The relative effect of gained extra-pair paternity versus ensured within-pair paternity on male fitness in collared flycatchers, Ficedula albicollis

Variation in a male ability to gain extra-pair paternity and maintain within-pair paternity are important sources of variation in male fitness in socially monogamous species and thus become the resource of sexual selection on male secondary sexual traits. While the maximum possible number of offspring sired within the social pair-bond is limited by the female’s clutch size, the possible number of sired extra-pair young could in theory by much higher. The crucial question then becomes why males not invest all their efforts into seeking matings with many females rather than investing in social pair bonds in terms of mate guarding and expensive paternal care of offpring? I examined the distribution and relationship between maintained within-pair paternity and gained extra-pair paternity in other males’ nests in a passerine bird, the collared flycatcher, in relation to age, body mass, tarsus length, and forehead patch size . There was no correlation between gaining extra-pair paternity in other males’ nests and keeping within-pair paternity within the own nest. Variation in maintained paternity has a slightly larger effect than gained paternity on variation in total male siring success (fitness) in our population. The overall number of offspring sired was not associated with body mass, patch size, or male age. These results indicate that ensuring paternity by mate guarding of a social mate may be important for male reproductive success