Birds with the brightest feathers may pay a cost for their showmanship; they go on to become poor fliers.
Males with the brightest plumage are thought to be more sexually attractive to female birds.
But a study of American goldfinches is the first to show that high levels of brightly coloured chemicals in feathers leads to a breakdown in flight muscles, which affects flight performance.
Details are published in the journal Naturwissenschaften. The discovery does not necessarily mean that the brightest birds are also the weakest, or least able to reproduce.
Instead it shows that having bright feathers comes at a real cost to male birds. That in turn means that bright feathers are an honest signal of quality.
Only the fittest males in the best condition, who are best able to cope with the negative effects, will take on enough brightly coloured chemicals to brighten their plumage.
Yellow health
In many animals, including fish and birds, males gain competitive or mating advantages by ingesting and using large quantities of pigments known as carotenoids.
These yellow, orange and red pigments are found naturally in the bird's diets, and they cannot be made by their bodies.
When eaten, carotenoids are converted to brighten otherwise dull feathers, creating bright plumages.
The beneficial effects of high levels of carotenoids are well documented by scientists: as antioxidants they are thought to improve bird's health, and the resulting bright feathers signal to female birds that males are healthy, have less parasites and a good diet.
But until now, scientists have not examined whether there is a downside to eating lots of carotenoids.
To do so, Professor Kristen Navara of the University of Georgia in Athens, US and colleagues at Auburn University, Alabama studied what happened to American goldfinch birds fed a diet rich in carotenoids.
Over two consecutive seasons, they fed wild caught goldfinches a high carotenoid diet for two months, followed by a normal diet for two months.
A control group of birds was consistently fed a diet low in carotenoids.
During the experiments, the researchers collected feathers from the birds to measure how much carotenoid pigment was taken up into the bird's plumage.
In the first year, they also tested for levels of an enzyme that might indicate muscle is being broken down in the birds.
In the second year, they followed this up by directly testing the bird's ability to fly by measuring the performance of the bird's flight muscles.
Muscle wasters
The results were clear.
Birds fed carotenoid supplements were significantly more colourful, having more strikingly yellow feathers.
However, birds fed this high-carotenoid diet also produced high levels of muscle-wasting enzymes, as the carotenoids became toxic, causing tissue damage.
They also performed less well during flight tests.
"The impairments were long-term and occurred two months after carotenoid supplementation had stopped," Prof Navara told the BBC.
"In a natural situation, this time period would correspond with the breeding season for male goldfinches.
"Impaired muscle performance during this time could decrease reproductive output overall." That means only birds in good enough condition to tolerate these negative effects will take on high levels of carotenoids, and hence have the showiest feathers.
"So when females choose males with bright colouration, they are choosing ones in good enough condition to withstand high levels of carotenoids," says Prof Navara.
"Until now high levels of carotenoids have been regarded as beneficial to songbirds, and out study suggests that other potentially detrimental effects need to be tested in other species to get a full picture
On the left, a sexier goldfinch with carotenoid-rich feathers
