AN EXPERIMENTAL-STUDY OF PATERNITY AND TAIL ORNAMENTATION IN THE BARNSWALLOW (HIRUNDO-RUSTICA)

Previous studies of the socially monogamous barn swallow (Hirundo rust ica) have shown that males that most frequently engage in extrapair co pulations and whose partners are least involved in copulations with ex trapair males are those with long tail ornaments. In this study, throu gh the use of three highly polymorphic microsatellite. markers, we ana lyze the relationships between length of tail ornaments of male barn s wallows and proportion of nestlings fathered in own broods, number of offspring fathered in broods of other pairs, and total number of offsp ring fathered, using both a correlational and an experimental approach . Consistent with our predictions, we show that males with either natu rally long or experimentally elongated tails have higher paternity (pr oportion of biological offspring in own broods), and they produce more biological offspring during the whole breeding season than males with naturally short or experimentally shortened tails. Males with natural ly long tails also had more offspring in extrapair broods than short-t ailed males, but the effect of tail manipulation on the number of offs pring fathered in extrapair broods, although being in the predicted di rection, was not statistically significant. Cuckolded males that did n ot fertilize extrapair females had smaller postmanipulation tail lengt h than cuckolders. We conclude that there is a causal, positive relati onship between male tail length and paternity. Since female barn swall ows have extensive control over copulation partners and heritability o f tail length is high, this study shows that female choice is a compon ent of selection for larger male ornaments. Benefits from extrapair fe rtilizations to females may arise because they acquire ''good'' genes for sexual attractiveness or high viability for their offspring.