How feather colour reflects its carotenoid content

1. Many birds sequester carotenoid pigments in colourful patches of feathers to advertise or compete for mates. Because carotenoids can be scarce in nature and serve valuable physiological functions, only the highest-quality individuals are thought to acquire or allocate more pigments for use in sexual displays. 2. A critical but rarely tested assumption of carotenoid-based signals is that the colour of pigmented feather patches directly reveals the total amount of carotenoids contained within them. 3. We studied the relationship between carotenoid-based coloration (hue, chroma and brightness) and the pigment content of tail feathers in wild-caught and captive male greenfinches (Carduelis chloris [Linnaeus]). Greenfinches incorporate two main carotenoids - canary xanthophylls A and B - into feathers to develop yellow patches of colour in their tail. 4. Variation in feather carotenoid content explained 32-51% of variation in chroma and hue of the yellow parts of tail feathers, while feather brightness was not significantly related to carotenoid concentration. Hence, chroma and hue appear good candidates to indicate feather carotenoid content. 5. Birds with the most colourful feathers deposited significantly more of both canary xanthophylls into plumage. Thus, there does not appear to be a specific biochemical strategy for becoming colourful in greenfinches; males instead follow the general decision rule to deposit as many xanthophylls as possible into feathers to become yellow.