sentences:  In Denmark, the average male tail length increased by 9% between 1984 and 2004, but it is possible that climatic changes may lead in the future to shorter tails if summers become hot and dry.Males with long streamers also have larger white tail spots, and since feather-eating bird lice prefer white feathers, large white tail spots without parasite damage again demonstrate breeding quality; there is a positive association between spot size and the number of offspring produced each season.The breeding season of the barn swallow is variable; in the southern part of the range, the breeding season usually is from February or March to early to mid September, although some late second and third broods finish in October.