Carotenoid-based Plumage Coloration Research Articles

Blood parasite infection differentially relates to carotenoid-based plumage and bill color in the American goldfinch.

Male and female American goldfinches (Spinus tristis) express condition-dependent carotenoid-based plumage and bill coloration. Plumage color is relatively static, as pigments incorporated into feathers during the spring molt cannot be mobilized thereafter. In contrast, bill color is dynamic, reflecting changes in condition over short time periods. Previous studies have shown that male and female ornaments, though similar in expression, are differentially related to measures of immunocompetence, suggesting that the relationship between ornamentation and parasite infection may differ between the sexes. In this study, we evaluate the relationship between condition-dependent ornamentation (plumage and bill color) and blood parasite infection in male and female American goldfinches. We captured goldfinches after completion of the pre-alternate molt and prior to the onset of nesting and assessed prevalence of Trypanosoma parasites via blood smears. Plumage color strongly predicted trypanosome infection: Birds with more colorful plumage were less likely to present infections. In contrast, we detected no relationship between infection and bill color, which in other studies has been shown to dynamically reflect current condition. Sex did not affect the relationship between infection status and either ornament. Together, these results suggest that physiological pathways linking carotenoid ornamentation and infection may vary even within a single species.

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.

Carotenoid-based plumage coloration reflects feather corticosterone levels in male house finches (Haemorhous mexicanus)

Indicator models of sexual selection predict that exaggerated traits communicate information about sender con- dition or quality to conspecific receivers. Environmental chal- lenges have often been considered as one such condition that could be encoded in an ornamental trait, and there is now extensive evidence showing how different stressors (e.g., nutri- tional, parasitological, and environmental) impact sexual signal elaboration. One of the primary means of assessing stress is by quantifying glucocorticoid (corticosterone or cortisol (CORT)) levels. For many ornaments, CORT impairs trait expression; however, the evidence is limited and mixed for one of the classic honest signals in animals, ornamental carotenoid color- ation. In a model species for studies of carotenoid ornamenta- tion (the house finch, Haemorhous mexicanus), we examined the relationship between male plumage redness and feather CORT levels, which serve as an integrated measure of hormone concentration during feather growth. We measured CORT in both tail (melanin-containing) and breast (carotenoid-contain- ing) feathers and found that CORT levels were not different between body regions, but they were negatively correlated with plumage hue, with redder birds having more CORT in feathers. Despiteopposingtraditionalviews onstressandornamentation, our results actually corroborate three other studies showing positive relationships betwee nc arotenoid coloration and CORT levels. Though the molecular mechanisms underlying such a relationship are still unclear, our results suggest that CORT should not be considered as a simple indicator of indi- vidual quality but rather as a mediator of complex allocation decisions or signals of metabolic activity that could link up with more elaborate expression of ornamental traits.

Pigments versus structure: examining the mechanism of age-dependent change in a carotenoid-based colour

Within-population colour variation is widespread in animals, yet the determinants of variable coloration have been relatively neglected by ecologists. Age-dependent expression of conspicuous coloration is prevalent, particularly in birds. Such patterns can be generated by multiple combinations of demographic heterogeneity or within-individual change; longitudinal analyses are necessary to establish the importance of these processes. Further, although pigment-based colours are composite traits, produced by multiple component mechanisms (e.g. feather microstructure and carotenoid pigmentation), the contributions of these mechanisms to components of age dependence are rarely considered, even though doing so may yield information about the ecological causes for age-dependent coloration. We used a large-scale, longitudinal study of carotenoid-based plumage coloration in great tits (Parus major) to show age dependence of plumage coloration is driven almost exclusively by within-individual effects in the first 2years of life. Using wavelength-specific analyses, we show that feather microstructure, while sensitive to annual variation, is independent of age, with increased carotenoid deposition driving changes in coloration. However, estimates of local carotenoid availability did not explain the change in coloration within individuals, suggesting that pigment availability may not be limiting. We thus show that it is individual-level changes in the pigment component of carotenoid-based coloration that determines age-dependent colour expression in great tits. More generally, our study highlights the utility of wavelength-specific analyses in determining the mechanisms underlying changes in expression of composite colour traits.

Seasonality of carotenoid-based plumage coloration: modelling wavelength-specific change through spectral reconstruction

Plumage coloration has provided important model systems for research on signal expression. Whilst it had previously been assumed that moulting provided the only mechanism to change plumage coloration, recent studies have shown plumage colours to be seasonally dynamic, with implications both for the quantification of expression and for any signalling role. However, the mechanistic processes underlying such change remain uncertain. Here, we describe within-moult shifts in expression of a carotenoid-based colour trait – the yellow ventral plumage of the great tit Parus major – over a nine-month timespan. We report that plumage chromaticity (‘colour’) – but not achromaticity (‘brightness’) – exhibits a marked seasonal decline, independent of sex, age or body condition, and at a constant rate across twelve environmentally heterogeneous plots within our study site. To gain a greater understanding of the mechanisms underlying this change we employed a spectral reconstruction approach, that generates predicted spectra for any timepoint within the sampling period. By comparing spectra for both early and late in the moult we show that the seasonal decline in chromaticity is driven by both a marked reduction in ultraviolet (UV) reflectance and, to a lesser extent, loss of active carotenoid pigments. Thus, our study shows that seasonal loss of chromaticity in the great tit is driven by altered reflectance primarily in the UV section of the spectrum, a finding made possible by the use of spectral compartmentalisation and multi-parallel modelling to produce reconstructed spectra. Whether change in plumage coloration influences signal function will depend on the dynamics of the signalling system but it could clearly inflate patterns such as assortative mating and should be considered in studies of colour expression.

Linking sexual selection and behavioral isolation in a colorful species of stream fish, Etheostoma barrenense

Linking sexual selection and behavioral isolation in a colorful species of stream fish, Etheostoma barrenense

Phytohaemagglutinin skin-swelling test in scarlet rosefinch males: low-quality birds respond more strongly

The phytohaemagglutinin (PHA) skin-swelling test is one of the most widely used methods for cell-mediated immunity measurement in immunoecology. Although several studies have investigated the condition-dependent traits associated with the magnitude of cutaneous inflammatory response to PHA, the results concerning signalling of the responsiveness through ornamental traits are still controversial. This is especially true for carotenoid-based feather ornamentation in birds. We therefore examined the linkage between several condition-dependent traits, including the red ornamental coloration of the plumage, and the magnitude of the PHA-induced immune response in scarlet rosefinch, Carpodacus erythrinus, males. Our results show two important aspects of the PHA-induced inflammation in this species. First, histological analysis showed that the swelling response was dependent on basophil activity. Second, the magnitude of the response (increase in patagium thickness) was associated with individual size, carotenoid-based ornamental coloration and a ptilochronological marker of feather growth at the time of moulting (mean growth bar width), thus mirroring the long-term quality of the individual. The positive linkage between the individual size or mean growth bar width and the PHA response suggests an association between the magnitude of the response and individual metabolic rate. However, as the magnitude of the response was also related negatively to ornament saturation and positively to ornament lightness, our results indicate stronger responsiveness in inferior males. Highly ornamented, healthier individuals recruited fewer basophils into the inflamed tissue causing less intense swelling. To our knowledge, this study is the first to show a negative association between carotenoid-based plumage coloration and the magnitude of the PHA-induced immune response.

Carotenoid-based plumage coloration in golden-crowned kinglets Regulus satrapa: pigment characterization and relationships with migratory timing and condition

Carotenoid-based ornamental coloration has long been proposed to honestly signal quality due to its dependence on individual condition. Because migration can be one of the most stressful periods of an animal's annual cycle, developing colourful plumage may be particularly challenging for species in which migration and moult periods overlap or occur sequentially. The purpose of this study was to investigate pigmentary and condition-dependent bases of carotenoid colour variation in a small migratory passerine, the golden-crowned kinglet Regulus satrapa (Family Regulidae). We captured 186 male and female kinglets of various ages during fall migration in southwestern Ontario, Canada and recorded arrival date, body condition index, fat and pectoral muscle scores, wing mite infestation, and feather growth rate as measures of condition. We quantified crown coloration using reflectance spectrometry and analyzed feather carotenoids using high-performance liquid chromatography. Yellow crown feathers of female kinglets contained only yellow hydroxycarotenoids, whereas orange feathers of males harboured a suite of eight carotenoid pigments. Males with longer wavelength orange crown hues deposited greater concentrations of ketocarotenoids, especially canthaxanthin. Female kinglets with longer wavelength crown hues and males with longer wavelength crown hues and more saturated crown coloration left for migration earlier in the year. Females with longer wavelength crown hues had fewer feather mites and tended to be in better condition. However, male kinglets with more saturated coloration possessed smaller pectoral muscles. This is the first study to identify plumage carotenoids in this North American bird family and to determine the pigmentary basis for both inter- and intrasexual colour variation. Our results provide further support for the condition-dependence of carotenoid coloration and suggest that ornamental elaboration in both sexes may encode information about fall condition and migratory performance.

Metal pollution indirectly increases oxidative stress in great tit ( Parus major) nestlings

Metals can cause oxidative stress by increasing the formation of reactive oxygen species (ROS), when there are insufficient amount of antioxidants to defend against the growing amount of free radicals. We aimed to find out the most reliable biomarkers to detect pollution-related oxidative stress in wild birds by comparing oxidative stress status in great tit ( Parus major) nestlings at populations in polluted and unpolluted areas. We also studied with experimental manipulations whether dietary carotenoid levels have any role in great tits’ antioxidant defence and whether their carotenoid-based plumage colour was connected to an oxidative stress status. We used antioxidants (GSH, carotenoids) and several antioxidant enzymes (GP, GR, GST, SOD, and CAT) as indicators of the oxidative stress. We found no direct connections between dietary metal exposure and antioxidant or antioxidant enzyme levels. The activity of GP was, however, slightly higher in the polluted environment. This was due to poorer condition and subsequently higher level of oxidative stress in the nestlings in the polluted area. We also found a positive association between GP and an ambient temperature during the nestling period, which may be due to higher metabolic activity of partly poikilothermic nestlings in warm weather. The activity of GST was positively related to the number of nestlings at the sampling time. Fledging success was better in an unpolluted area, where also the nestling body mass was higher. Carotenoid treatment increased the plasma carotenoid concentrations 2.1 fold in carotenoid-supplemented birds, but was not associated with the oxidative stress biomarkers or metal levels. The yellow plumage colour was associated with dietary carotenoid levels in both study areas, but not with the metal exposure or the oxidative stress status. Our results suggest that at the exposure levels found in our study area, the enzyme activities do not indicate metal-related oxidative stress. Instead, GP can be used as an indicator of growth related oxidative stress, which is greater in the polluted area. The activity of this enzyme was, however, not directly related to metal exposure, but more likely to some secondary pollution-related change in the nestling condition.

Carotenoids bolster immunity during moult in a wild songbird with sexually selected plumage coloration

Carotenoid-based colours in animals are valuable models for testing theories of sexual selection and life-history trade-offs because the pigments used in coloration are chemically tractable in the diet and in the body, where they serve multiple purposes (e.g. health enhancement, photoprotection). An important assumption underlying the hypothesized signalling value of carotenoid coloration is that there is a trade-off in carotenoid pigment allocation, such that not all individuals can meet the physiological/morphological demands for carotenoids (i.e. carotenoids are limited) and that only those who have abundant supplies or fewer demands become the most colourful. Studies of carotenoid trade-offs in colourful animals have been limited largely to domesticated species, which may have undergone artificial selection that changed the historical/natural immunomodulatory roles of carotenoids, to young animals lacking carotenoid-based signals or to species displaying carotenoid-based skin and bare parts. We studied the health benefits of carotenoids during moult in house finches (Carpodacus mexicanus), which display sexually selected, carotenoid-based plumage coloration. We manipulated dietary carotenoid availability during both winter (nonmoult) and autumn (moult) in captive males and females and found that carotenoid-supplemented birds mounted stronger immune responses (to phytohemagglutinin injection and to a bacterial inoculation in blood) than control birds only during moult. This study provides experimental, seasonal support for a fundamental tenet of Lozano's ‘carotenoid trade-off’ hypothesis and adds to a growing list of animal species that benefit immunologically from ingesting higher dietary carotenoid levels.

Age, sex and beauty: methodological dependence of age- and sex-dichromatism in the great tit Parus major

Carotenoid-based colour expression is frequently involved in sexual dichromatism, particularly in bird plumage, suggesting a role in sexual selection. Despite much work on expression of the carotenoid-based ventral plumage coloration of the great tit (Parus major), which represents a popular model in evolution and ecology, a consensus on even the most basic demographic patterns of variation (e.g. age and sex differences) is lacking. This may reflect the use of variable methods for analysing colour variation, although what is not clear, either in this case or in general, is the extent to which these alternative methods are equally effective at describing age- and sex-related dichromatism. Using data obtained over 4 years from a large sample of free-ranging great tits, we examined how colour-scoring methodology influences estimates of age- and sex-related dichromatism. We compare: (1) principal components analysis-derived scores; (2) tristimulus colour variables; (3) a visual model-independent, carotenoid-focussed colour score; and (4) two colour scoring methods based on avian visual models, examining how they assess colour variation with respect to age and sex to determine how methodology may influence results. We demonstrate clear age- and sex-dependent expression of this colour trait, both in our own data and in meta-analyses of results from great tit populations across Europe, and discuss the merits of the various colour scores, which yield very different estimates of the extent of age- and sex-dependent dichromatism. We show variation is likely to be visible to conspecifics and propose a novel, visual model-derived scoring system for describing variation in carotenoid-based colour patches, where the perceived signal is divided into independent chromatic and achromatic components, in line with current understanding of visual perception. The present study highlights the impact of colour-scoring methodology and shows that, as novel measures continue to be developed, researchers should consider carefully how they quantify colour expression. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 777–796.

Dissecting Carotenoid from Structural Components of Carotenoid‐Based Coloration: A Field Experiment with Great Tits (Parus major)

Carotenoid-based yellowish to red plumage colors are widespread visual signals used in sexual and social communication. To understand their ultimate signaling functions, it is important to identify the proximate mechanism promoting variation in coloration. Carotenoid-based colors combine structural and pigmentary components, but the importance of the contribution of structural components to variation in pigment-based colors (i.e., carotenoid-based colors) has been undervalued. In a field experiment with great tits (Parus major), we combined a brood size manipulation with a simultaneous carotenoid supplementation in order to disentangle the effects of carotenoid availability and early growth condition on different components of the yellow breast feathers. By defining independent measures of feather carotenoid content (absolute carotenoid chroma) and background structure (background reflectance), we demonstrate that environmental factors experienced during the nestling period, namely, early growth conditions and carotenoid availability, contribute independently to variation in yellow plumage coloration. While early growth conditions affected the background reflectance of the plumage, the availability of carotenoids affected the absolute carotenoid chroma, the peak of maximum ultraviolet reflectance, and the overall shape, that is, chromatic information of the reflectance curves. These findings demonstrate that environment-induced variation in background structure contributes significantly to intraspecific variation in yellow carotenoid-based plumage coloration.

Oxidative stress and information content of black and yellow plumage coloration: an experiment with greenfinches

Carotenoid and melanin pigments in the plumage of birds are hypothesized to be sensitive to oxidative stress. We manipulated oxidative status of captive greenfinches (Carduelis chloris L.) by the administration of buthionine sulfoximine (BSO), a selective inhibitor of the synthesis of glutathione (GSH), an intracellular antioxidant. Half of the birds in the treated group, as well as in the control group, also received dietary carotenoid (lutein) supplementation. BSO treatment reduced erythrocyte GSH levels and caused oxidative damage as indicated by the increased concentration of plasma malondialdehyde (MDA), an end product of lipid peroxidation. BSO treatment also reduced the brightness (i.e. increased blackness) of the tips of tail feathers grown during the experiment. These results show that a low systemic GSH level is required for development of eumelanin plumage coloration and that such a low GSH level is also potentially dangerous for the organism. Carotenoid supplementation increased plasma carotenoid levels and chroma of the yellow parts of the feathers grown during the experiment. However, carotenoid supplementation did not reduce plasma MDA levels. Manipulation of GSH did not affect plasma carotenoids or carotenoid-based plumage coloration. These findings argue against the antioxidant function of lutein in vivo and carotenoid signaling of antioxidant status.

Variation in Plumage Coloration of Northern Cardinals in Urbanizing Landscapes

Abstract Biologists know relatively little of how one of the most important avian phenotypic signals, feather coloration, may be affected by anthropogenic changes resulting from urbanization. We examined the relationship between urbanization and carotenoid-based plumage color of Northern Cardinals (Cardinalis cardinalis) in 13 riparian forests distributed across a rural-to-urban landscape gradient in central Ohio, USA. Feathers and morphometric measurements were collected from breeding territorial males (n = 129) and females (n = 145) during March–August 2006–2008. Plumage brightness of males, but not females, increased with body condition (i.e., size-adjusted mass) and declined with amount of urbanization surrounding forests in which cardinals bred. The extent plumage coloration reflected condition was partially mediated by landscape composition. Specifically, the relationship between brightness and body condition was most pronounced in the most rural landscapes. The interdependency of male coloratio...

Carotenoid Composition of Invertebrates Consumed by Two Insectivorous Bird Species

Dietary carotenoids are important pigments, antioxidants, and immune-stimulants for birds. Despite recent interest in carotenoids in bird ecology, we know surprisingly little about the carotenoid content of invertebrates consumed by birds. We compared carotenoid (lutein, beta-carotene, and total) concentrations in invertebrates brought to nestlings by two insectivorous passerines, the great tit, Parus major and the pied flycatcher, Ficedula hypoleuca. We also compared carotenoid levels between environments that were either polluted by heavy metals or were not polluted, because the carotenoid-based plumage color of P. major nestlings is affected by environmental pollution. Lepidopterans were the most carotenoid-rich food items and contained the largest proportion of lutein. There were no differences in carotenoid concentrations in the food items of the two bird species but P. major nestlings obtained more carotenoids from their invertebrate diet than F. hypoleuca nestlings because the P. major diet had a higher proportion of lepidopteran larvae. In polluted areas, P. major nestlings consumed lower levels of dietary carotenoids than in unpolluted areas because of temporal differences in caterpillar abundance between polluted and unpolluted sites. Our study suggests that pollution-related difference in nestling plumage color in P. major is related to varying dietary proportion of lutein-rich food items rather than pollution-related variation in insect carotenoid levels.

Relationships between territory quality and carotenoid-based plumage colour, cell-mediated immune response, and body mass in Great Tit <I>Parus major</I> nestlings

Plumage colour is classified as pigmentary or structural, depending on whether it is caused by pigments or by feather microstructure. However, recent findings indicate that carotenoid-based plumage colouration also reflects at UV-blue wavelengths and that the underlying structure is related to the reflectance properties of the yellow feathers. Thus, yellow plumage is based on interactions between structural and pigmentary components. This study investigat- ed the relationships among the vegetation structure of breeding territories, both components of plumage colour, T-cell- mediated immune response and body mass of nestling Great Tits Parus major. By using a model of avian visual percep- tion, we found that, while plumage yellowness was associated with mature vegetation, plumage brightness and UV- blue reflectance were related to immature habitats in territories. We noted considerable variability in the development of carotenoid-based colour components under different environmental conditions, as plumage yellowness, but not brightness or UV-blue reflectance, depends on the availability of carotenoids, which is assumed to be high in mature territories with high food abundance. Territorial features denoting mature territories were also related to high body mass and immune response in nestlings, but none of the colour components were related to these variables of the veg- etation structure, suggesting that habitat quality is related to nestling body mass and immune response through mech- anisms different from those through which it is related to colour.

Carotenoid-based plumage colouration is associated with blood parasite richness and stress protein levels in blue tits (Cyanistes caeruleus)

Carotenoids are molecules that birds are not able to synthesize and therefore, must be acquired through their diet. These pigments, besides their function of giving birds red and yellow colouration when deposited in feathers, seem to act as immune-stimulators and antioxidants in the organism. Hence, only the healthiest individuals would be able to express carotenoid-based ornaments to a larger extent without compromising the physiological functions of carotenoids. Various studies have reported that birds infected by parasites are paler than those uninfected, but, to our knowledge, none of them has assessed the possible effect of multiple infections by blood parasites on plumage colour. By comparing the yellow colour in the breast plumage of blue tits, Cyanistes caeruleus, between birds infected by different numbers of blood parasite genera, we found that those birds infected by more than one genus were paler than those parasitized just by one. In addition, we examined the potential role of carotenoid-based plumage colour of blue tits as a long-term indicator of other parameters of health status, such as body condition and immunoglobulin and heat shock protein (HSP) levels. Our results indicate that more brightly coloured birds had lower HSP70 levels than paler birds, but we did not find any significant association between colour and body condition or immunoglobulin levels. In addition, we found a positive significant association between Haemoproteus density of infection and HSP60 levels. Overall, these results support the role of carotenoid-based colours as indicators of health status in blue tits and show detrimental effects of parasitism on this character.

The Relationship Between Carotenoid-Based Coloration and Pairing, Within- and Extra-Pair Mating Success in the American Redstart

This study examines the relationship of carotenoid-based plumage coloration to mating and reproductive success in a migratory songbird, the American Redstart (Setophaga ruticilla). Adult male redstarts have several highly variable orange carotenoid-based patches of plumage on their wings, flanks, and tail. We tested the prediction that males with larger and more intensely orange plumage patches have higher pairing success, as well as higher within-pair, extra-pair, and total reproductive success. We also quantified nestling-provisioning rates, to determine if carotenoid-based coloration was related to males' provisioning behavior. Males that paired successfully had significantly brighter orange flanks than unmated males. Paternity testing with microsatellites showed that 64% of broods (18 of 28) had extra-pair young and 44% of nestlings (36 of 81) were extra-pair. We found no relationship between carotenoid-based coloration and within- or extra-pair paternity. Contrary to our predictions, males with less saturated orange flanks sired more total young than more saturated males. Males with brighter orange flanks also provisioned nestlings at rates lower than did less bright males. These unexpected results highlight the need for further research on the mechanisms governing the production and maintenance of carotenoid-based signals in redstarts, as well as incorporating the ecology of migratory species into the study of carotenoid-based signals, which heretofore has been based primarily on nonmigratory species.

Differential ability of carotenoid C4-oxygenation in yellow and red bishop species (Euplectes spp.)

Male breeding plumages of African widowbirds and bishops (Euplectes spp.) show striking variation in carotenoid-based plumage coloration, with saturated yellow or orange-red patches of different size. Yet, from observations and experiments, agonistic signaling appears to have been a generalized sexual selection pressure for redness in the genus. Recent results show that yellow and red widowbird colors derive from distinctly different pigment profiles, and suggest that species vary in their ability to metabolize ingested carotenoids. We used reflectance spectrometry and High Performance Liquid Chromatography (HPLC) to describe the species-specific colors and plumage carotenoids of the congeneric yellow-crowned bishop (E. afer) and southern red bishop (E. orix). Results show that the yellow rump color of E. afer primarily derives from direct-deposited, dietary yellow pigments, i.e. lutein and zeaxanthin. In the red breast of E. orix, these are complemented by smaller amounts of derived red C4-keto-carotenoids: mainly alpha-doradexanthin, but also beta-doradexanthin, canthaxanthin, astaxanthin and adonirubin. We also performed a diet supplementation experiment to investigate the relative importance of nutritional and metabolic constraints in determining the differential occurrence of C4-keto-carotenoids, and thus red plumage color, in the two species. Our results indicate that E. orix, but not E. afer, can manufacture red C4-keto-carotenoids (alpha-doradexanthin and canthaxanthin) from yellow dietary precursors (lutein and beta-carotene).

Brighter Great Tit parents build bigger nests

Nest size is commonly believed to be naturally selected,and to covary with requirements of developing eggs andnestlings (Collias & Collias 1984). In many altricial birdspecies, it is an important component of parental effort,as parents devote an important amount of energy andtime to nest building, but by doing so they expose them-selves and their offspring to a substantial predation risk(Martin et al. 2000). In this paper, we studied nest-sizevariation in a Great Tit Parus major population in rela-tion to breeding factors and parental carotenoid-basedplumage characteristics. The Great Tit is a secondary-cavity nesting species and females are solely responsiblefor building the nest (Gosler 1993). Sexual signalling viacarotenoid-based plumage coloration is known to occurin this species (Hill & McGraw 2006). We found thatplumage chroma of female and especially male GreatTits was positively related to nest size, suggesting greaterfemale investment in nest building when mated to malesin good condition.Nest-building behaviour and ultimately the nest itselfare often associated with courtship and pair formation(Collias & Collias 1984, Soler et al. 1998a). Further,nest-building behaviour may be involved in post-matingsexual communication between partners (Soler et al.1998a). In this way, individuals may increase their nest-building effort to signal their quality, or willingness toinvest in the current reproductive event (e.g. Soler et al.2001). Likewise, they may respond to their partner’scharacteristics in a similar way, to promote their invest-ment. However, knowledge of the use of the nest as apost-mating sexual display is limited, and most empiricalevidence comes from species where males are responsi-ble for nest building (Moreno et al. 1994, Hoi et al.1996, Soler et al. 1998b, 2001).Plumage coloration is known to be a sexually selectedcharacter signalling individual quality, and is ofteninvolved in pair formation and courtship (Hill &McGraw 2006). In particular, carotenoid-based plumagecoloration has been the focus of numerous recent stud-ies, and as carotenoids can only be obtained through thediet, they can be reliable indicators of the bearer’s feed-ing efficiency (Senar & Escobar 2002, Senar et al. 2008),body condition (Saks et al. 2003a, 2003b, Ferns & Hins-ley 2008) and survival (Horak et al. 2001). Thus, carot-enoid-based plumage coloration can reflect individualparenting ability (Senar et al. 2002, Doutrelant et al.2008), and consequently could be expected to affect sev-eral components of parental investment and resourceallocation (Clutton-Brock & Godfray 1991). In this way,nest-size variation may be related to parental quality,which in turn may be reflected by the parents’ plumagecoloration.METHODSThe study was performed in a forest area (c.4km