Extrapair paternity in relation to sexual ornamentation, arrival date, and condition in a migratory bird

We tested the novel hypothesis that arrival date in migratory birds represents a reliable indicator of male quality that can be used by females as a cue in extrapair mating decisions. Secondary sexual characters are often condition-dependent, and competition for early arrival leads to condition-dependent migration. Hence, both secondary sexual characters and arrival date are predicted to be condition-dependent indicators of male phenotypic quality. We studied the relationship between expression of a secondary sexual character, arrival date, and condition, respectively, and extrapair paternity in a Spanish population of barn swallows, Hirundo rustica. By using microsatellite markers to determine paternity, we showed that 17.8% of all offspring (N = 674) and 32.4% of all broods (N = 170) were due to extrapair paternity. Quasi-parasitism (in which the male nest owner fathered the offspring, but the eggs were laid by another female) occurred in 2.6% of all nestlings and 2.9% of all broods. Individuals were consistent in the frequency of extrapair paternity among first, second, and third broods. Males with long outermost tail feathers, arriving early and in prime body condition, had little extrapair paternity in their nests. This was also the case when controlling for the confounding effects of male age. Partial correlation analysis was used to investigate the direct and indirect effects of tail length, arrival date, and body condition on extrapair paternity. Body condition accounted for most of the variance in extrapair paternity, whereas tail length and arrival date accounted for a smaller proportion of the variance. Body condition was strongly correlated with tail length and arrival date. However, because females cannot directly assess condition or arrival date (males arrive before females), females may obtain an indirect measure of condition and migration ability from tail length and other phenotypic traits of males. This suggests that extrapair paternity depends on the effects of condition, through its indirect effects on arrival date, tail length, and other variables.