Female birds find males with the brightest beaks the most attractive for good reason: these are also the healthiest birds.
This has been confirmed by new studies from UK and French researchers.
Zebra finches (Taeniopygia guttata) on a wire
They have shown how male birds with very red or orange beaks are the ones that take in very high levels of nutrients, carotenoids, which can boost the immune system.
Birds with duller beaks - those more likely to be passed over by females looking for a mate - have immune systems that are stressed.
The research, done on zebra finches and blackbirds, has been published in the journal Science.
In the British study, the scientists varied the finches' food intake of carotenoid pigments, the naturally occurring nutrients that are responsible for the red, yellow and orange colouration patterns in animals and in foods such as carrots.
They measured the effects on beak colour, sexual attractiveness and the immune system.
They found that the male zebra finches that ate more carotenoids developed redder beaks and became more sexually attractive to the females. These males also had substantially enhanced immune defences.
"Carotenoids are more than just colourful pigments - they are used in the body to neutralise harmful free radicals and to stimulate the immune system," explained team leader Dr Jonathan Blount of Glasgow University.
"Until now it hasn't been clear whether all of these birds have sufficient carotenoids to maintain their sexual attractiveness and at the same time keep their immune system in top working order.
"Our research suggests that this isn't the case - only those males with fewest parasites and diseases are likely to be able to devote sufficient carotenoids to their appearance to produce the best sexual display.
Before and after: Colour drained from the beak in three weeks
The French researchers approached carotenoids and immune function from a different direction. They started with a baseline carotenoid balance based on the blackbirds' beak colour.
When they taxed the blackbird immune systems - by injecting sheep blood into the birds - their beak colour dulled due to carotenoid declines.
"In blackbirds, dynamic reallocations of carotenoids from the beak to the immune system appear to convey a continual update on male health," said Frank CÚzilly, a co-author and professor at Bourgogne University, Dijon.
"Reallocations of carotenoids were observed in three weeks' time. We didn't think the answer could be so quick."
While carotenoid-based beak colour is probably not the only factor these birds consider when selecting a mate, "It is now clear that carotenoid levels are linked to sexual attractiveness and immune function," said Dr Blount.
Such birds would have an advantage in the evolutionary "marketplace". Animals with stronger immune systems would naturally be better able to fight off disease. They may have more efficient physiologies, may be better at finding nutritious foods, or both.