Choosing a mate by the length of his tail

Swallows are migratory birds with European swallows spend the winter in Africa south of the Sahara where they can be found in countries like Kenya and Tanzania. Their British counterparts on the other hand prefer spending their winter in South Africa.

For a distance, male and female swallows are indistinguishable. On closer scrutiny the male seems to have longer streamers on his forked tail. It is not clear whether this gives him any special aerial advantage but the females seem to have a liking of it.

The length and symmetry of a male swallow’s tail is what the female considers attractive. Growing a long tail must surely demand a great deal of energy that only a strong bird can afford.

Females therefore prefer mates with long streamers and these males are often the first to find breeding partners in spring. Consequently long tailed males tend to father more offspring than shorter tailed swallows.