Carotenoids are potent guardians of health. With their impressive antioxidant powers they constantly protect us from the ravages of free radical damage. Most species benefit from the physiological effects of carotenoids, but while the beneficial effects of carotenoids for adult birds are relatively well established, their effects on chicks were less clear. Clotilde Biard and Anders Møller from the Université Pierre et Marie Curie, France, were intrigued by the role that carotenoids may play in wild chick development. Would blue and great tit chicks that were kept well supplied with carotenoids have a head start over nest mates on a regular diet? Biard headed out into the Forêt d'Orient to see how chicks on a carotenoid supplemented diet fared (p. 1004).

But getting access to the tiny newborns required a degree of agility; Biard had to clamber 3 m up into trees to retrieve the youngsters from their nesting boxes. Rapidly developing a head for heights, Biard gave half of the chicks in each nest box a 0.50 ml dose of carotenoid-spiked sunflower oil, while the others were given untreated oil, repeating the supplement every two days until the youngsters fledged at 16 days of age. On the day before the chicks left home, Biard measured their weights and tarsus lengths, collected a 100 ml blood sample from a vein in the chick's wing and collected 6 yellow feathers from the breast to see whether the carotenoid boost had benefited them. She also checked how strong the chick's immune system was by injecting a small amount of antigen into the chick's wing, and measured the degree of inflammation 19 hours later.

Back in the Paris lab, Biard began analysing the blood samples to see if the carotenoid boost had improved the chick's health. But when she looked at the proportion of red and white blood cells in each sample there was no difference between the two sets of chicks. Checking the fledgling's feather colour with a spectrometer, Biard saw that the carotenoid-fed great tits' feathers were much brighter than those of unsupplemented great tits. However, the blue tits' plumage hadn't benefited from the carotenoid dietary supplement at all. And when she compared the degree of inflammation produced by the antigen injection, that was also unaffected by the extra carotenoids in the chicks' diet: carotenoids had not improved the youngsters' immune response. Teaming up with Peter Surai at the Avian Research Centre in the Scottish Agricultural College, Biard analysed the levels of antioxidants in the bird's blood and was surprised that the plasma carotenoid concentration was essentially the same in carotenoid supplemented chicks and chicks on a regular diet.

The only detectable benefit to the chick's condition that Biard found was that the smallest chicks on the carotenoid enhanced diet were heavier than equally sized unsupplemented chicks, giving the heavy guys a much greater chance of survival than their lighter nest-mates. `It seems that dietary availability of carotenoids may have important fitness consequences for tits' says Biard. `The picture is far more complicated than we initially thought' she adds.