SEXUAL SELECTION IN THE MONOGAMOUS BARN SWALLOW (HIRUNDO RUSTICA). I. DETERMINANTS OF TAIL ORNAMENT SIZE.

The proportion of phenotypic variance in the length of the sexually selected tail of the monogamous barn swallow Hirundo rustica that is attributable to genetic variance was studied in the field in Denmark during a seven-year period. Tail length was on average 20% greater in males than in females. Tail length correlated with wing length, but not with other morphological traits. Tail length increased with the first molt, but remained constant during subsequent years. Changes in tail length between years, owing to molt were significantly affected by sex and by degree of infection with an haematophagous mite (Ornithonyssus bursa). There were significant differences in sexual size dimorphism between years, apparently as a result of environmental conditions in the African winter quarters during molt. Tail length was a highly repeatable morphological trait, and standardization of tail length for age effects only marginally increased repeatability. Heritability of tail length as estimated from regression of values for sons on those of their fathers was 0.59. This suggests that secondary sexual traits affected by strong directional selection still may show a statistically significant heritability.