THEY say that personality is more important than looks to women, but try telling that to a North American barn swallow.

Scientists found that female barn swallows will cheat on their partners if they spy a more handsome suitor.

The female bird constantly compares her mate with other males. Should a more attractive proposition come along, the pair will build themselves a secret love nest.

American researchers were able to influence the birds’ mating habits by painting the throats, breast and belly feathers of randomly selected males with a red marker. For the female North American barn swallow, a red breast is best.

Rebecca Safran, of Cornell University in Ithaca, New York, who led the study, said: “The bad news for male swallows is the mating game is never over. It is dynamic and continual. This is something that most humans can relate to — think of how much time and money we spend on our looks and status long after we have established relationships.”

Male swallows have a wash of reddish-chestnut colour from throat to belly, which varies between individuals.

Like many songbirds, half of all males typically care for at least one chick fathered by another bird. Sometimes males rear an entire nest of illegitimate young. All 30 females studied remained socially paired with their original mates, but they were sexually active with other males. After all the pairs had laid their first set of eggs, Dr Safran removed the eggs so that the females would mate again. A number of males were then chosen for “smartening up”. DNA tests showed that males with enhanced colour fathered a larger percentage of offspring.

Dr Safran, whose findings appear in the journal Science, said: “The study shows that the females pay close attention to these signals and that they respond quickly to changes in their mate’s appearance.”

She believes that the red colour indicates aspects of a male’s quality, such as health, status, or ability to raise young.

The findings may point to why evolution has allowed decorations such as the peacock’s tail to remain.

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