Extra-pair paternity results from female preference for high-quality males in the blue tit

EXTRA-PAIR copulations (EPCs) seem to be one of the most widespread alternative reproductive behaviours by which male birds can increase their fitness 1,2. In many species females actively solicit or freely engage in EPCs 3–5 , which suggests that they benefit from them. Of the eight hypothetical benefits proposed 2,6 , the most likely are genetic 2. Often females engage in EPCs with more dominant males 3,7 or with males with more elaborate ornaments 8,9 . In species in which paternity was assigned, extra-pair young were divided asymmetrically between males 10–12 . Here, combining detailed behavioural work with DNA-fingerprinting of an entire population, we present evidence that such an asymmetry is indeed caused by female behaviour, and that 'attractive' males do not suffer lost paternity, survive better and recruit more young. Our results support the genetic quality hypothesis.