Extrapair paternity in the blue tit (Parus caeruleus) : female choice, male charateristics, and offspring quality

Extrapair paternity is common in many birds, and it is now generallyaccepted that female choice plays an important role. However,die benefits that females obtain from extrapair paternity aremuch less dear. To test the hypothesis that females obtain indirectfitness benefits, we studied paternity in a blue tit populationover 4 years. Extrapair paternity occurred in 31-47% of allnests and accounted for 11-14% of all offspring. Most malesthat fathered extrapair young did not lose paternity themselves,males never "exchanged" paternity, and within nests the extrapairoffspring were usually fathered by a single male. Comparisonsbetween males that did and did not lose paternity and pairwisecomparisons between the extrapair male(s) and the within-pairmale showed that successful males had longer tarsi and sangon average longer strophes during the dawn chorus. Successfulmales weighed less (relative to their size) during the nettlingstage, but nevertheless they survived better. Male age did notinfluence their likelihood of losing paternity, but extrapairmales were usually older than the within-pair male they cuckolded.Within nests with mixed paternity, extrapair young were morelikely to survive than within-pair young in cases of partialbrood mortality. Our data also suggest that extrapair offspringwere more likely to be males. Because extrapair males were usuallyclose neighbors, male quality should be considered relativeto the quality of the neighbors. Despite this, we found consistencyin female choice over years. Our observations provide supportfor the hypothesis that female blue tits engage in extrapaircopulations to obtain good genes for their offspring.