Female birds go for males with the brightest beaks, a sign of his healthy immune system, according to a pair of studies.
British and French researchers said male birds with the most colourful beaks have high levels of nutrients that can boost the immune system.
The researchers experimented with the birds' diet by increasing the amount of carotenoids, nutrients found in fruits and vegetables that give the bird beaks a strong, reddish colour. They cannot naturally produce the compound and must get it from their diet.
Naturalists know males often sport the showiest features, from peacocks with their long, dazzling tales, to songbirds with the best tune. The hypothesis is males show off to prove they have the extra resources needed to advertise their fitness. A sickly male couldn't afford the weight of a heavy tail, for example.
Canadian biologist George Lozano first proposed the idea during his doctoral research at McGill University in 1994.
The two new studies confirm the assumption by directly measuring how brighter beaks show off a healthier immune system.
In one study, evolutionary ecologist Jonathan Blount of the University of Glasgow in Scotland and his colleagues used 10 pairs of Zebra finch brothers. One bird in each pair was given drinking water enriched with cartenoids; all the birds ate the same grain for food.
Within eight weeks, the beaks of the treated males become a deeper red than their untreated brothers, Blount said.
When a female finch was placed in the a cage with the brothers, each male started singing to get her attention. Nine of the 10 females finches chose males with the brightest beaks.
The researchers also tested the immune function of the birds and found the redder males had stronger immune responses. Birds with strong immune systems are better able to fight off disease and parasites.
Blount said for the mate selection system to work, the signal must be something that a male bird could not fake. If successful, females who select healthy mates will be more likely to produce more healthy chicks.
In the French experiment, researchers tested how a stress to the immune system would affect the beak colour of European blackbirds, whose bills range from yellow to a prized orange.
When Bruno Faivre and colleagues at the University of Bourgogne in France taxed the blackbird immune systems, the birds' beak colour dulled.
Faivre said beak colour is probably not the only factor blackbirds use as a sexual signal. But bill colour may signal a change in caretenoids more quickly than plumage would.
The studies appear in Friday's issue of the journal Science.