High heritable variation of a male secondary sexual character revealed by extra-pair fertilization in the barn swallow

Females may accrue indirect benefits in terms of sexual attractiveness or 'good genes' for their offspring by preferring males with heritable ornamental traits. Since existing heritability estimates may be affected by environmental and maternal effects, there is a need for unbiased estimates. Extra-pair fertilization was used in barn swallows, Hirundo rustica, to study heritability while controlling for such confounding effects. Heritability of tail length, a male ornament, and other morphological characters were estimated by comparing resemblance between sons and their biological father to that between extra-pair sons and their foster father. The slopes of linear regressions of extra-pair sons on their foster father did not differ from 0. However, slopes of regressions of biological sons on their parents were significantly greater than 0 for tail and wing length (both parental sexes), and bill and tarsus length (mothers only). Extra-pair sons had longer tails than their foster father, but did not differ in this respect from within-pair sons, suggesting that females benefit in terms of sexual attractiveness of their sons, via extra-pair fertilization. Tail length is a reliable signal of male quality in the barn swallow. Hence, heritable variation in ornaments allows females to adaptively select for exaggeration of these traits.