Sexual Signals of the Male American Goldfinch

Male American goldfinches (Carduelis tristis) exhibit conspicuous yellow plumage, orange bills, and black caps during the breeding season. These secondary sexual characteristics may serve as criteria by which females evaluate males as potential mates because the traits vary among individuals and are likely to be costly. We quantified plumage and bill color and cap characteristics of wild, free‐ranging American goldfinches during the breeding season and tested for relationships between those features, body condition and individual genetic diversity in males. Overall, male and female goldfinches were highly sexually dichromatic, with plumage saturation and brightness and bill brightness contributing strongly to the dimorphism. Body condition decreased significantly with Julian date, even over the 2‐wk period immediately prior to the onset of nesting during which we collected our color and cap measurements. Principal components describing color of the back and the bill significantly predicted date‐corrected body condition based on quadratic regressions, suggesting that there is reliable information in back and bill color that females could use when choosing mates. In a subset of captive males, we found that bill hue faded from orange to yellow within 24 h of capture, suggesting that bill color may reflect short‐term changes in health status or carotenoid availability. Individual genetic diversity based on a panel of eight microsatellite loci was correlated with back brightness and perhaps with cap symmetry. Based on the results of this field study, ornamentation of male American goldfinches appears to signal both long‐ and short‐term aspects of phenotypic quality.     

Ornamental bill color rapidly signals changing condition

Ornamental bill color is postulated to function as a condition‐dependent signal of individual quality in a variety of taxonomically distant bird families. Most red, orange, and yellow bill colors are derived from carotenoid pigments, and carotenoid deposition in ornamentation may trade off with their use as immunostimulants and antioxidants or with other physiological functions. Several studies have found that bill color changes in response to physiological perturbations, but how quickly such changes can occur remains unclear. We tested the hypothesis that carotenoid‐based orange bill color of American goldfinches Spinus tristis responds dynamically to rapid changes in physiological stress and reflects short‐term changes in condition. We captured male and female goldfinches and measured bill color in the field and again under captive conditions several hours later. The following day, the captive birds were injected with either immunostimulatory lipopolysaccharide (LPS) or a control saline and changes in bill color were measured over a five day period. Yellow saturation of the bill decreased within 6.5 h between the field and captivity measures on the first day, presumably in response to capture stress. Over the longer experimental period, bill hue and luminance decreased significantly, whereas saturation significantly increased in both LPS and control groups. Bill hue and luminance decreased significantly more in birds treated with LPS than in control birds. Among LPS treated birds, individuals expressing high bill color at the beginning of the experiment lost more color than ‘low‐color’ birds, but still retained higher color at the end of the experiment, suggesting that birds that invest heavily in bill coloration are able to sustain high costs in the face of a challenge. Bill color change may have resulted from rapid reallocation of carotenoids from ornamentation to immune function. However, the complex shifts in bill color over time suggest that bill color may be influenced by multiple carotenoid compounds and/or changes in blood flow or chemistry in vessels just beneath the translucent keratinized outer layer of the bill. We conclude that bill color is a dynamic, condition‐dependent trait that strategically and reliably signals short‐term fluctuations in physiological condition.     

Dietary carotenoid pigments and immune function in a songbird with extensive carotenoid-based plumage coloration

Carotenoid pigments can directly enhance the immune responsesof vertebrates, and they are used by many animals to createornamental color displays. It has been hypothesized that thesetwo functions of carotenoid pigments are linked: animals musttrade off use of carotenoid pigments for immune function versusornamental display. We tested two key predictions of this hypothesiswith captive American goldfinches, Carduelis tristis, a specieswith extensive carotenoid-based plumage coloration. First, wetested whether the immune systems of male goldfinches are carotenoidlimited during molt by supplying treatment groups with low,approximately normal, or high dietary access to lutein and zeaxanthin.Dietary treatment had a significant effect on plumage and billcolor but not on immunocompetence. We compared the cell-mediatedand humoral immune responses and the course of disease afterinfection for males in the different treatments. We observedno significant effect of the carotenoid content of diet on immuneresponse or disease resistance. Second, we tested whether therewas a positive relationship between immune function and expressionof ornamental coloration by comparing both the pre- and posttreatmentplumage coloration of males to their immune responses. We failedto find the predicted trade-off between ornament display andimmune function. These findings do not support the hypothesisthat songbirds with extensive carotenoid-based plumage displaystrade off the use of carotenoids for ornamentation versus immunefunction.     

Honesty of a dynamic female aggressive status signal: baseline testosterone relates to bill color in female American goldfinches

Status signals are linked to fighting ability and enable competitors to gain access to resources without risking injury in aggressive combat. The relationship between testosterone (T), a hormone that mediates aggression, and signals of status is well studied in males, but little is known about the relationship between T and female signals of status. Female and male American goldfinches Spinus tristis express a dynamic carotenoid‐based orange bill color during the breeding season and previous work has demonstrated that females use orange bill color to communicate competitive ability during intrasexual competition. We test the hypothesis that female bill color reflects baseline T, which would allow receivers to directly assess a competitor's aggressive potential. We found a positive relationship between T and bill coloration in females, indicating that bill color has the ability to signal female competitive status. This finding is consistent with the hypothesis that female bill color is a reliable signal of fighting ability, and indicates that females, like males, may use coloration to signal their hormonally mediated aggressive potential.     

Carotenoid-based ornaments of female and male American goldfinches (Spinus tristis) show sex-specific correlations with immune function and metabolic rate

Conspicuous ornamentation has been linked to immunological and physiological condition in males of many species. In species where both sexes are ornamented, it is unclear whether the signal content of ornaments differs between males and females. We examined the immunological and physiological correlates of carotenoid-based bill and plumage ornamentation in American goldfinches Spinus tristis, a species in which bright orange bills are sexually monomorphic but yellow plumage is sexually dimorphic during the breeding season. Because bill color is dynamic over short periods while plumage color is static over longer time frames, we tested whether these signals have the potential to provide temporal information about immunity and condition. In both sexes, bill color (but not plumage color) was negatively related to leukocyte differential, a measure of recent stress, while plumage color (but not bill color) was positively related to resting metabolic rate. In females, bill color also positively correlated with immunoglobulin Y, a component of acquired immunity, while plumage color positively predicted natural antibody levels, a component of innate immunity. In males, neither bill color nor plumage color predicted immune function, suggesting that the mechanisms underlying these signals vary with sex. Our results demonstrate that dynamic signals such as bill coloration do not merely reflect the same information provided by static signals but that these two classes of signal provide information about different temporal aspects of phenotypic quality. Furthermore, our results indicate that a signal expressed in both sexes has the potential to provide different information depending on the sex of the bearer.     

Winter Plumage Coloration in Male American Goldfinches: Do Reduced Ornaments Serve Signaling Functions in the Non‐Breeding Season?

The signaling role of sexual ornaments that are displayed during the mating season is well known for many species, but dimorphisms that occur in the non‐breeding season have received much less attention, particularly when individuals only partially express their breeding condition during reproductively inactive periods. I assessed variation in the expression of colorful breeding and non‐breeding plumages in male American goldfinches (Carduelis tristis), a species in which males molt out of their colorful breeding ornaments in the fall but still display reduced carotenoid‐ and melanin‐based sexual dichromatism during the winter. I found that variability in the saturation of carotenoid‐based plumage pigmentation did not differ significantly between the breeding and non‐breeding seasons. Moreover, the area of melanin coloration in the cap was more variable in winter than when it is fully displayed during breeding. I also detected a significant positive correlation between the extent of melanin coloration during the winter and the saturation of non‐breeding carotenoid‐based plumage. Because of such variation in and correlated expression of these two color ornaments in winter, it is conceivable that male goldfinches display these hints of non‐breeding coloration for use as conspecific social or sexual signals. Natural selection pressures like predation and energetic demands are traditionally thought of as factors that restrict sex ornaments to breeding times alone, but this should not preclude animals from simply reducing their exaggerated features during winter and finding an expression optimum that balances signal costs and value. Such ‘remnant’ winter ornaments might be expected to evolve not only in animals that live in large non‐breeding groups (e.g. status‐signaling systems) but also in those where mates begin associating before breeding onset.     

The effect of dietary carotenoid access on sexual dichromatism and plumage pigment composition in the American goldfinch

We investigated potential dietary and biochemical bases for carotenoid-based sexual dichromatism in American goldfinches (Carduelis tristis). Captive male and female finches were given access to the same type and amount of carotenoid pigments in the diet during their nuptial molt to assess differences in the degree to which the two sexes incorporated ingested pigments into their plumage. When birds were fed a uniform, plain-seed diet, or one that was supplemented with the red carotenoid canthaxanthin, we found that males grew more colorful plumage than females. HPLC analyses of feather pigments revealed that male finches incorporated a higher concentration of carotenoids into their pigmented feathers than females. Compared to females, males also deposited significantly more canary xanthophyll B into feathers when fed a plain-seed diet and a greater concentration and proportion of canthaxanthin when fed a carotenoid-supplemented diet. These results indicate that sex-specific expression of carotenoid pigmentation in American goldfinches may be affected by the means by which males and females physiologically utilize (e.g. absorb, transport, metabolize, deposit) carotenoid pigments available to them in the diet.     

Colourful male European Serins Serinus serinus are more careful with their plumage

Ornamental traits typically advertise individual condition and can be costly to maintain. Plumage maintenance behaviour can increase plumage quality and positively influence female mate preference. We investigated this prediction by performing female mate‐choice trials and measuring male plumage maintenance behaviour in the European Serin Serinus serinus. More colourful males spent more time in plumage maintenance than less colourful males, but this was not correlated with body condition or ectoparasite load. Females preferred more colourful males, although this was not influenced by male plumage maintenance. Our findings highlight the fact that investment in plumage maintenance depends on individual coloration and this may reinforce the honesty of ornamental plumage and convey positive information for mate choice.     

Independent sources of condition dependency and multiple pathways determine a composite trait: lessons from carotenoid‐based plumage colouration

Many colour ornaments are composite traits consisting of at least four components, which themselves may be more complex, determined by independent evolutionary pathways, and potentially being under different environmental control. To date, little evidence exists that several different components of colour elaboration are condition dependent and no direct evidence exists that different ornamental components are affected by different sources of variation. For example, in carotenoid‐based plumage colouration, one of the best‐known condition‐dependent ornaments, colour elaboration stems from both condition‐dependent pigment concentration and structural components. Some environmental flexibility of these components has been suggested, but specifically which and how they are affected remains unknown. Here, we tested whether multiple colour components may be condition dependent, by using a comprehensive 3 × 2 experimental design, in which we carotenoid supplemented and immune challenged great tit nestlings (Parus major) and quantified effects on different components of colouration. Plumage colouration was affected by an interaction between carotenoid availability and immune challenge. Path analyses showed that carotenoid supplementation increased plumage saturation via feather carotenoid concentration and via mechanisms unrelated to carotenoid deposition, while immune challenge affected feather length, but not carotenoid concentration. Thus, independent condition‐dependent pathways, affected by different sources of variation, determine colour elaboration. This provides opportunities for the evolution of multiple signals within components of ornamental traits. This finding indicates that the selective forces shaping the evolution of different components of a composite trait and the trait's signal content may be more complex than believed so far, and that holistic approaches are required for drawing comprehensive evolutionary conclusions.     

Carotenoid‐based plumage colouration is predicted by age and parasites in the male European serin

A fundamental assumption of theories on the evolution of sexual signals is that they should be costly to produce in order to honestly signal the quality of the sender. The expression of carotenoid‐based plumage signals is considered to be condition‐dependent, due to the role of carotenoids functioning as pigments and as health modulators. We assessed carotenoid‐based plumage colouration in relation to male condition in a free living population of male European serins Serinus serinus during the breeding season. Male serins were trapped for morphometric and colouration measurements, during a four‐year field study, in order to evaluate the signalling value of colouration in relation to body condition and parasites level. We compared two different forms of colour quantification based on spectral data – the most commonly used tristimulus colour variables and physiological models of avian colour vision – and found that they were highly correlated for this species. We investigated the signalling value of male plumage colouration and it was found to be related to age and ectoparasite load. Plumage double cone and patch size were negatively related to parasites level, whereas SWS ratio was positively related to parasites and age. Colouration was also related with the time since moult. Our results indicate that the colour expression of serin's plumage is age dependent and is related, in complex ways, with the ability to cope with parasitic infection.     

High brood patch temperature of less colourful,less pheomelanic female Barn Swallows Hirundo rustica

In birds, even a minor difference in egg temperature (1–1.5 °C) has been shown to affect the fitness of offspring by changing hatching success, incubation period and nestling quality. Female, but not male, passerines develop brood patches. Thus if there are traits, such as plumage ornamentation, that indicate optimal egg temperature, males should pair with females that exhibit those traits. However, no study has yet investigated the relationship between female brood patch temperature, which would directly affect egg temperature, and female plumage ornamentation. In this study, we examined the surface temperature of female brood patches during nocturnal incubation and examined its relationship with female plumage ornaments in Asian Barn Swallows Hirundo rustica gutturalis. After controlling for ambient air temperature, brood patch temperature was negatively associated with colour saturation of the female throat patch. No other female ornaments, such as tail‐length, white tail spots or throat patch size, predicted brood patch temperature. When oral (mouth) temperature was statistically controlled, females with less colourful throats and longer tails showed higher brood patch temperature, indicating that these females had hotter brood patches in relation to the temperature of other body parts. Furthermore, we found a negative relationship between pheomelanin pigmentation and brood patch temperature after controlling for ambient air temperature or oral temperature. To our knowledge, this is the first study to show that female ornaments can predict the absolute/relative thermal investment in brood patches. This relationship, together with other aspects of female quality, may affect male mate preference and female ornamentation.     

Conditions on the Mexican moulting grounds influence feather colour and carotenoids in Bullock's orioles (Icterus bullockii)

Carotenoid‐based plumage coloration plays a critical role for both inter‐ and intrasexual communication. Habitat and diet during molt can have important consequences for the development of the ornamental signals used in these contexts. When molt occurs away from the breeding grounds (e.g., pre‐alternate molt on the wintering grounds, or stopover molt), discerning the influence of habitat and diet can be particularly important, as these effects may result in important carryover effects that influence territory acquisition or mate choice in subsequent seasons. Several species of songbirds in western North America, including the Bullock's oriole (Icterus bullockii), migrate from the breeding grounds to undergo a complete prebasic (post‐breeding) molt at a stopover site in the region affected by the Mexican monsoon climate pattern. This strategy appears to have evolved several times independently in response to the harsh, food‐limited late‐summer conditions in the arid West, which contrast strongly with the high productivity driven by heavy rains that is characteristic of the Mexican monsoon region. Within this region, individuals may be able to optimize plumage coloration by molting in favourable areas characterized by high resource abundance. We used stable isotope analysis (δ¹³C, δ¹⁵N) to ask whether the diet and molt habitat/location of Bullock's orioles influenced their expression of carotenoid‐based plumage coloration as well as plumage carotenoid content and composition. Bullock's orioles with lower feather δ¹⁵N values acquired more colorful plumage (orange‐shifted hue) but had feathers with lower total carotenoid concentration, lower zeaxanthin concentration, and marginally lower canthaxanthin and lutein concentration. Examining factors occurring throughout the annual cycle are critical for understanding evolutionary and ecological processes. Here, we demonstrate that conditions experienced during a stopover molt, occurring hundreds to thousands of kilometers from the breeding grounds, influence the production of ornamental plumage coloration, which may carryover to influence inter‐ and intrasexual signaling in subsequent seasons.     

Individual quality and carotenoid‐based plumage ornaments in male red bishops (Euplectes orix): plumage is not all that counts

Males in many bird species develop elaborate carotenoid‐based plumage ornaments that play an important role as signals of individual quality in intra‐ or intersexual selection. In the present study, we investigated which of several factors related to male condition and health affect the brightness and coloration of the carotenoid‐based orange–red breeding plumage in males of the red bishop (Euplectes orix), a polygynous and sexually dimorphic weaverbird species. The study revealed a very complex pattern, with the relationships between plumage traits and both heterophil‐to‐lymphocyte ratio and blood parasite load varying considerably among seasons, suggesting a strong influence of environmental conditions. Furthermore, overall condition of males strongly affected the association pattern between plumage traits and other factors, with males in bad condition being forced to allocate resources away from plumage elaboration to body maintenance or the enhancement of immune functions, whereas males in good condition can afford to invest in plumage ornamentation without obvious detrimental effects on health. Thus, females cannot rely on plumage characteristics alone to gather information on male quality, but have to assess additional traits that advertise general male health status. Perhaps surprisingly, testosterone levels were not related to male plumage characteristics. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 384–397.     

Multiple Plumage Ornaments as Signals of Intrasexual Communication in Golden‐Winged Warblers

Avian plumage represents some of the greatest diversity in integument coloration of all animals. Plumage signals are diverse in function, including those that allow for assessing potential mates or the mitigation of agonistic interactions between rivals. Many bird species possess multiple ornamental traits that have the potential to serve as multiple or redundant signals. For example, male golden‐winged warblers (Vermivora chrysoptera) have brilliant carotenoid‐based yellow crowns, melanin‐based black throats, and structurally based white patches on their outer tail feathers. Using a correlative approach, we investigated whether plumage ornaments have the potential to reliably signal ability to acquire higher quality territory, aggressive response to simulated territorial intrusions, and reproductive success. We found that both crown chroma and tail brightness were significantly related to habitat quality and aggression; more ornamented birds held territories with higher quality habitat and were less aggressive toward simulated conspecific stimuli. Older birds sang less threatening songs than younger birds and were more likely to sing their mate attraction song type (type 1) rather than songs typically reserved for agonistic interactions (type 2). Finally, despite our previous research demonstrating that habitat strongly predicts reproductive success in this warbler population, we found no evidence of a direct link between ornamentation and reproductive success. Overall, these data suggest that younger males, and those with lower quality ornaments, compensate with more aggressive behaviors. Additional research is needed to investigate the dynamics between behavioral traits and ornaments to better understand complex signaling and how golden wing signals function in conspecific interactions (male–male interactions and mate‐choice).     

Coccidial infection does not influence preening behavior in American goldfinches

Preening behavior in birds is important for the maintenance of thermoregulatory and ornamental functions of plumage. It has been repeatedly demonstrated that birds trade off time between plumage maintenance and other activities. However, the condition-dependent constraints of preening remain virtually unstudied. Here, we present the first experimental test of the hypothesis that intestinal parasite infection impairs preening activity. We studied male American goldfinches (Spinus tristis), a species with carotenoid-based plumage coloration. Following pre-alternate (spring) molt, we manipulated the health of males by infecting some birds with Isospora spp. coccidia and keeping others free of the infection. Although the goldfinches increased preening throughout the captive period, we found no significant effect of coccidial treatment on preening behavior. The effect of coccidia on plumage maintenance may be more pronounced under natural conditions where birds have limited access to food and engage in more activities that might limit time available for preening.     

Sexually selected male plumage color is testosterone dependent in a tropical passerine bird, the red-backed fairy-wren (Malurus melanocephalus)

Background

Sexual signals, such as bright plumage coloration in passerine birds, reflect individual quality, and testosterone (T) may play a critical role in maintaining signal honesty. Manipulations of T during molt have yielded mixed effects on passerine plumage color, in most cases delaying molt or leading to production of drab plumage. However, the majority of these studies have been conducted on species that undergo a post-nuptial molt when T is low; the role of T in species that acquire breeding plumage during a pre-nuptial molt remains largely unexplored.

Methodology/Principal Findings

We experimentally tested the effects of increased T on plumage color in second-year male red-backed fairy-wrens (Malurus melanocephalus), a species in which after-second-year males undergo a pre-nuptial molt into red/black (carotenoid and melanin-based) plumage and second-year males either assume red/black or brown breeding plumage. T treatment stimulated a rapid and early onset pre-nuptial molt and resulted in red/black plumage acquisition, bill darkening, and growth of the sperm storage organ, but had no effect on body condition or corticosterone concentrations. Control males molted later and assumed brown plumage. T treated males produced feathers with similar but not identical reflectance parameters to those of unmanipulated after-second-year red/black males; while reflectance spectra of red back and black crown feathers were similar, black breast feathers differed in UV chroma, hue and brightness, indicating a potentially age and plumage patch-dependent response to T for melanin- vs. carotenoid-pigmentation.

Conclusions/Significance

We show that testosterone is the primary mechanism functioning during the pre-nuptial molt to regulate intrasexually variable plumage color and breeding phenotype in male red-backed fairy-wrens. Our results suggest that the effects of T on plumage coloration may vary with timing of molt (pre- vs. post-nuptial), and that the role of T in mediating plumage signal production may differ across age classes, plumage patches, and between pigment-types.     

Mutual ornamentation and the parental behaviour of male and female Collared Flycatchers Ficedula albicollis during incubation

One of the benefits of mate choice based on sexually selected traits is the greater investment of more ornamented individuals in parental care. The choosy individual can also adjust its parental investment to the sexual signals of its partner. Incubation is an important stage of avian reproduction, but the relationship between behaviour during incubation and mutual ornamentation is unclear. Studying a population of Collared Flycatchers Ficedula albicollis, we monitored the behaviour of both sexes during incubation in relation to their own and their partner's plumage traits, including plumage‐level reflectance attributes and white patch sizes. There was a marginally positive relationship between male feeding rate during incubation and female incubation rate. Female but not male behavioural traits were associated with the laying date of the first egg and clutch size. The behaviour of the two sexes jointly determined the relative hatching speed of clutches and the hatching success of eggs. Females with larger white wing patches spent less time incubating eggs and left the nestbox more frequently. Males with larger white wing patches fed females less frequently, whereas males with brighter white plumage areas visited the nestbox more regularly without feeding. Females tended to leave the nest less often when mated to males with larger wing patches, and females spent less time incubating when males had more UV chromatic plumage. The behaviour of both partners during incubation therefore predicted hatching patterns and was correlated with their own and sometimes with their partner's plumage ornamentation. These results call for further studies of mutual ornamentation and reproductive effort during incubation.     

Color expression in experimentally regrown feathers of an overwintering migratory bird: implications for signaling and seasonal interactions

Plumage coloration in birds plays a critical role in communication and can be under selection throughout the annual cycle as a sexual and social signal. However, for migratory birds, little is known about the acquisition and maintenance of colorful plumage during the nonbreeding period. Winter habitat could influence the quality of colorful plumage, ultimately carrying over to influence sexual selection and social interactions during the breeding period. In addition to the annual growth of colorful feathers, feather loss from agonistic interactions or predator avoidance could require birds to replace colorful feathers in winter or experience plumage degradation. We hypothesized that conditions on the wintering grounds of migratory birds influence the quality of colorful plumage. We predicted that the quality of American redstart (Setophaga ruticilla) tail feathers regrown after experimental removal in Jamaica, West Indies, would be positively associated with habitat quality, body condition, and testosterone. Both yearling (SY) and adult (ASY) males regrew feathers with lower red chroma, suggesting reduced carotenoid content. While we did not observe a change in hue in ASY males, SY males shifted from yellow to orange plumage resembling experimentally regrown ASY feathers. We did not observe any effects of habitat, testosterone, or mass change. Our results demonstrate that redstarts are limited in their ability to adequately replace colorful plumage, regardless of habitat, in winter. Thus, feather loss on the nonbreeding grounds can affect social signals, potentially negatively carrying over to the breeding period.     

The effects of elevated testosterone on plumage hue in male House Finches

The majority of studies examining the role of hormones in the proximate mechanisms of plumage coloration in birds have focused on intersexual differences (plumage dichromatism) and on structural- or melanin-based plumage coloration. The relationship between hormones and carotenoid-based plumage color, and in particular intrasexual plumage color variation, has received little attention. We manipulated testosterone levels of both captive and wild male House Finches to determine whether testosterone influences the expression of male plumage color in this species. We found that in captive male House Finches elevated testosterone delayed molt and resulted in drabber, less red plumage, even when birds were supplemented with dietary carotenoids. Elevated testosterone also resulted in drab plumage color in wild males, and appeared to delay molt in wild birds as well. Wild males implanted with testosterone showed wide variation in expression of plumage coloration. Those implanted early in the year molted plumage similar in color to their pre-treatment plumage, but those implanted later molted substantially duller plumage, possibly because delayed molt resulting from elevated testosterone caused these males to molt when carotenoid pigments were not available in sufficient amounts. These observations have the potential to explain previously reported relationships between plumage color and behavior in male House Finches, and highlight the importance of considering the proximate mechanisms of plumage coloration in avian sexual selection.     

The influence of carotenoid acquisition and utilization on the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis).

Birds display a tremendous variety of carotenoid-based colors in their plumage, but the mechanisms underlying interspecific variability in carotenoid pigmentation remain poorly understood. Because vertebrates cannot synthesize carotenoids de novo, access to pigments in the diet is one proximate factor that may shape species differences in carotenoid-based plumage coloration. However, some birds metabolize ingested carotenoids and deposit pigments that differ in color from their dietary precursors, indicating that metabolic capabilities may also contribute to the diversity of plumage colors we see in nature. In this study, we investigated how the acquisition and utilization of carotenoids influence the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis). We supplemented the diet of captive goldfinches with red carotenoids to determine whether males, which are typically yellow in color, were capable of growing red plumage. We also deprived cardinals of red dietary pigments to determine whether they could manufacture red carotenoids from yellow precursors to grow species-typical red plumage. We found that American goldfinches were able to deposit novel pigments in their plumage and develop a striking orange appearance. Thus, dietary access to pigments plays a role in determining the degree to which goldfinches express carotenoid-based plumage coloration. We also found that northern cardinals grew pale red feathers in the absence of red dietary pigments, indicating that their ability to metabolize yellow carotenoids in the diet contributes to the bright red plumage that they display.