Among bird species in which males contribute to nest building, sexual selection has favored larger nests. I investigated determinants of nest size in the barn swallow Hirundo rustica and how nest size changed during the period 1977–2003, when tail length (a male secondary sexual character) increased by more than 1.2 standard deviations. Males with short tails contributed more to nest building than long-tailed males, signaling their future investment in food provisioning of offspring. Pairs of barn swallows were consistent in nest size when build ing new nests the same or different years, and level of phenotypic plasticity in nest size was small and could not account for temporal patterns in nest size. Offspring resembled their parents with respect to nest size, indicating a significant heritability of nest size, independent of whether offspring were reared by their parents or by foster parents, and there was a significant negative genetic correlation between male tail length and outer nest volume and amount of nest material. The temporal increase in male tail length was associated with a decrease in nest size, with the amount of nest material in 2003 on average being less than a third of the amount used in 1977. Temporal change in nest size could be accounted for by indirect selection on tail length causing change in nest size to match that predicted from change in tail length and the genetic correlation between male tail length and nest size.
