Functional Ecology 2006 20, 678–688678© 2006 The Authors.Journal compilation© 2006 British Ecological SocietyBlackwell Publishing LtdCarotenoid accumulation strategies for becoming a colourful House Finch: analyses of plasma and liver pigments in wild moulting birdsK. J. MCGRAW,† P. M. NOLAN* and O. L. CRINO‡School of Life Sciences, Arizona State University, Tempe, AZ 85287–4501, USA Summary1. Male House Finches (Carpodacus mexicanus) colour their sexually selected plumagewith carotenoid pigments, and there has been much interest in the factors that affecttheir ability to become bright red rather than drab yellow.2. There is good support for the notions that health, nutritional condition and totalcarotenoid intake influence colour expression, but there are also suggestions thatacquiring particular types of carotenoids from the diet may be important for developingred plumage.3. We used high-performance liquid chromatography (HPLC) to analyse the types andamounts of endogenous (in plasma and liver) and integumentary (in newly grown feathers)carotenoids in a wild, native population of moulting male and female House Finchesfrom the south-western United States to determine the carotenoid-accumulationstrategies for becoming optimally colourful.4. Four plant carotenoids – lutein, zeaxanthin, β-cryptoxanthin and β-carotene – weredetected in plasma and liver. However, as was found previously, 11 carotenoids wereobserved in colourful plumage, with xanthophylls (e.g. lutein, dehydrolutein) predom-inant 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 didnot necessarily accumulate more of all other types, suggesting that individuals are notemploying a simple ‘more is better’ strategy for coloration. Instead, when forwardstepwise regression was used to examine the ability of individual types of carotenoidsin plasma and liver to explain variation in red plumage pigments and plumageredness, we found that the lone variable remaining in all models was β-cryptoxanthinconcentration.6. This supports the idea that, unlike some other songbirds (e.g. yellow Carduelisfinches), there is a specialized biochemical strategy that male House Finches follow tobecome red and most sexually attractive – to accumulate as much β-cryptoxanthin inthe body as possible. β-Cryptoxanthin is a less common dietary carotenoid than thetypical xanthophylls and carotenes in grains and fruits and may be limited enough inthe diet that, to become colourful, House Finches might adopt selective foragingstrategies for the most β-cryptoxanthin-rich foods.Key-words: Carpodacus mexicanus, ketocarotenoids, plumage coloration, sexual selection, xanthophylls Functional Ecology (2006) 20, 678–688 doi: 10.1111/j.1365-2435.2006.01121.x IntroductionThe variable red-to-yellow plumage of male HouseFinches (Carpodacus mexicanus) has emerged as a classicexample of a sexually selected indicator of qualityin birds (reviewed in Hill 2002). Numerous field andlaboratory studies demonstrate female mate choice formale House Finches with the reddest plumage (e.g. Hill†Author to whom correspondence should be addressed.E-mail: [email protected]*Present address: Department of Biology, Ithaca College,Ithaca, NY 14850, USA.‡Present address: Department of Zoology, University ofFlorida, Gainesville, FL, 32611, USA.679Carotenoids in moulting house finches© 2006 The Authors.Journal compilation© 2006 British Ecological Society, Functional Ecology, 20, 678–6881990, 1991) and the associated reproductive benefits(e.g. earlier timing of breeding, increased youngproduction) reaped by colourful males and their mates(Hill et al. 1994, 1999; McGraw et al. 2001). Moreover,a series of correlational and experimental studiesindicate that males must ingest sufficient amounts ofcarotenoids from food to become colourful (Hill 1992,1993; Hill et al. 2002) as well as maintain adequateoverall nutrition (Hill & Montgomerie 1994; Hill2000) and health (Thompson et al. 1997; Brawneret al. 2000; Hill et al. 2004) during feather growth.There are indications from other work, however,that acquiring certain types of carotenoids from foodsmight also be critical for developing red, as opposed toyellow, coloration. Without exception, House Fincheshoused in captivity on a normal seed diet during moultgrow yellow plumage (Hill 1992, 2002), and it was onlywhen additional sources of non-xanthophyll carotenoids(e.g. synthetic red carotenoids, tangerine juice) wereadded to their diet that they grew red feathers (Hill1992, 2000). More recent biochemical studies of thecarotenoid components in House Finch feathers (Inouyeet al. 2001) show that important red carotenoids absentfrom other tissues and fluids in these birds (McGraw2004) are what determine red coloration and thus mustbe manufactured by birds from precursor moleculesnot commonly available to them. Hill (2000, 2002)speculated that it was the ability of birds to accumulatethe precursors of red pigments in feathers – candidatemolecules such as β-cryptoxanthin (based on purportedchemical transformations) – that controls, at the pigmentlevel, whether birds could acquire red plumage or not(also see Stradi et al. 1996 for similar predictions forother red-coloured cardueline finches). However, todate, there are no published studies on variation in thetypes of carotenoids that free-ranging House Finchesingest and accumulate in the body or on the extentto which ingested carotenoids predict the pigmentcomposition and expression of colourful plumage.We studied carotenoid accumulation strategiesduring moult in a native population of House Finchesfrom the desert south-western United States. Wecollected blood from live birds (to obtain a recentrecord of dietary-carotenoid assimilation), liver tissueand feathers from freshly euthanized animals, andanalysed carotenoid content of all samples so thatwe could ask the following questions about therelationships between pigment accumulation andcoloration:1. How do levels of different carotenoids covarywithin plasma or liver? In those bird species forwhich the biochemical basis of carotenoid colorationhas been investigated (e.g. American Goldfinch[Carduelis tristis], McGraw & Gregory 2004;Greenfinch [Carduelis chloris], Saks et al. 2003),birds tended to accumulate more of all types