Sexual differences in levels of blood carotenoids in Cirl Buntings Emberiza cirlus

Carotenoids are responsible for the bright red and yellow plumage of birds. These substances cannot be synthesised by birds and must be obtained from the diet and transported through the blood to the feathers. The carotenoid content of blood was measured in Girl Buntings Emberiza cirlus, a species w ith sexually dimorphic yellow plumage. Carotenoid concentration, estimated from the coloration of the plasma, was higher in males than in females and juveniles. These differences were unrelated to the greater incidence of fea ther moult in males. Although the reasons for the differences are not under stood, the results of this and two previous studies suggest that, in specie s with sexually dichromatic plumage, the quantity of carotenoids transporte d in the blood is higher in the more brightly coloured sex.