely tuned.