Carotenoid accumulation strategies for becoming a colourful House Finch: Analyses of plasma and liver pigments in wild moulting birds

1. Male House Finches (Carpodacus mexicanus) colour their sexually selected plumage with carotenoid pigments, and there has been much interest in the factors that affect their ability to become bright red rather than drab yellow. 

2. There is good support for the notions that health, nutritional condition and total carotenoid intake influence colour expression, but there are also suggestions that acquiring particular types of carotenoids from the diet may be important for developing red plumage. 

3. We used high-performance liquid chromatography (HPLC) to analyse the types and amounts of endogenous (in plasma and liver) and integumentary (in newly grown feathers) carotenoids in a wild, native population of moulting male and female House Finches from the south-western United States to determine the carotenoid-accumulation strategies for becoming optimally colourful. 

4. Four plant carotenoids - lutein, zeaxanthin, β-cryptoxanthin and β-carotene - were detected in plasma and liver. However, as was found previously, 11 carotenoids were observed in colourful plumage, with xanthophylls (e.g. lutein, dehydrolutein) predominant in yellow feathers and ketocarotenoids (e.g. adonirubin, 3-hydroxy-echinenone) in red feathers. This indicates endogenous modification of ingested carotenoids. 

5. Birds that accumulated more of one type of carotenoid in plasma and liver did not necessarily accumulate more of all other types, suggesting that individuals are not employing a simple 'more is better' strategy for coloration. Instead, when forward stepwise regression was used to examine the ability of individual types of carotenoids in plasma and liver to explain variation in red plumage pigments and plumage redness, we found that the lone variable remaining in all models was β-cryptoxanthin concentration. 

6. This supports the idea that, unlike some other songbirds (e.g. yellow Carduelis finches), there is a specialized biochemical strategy that male House Finches follow to become red and most sexually attractive - to accumulate as much β-cryptoxanthin in the body as possible. β-Cryptoxanthin is a less common dietary carotenoid than the typical xanthophylls and carotenes in grains and fruits and may be limited enough in the diet that, to become colourful, House Finches might adopt selective foraging strategies for the most β-cryptoxanthin-rich foods.