The male barn swallow returns to the breeding grounds before the females and selects a nest site, which is then advertised to females with a circling flight and song. Plumage may be used to advertise: in some populations, like in the subspecies "H. r. gutturalis", darker ventral plumage in males is associated with higher breeding success. In other populations, the breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female. Males with longer tail feathers are generally longer-lived and more disease resistant, females thus gaining an indirect fitness benefit from this form of selection, since longer tail feathers indicate a genetically stronger individual which will produce offspring with enhanced vitality. Males in northern Europe have longer tails than those further south; whereas in Spain the male's tail streamers are only 5% longer than the female's, in Finland the difference is 20%. 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.