Why are birds' eggs colourful? Eggshell pigments co‐vary with life‐history and nesting ecology among British breeding non‐passerine birds

The colourful appearance of bird eggshells has long fascinated biologists and considerable research effort has focused on the structure and biochemistry of the avian eggshell matrix. The presence of tetrapyrrole pigments was identified nearly a century ago. Surprisingly, how the concentrations of avian eggshell pigments vary among related species, and whether this variability is associated with either eggshell appearance and/or species life‐history traits, remains poorly understood. We quantified the concentrations of the two key eggshell pigments, protoporphyrin IX and biliverdin, from a diverse sample of eggshells stored at the Natural History Museum, Tring, UK. We explicitly tested how these two pigments are associated with physical measures of eggshell coloration and whether the pigment concentrations and colour diversity co‐vary with phylogenetic affiliations among species. We also tested a series of comparative hypotheses regarding the association between the concentrations of the two pigments and specific life‐history and breeding ecology traits. Across species, the average concentrations of protoporphyrin and biliverdin were positively correlated, and both strongly co‐varied with phylogenetic relatedness. Controlling for phylogeny, protoporphyrin concentration was associated with a higher likelihood of cavity nesting and ground nesting, whereas biliverdin concentration was associated with a higher likelihood of non‐cavity nesting habit and bi‐parental provisioning. Although unlikely to be explained by a single function, the breeding ecology and life history‐dependence of eggshell pigment concentrations in these comparative analyses implies that related species share pigment strategies, and that those strategies relate to broad adaptive roles in the evolution of variation in avian eggshell coloration and its underlying mechanisms. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 657–672.     

Blue‐green eggshell coloration reflects yolk antioxidant content in spotless starlings Sturnus unicolor

The hypothesis that eggshell colouration is a sexually selected trait of female birds is based on the fact that biliverdin, the pigment responsible for blue‐green colours of the eggshell, is a potent antioxidant and that only females with high antioxidant capacity can deposit higher concentrations of biliverdin as eggshell pigment. Antioxidants (e.g. carotenoids, vitamins) are also abundant in the egg yolk, which serve as nutrient reserves for the developing embryo, and eggshell colour intensity may reflect maternal investment in yolk antioxidants. Here, we test the relationship between blue‐green eggshell colour intensity and concentration and amount of carotenoids, vitamin A, and vitamin E in the egg yolk of spotless starling Sturnus unicolor, a species for which we have previously shown good evidence of sexual selection driving egg coloration. As could be extrapolated from the hypothesis of sexual selection driving the evolution of blue‐green eggshells, we found that eggshell colour intensity was positively related to the concentration and amount of carotenoids and vitamin E in the yolk. Thus, mothers may use egg colour intensity to signal to fathers the antioxidant status of their offspring. Moreover, we provide evidence suggesting that maternal yolk investment in more coloured eggs can also explain the detected association between feeding decisions of males and egg colour intensity that we have found previously in this species.     

Eggshell pigment composition covaries with phylogeny but not with life history or with nesting ecology traits of British passerines

No single hypothesis is likely to explain the diversity in eggshell coloration and patterning across birds, suggesting that eggshell appearance is most likely to have evolved to fulfill many nonexclusive functions. By controlling for nonindependent phylogenetic associations between related species, we describe this diversity using museum eggshells of 71 British breeding passerine species to examine how eggshell pigment composition and concentrations vary with phylogeny and with life‐history and nesting ecology traits. Across species, concentrations of biliverdin and protoporphyrin, the two main pigments found in eggshells, were strongly and positively correlated, and both pigments strongly covaried with phylogenetic relatedness. Controlling for phylogeny, cavity‐nesting species laid eggs with lower protoporphyrin concentrations in the shell, while higher biliverdin concentrations were associated with thicker eggshells for species of all nest types. Overall, these relationships between eggshell pigment concentrations and the biology of passerines are similar to those previously found in nonpasserine eggs, and imply that phylogenetic dependence must be considered across the class in further explanations of the functional significance of avian eggshell coloration.     

Eggshell coloration is an indicator of dietary calcium in Common Pheasants (Phasianus colchicus)

According to the structural-function hypothesis, the eggshell pigment protoporphyrin, deposited at weak spots, can strengthen the shell structure when calcium is lacking in avian species. However, this hypothesis has not been tested in species that produce pigmented eggs of uniform ground colour without spotting patterns. We tested the structural-function hypothesis using 435 eggs produced on seven calcium diets (0.2–4.5%) given to Common Pheasants Phasianus colchicus, a species that produces a large number of eggs on a low-calcium diet with unspotted eggshells composed of a uniform ground colour of mainly protoporphyrin. We found that pheasants on the lowest calcium diet (0.2%), which had thinner eggshells, produced eggs containing more protoporphyrin-based coloration than four of six other diets, suggesting this species employs pigmentation as ground colour to strengthen eggshells when available calcium is low. Our results provide the first, at least partial support for a structural function for eggshell pigments producing ground colour without spotting in a species that is often calcium-limited. This pattern may be more widespread in other ground-nesting taxa that also produce large numbers of eggs with protoporphyrin-based ground colour and are potentially limited by calcium during breeding.     

Effects of food limitation on the intensity of blue‐green and brown eggshell coloration: an experimental study with the canary

Variation in blue‐green and brown coloration of avian eggshells could be affected by several factors, including environmental nutritional constraints. Better availability of nutrients could influence the synthesis and deposition of pigments into the eggshell, so we may expect a link between the food availability during egg formation, the body condition of the female and intensity of eggshell coloration. This hypothesis has received mixed support so far: in some bird species a positive correlation between female body condition and eggshell blue‐green coloration could be demonstrated, but other studies failed to find a significant link. In this study, we experimentally limited the food availability for domestic canaries Serinus canaria prior to and during egg laying, and examined its effects on the biliverdin‐ and protoporphyrin‐based eggshell coloration. Treatment had no significant effect on eggshell blue‐green chroma and biliverdin concentration. However, we found a significant positive relationship between female body condition and eggshell background blue‐green chroma in the control group, but not in the food restricted group. Females possibly experiencing a decline in antioxidant capacity due to food limitation may not be able to produce a blue‐green eggshell colour intensity reliably indicating their body condition. Furthermore, food‐restricted canary females laid eggs with significantly higher eggshell brown chroma, spot intensity, and protoporphyrin concentration. Therefore, our results suggest that limitation in actual nutrient availability increases deposition of protoporphyrin into the eggshell, and it may also modify the association between female body condition and intensity of blue‐green eggshell coloration.     

Eggshell spot scoring methods cannot be used as a reliable proxy to determine pigment quantity

Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin which is produced during biosynthesis of blood haem. Its functional significance has only received empirical attention in recent years. This interest has generated a number of hypotheses of which some remain untested partly because the quantification of protoporphyrin is analytically challenging and can be prohibitively expensive. Many studies have therefore used the extent of eggshell spotting as a proxy for total eggshell protoporphyrin concentration, although this has not been formally tested. Pigment scoring involves recording visible eggshell pigment attributes, such as spot intensity, distribution and size. Since even immaculate eggs can contain some protoporphyrin, there remains doubt over the degree to which visible pigment correlates with total pigment content of the shell. In this study, we test whether visible pigment scoring can be used as a proxy for protoporphyrin concentration of an eggshell. We use pigmented eggshells of two common British passerine species to compare eggshell spot intensity, distribution and spot size (as used by the visual pigment scoring method) with direct measures of eggshell protoporphyrin concentration. In addition, we compared an alternative method of pigment scoring, the pixel pigment scoring method, using a computer programme to quantify the number of pixels exceeding a specified colour threshold. We demonstrate that although results from both scoring methods were positively correlated with eggshell protoporphyrin concentrations, the correlations were not sufficiently strong to be used as surrogates in studies where actual pigment concentrations are required.     

Eggshell characteristics and yolk composition in the common cuckoo Cuculus canorus: are they adapted to brood parasitism?

The developmental rate of cuckoo embryos and their hatching size is greater than that of host species, which may require more nutrient resources in the egg and more intensive gas exchange during development. In the present study, we compared various egg characteristics of a brood parasite, the common cuckoo Cuculus canorus, and its frequent host, the great reed warbler Acrocephalus arundinaceus. As maternally‐derived testosterone is known to enhance growth rate of embryos and hatchlings, cuckoo eggs are expected to contain higher concentration of testosterone than host eggs. In addition, we expected higher concentration of antioxidants in cuckoo eggs to protect embryos from oxidative stress associated with accelerated growth. Our results showed that cuckoo eggs had thicker shells and higher pore density than great reed warbler eggs. Yolk was significantly heavier in cuckoo eggs and contained higher concentrations of carotenoids and vitamin E, however, yolk androgen and immunoglobulin concentrations were lower in cuckoo eggs as compared to great reed warbler eggs. We also examined whether eggshell colour was associated to egg quality, and detected a positive association between blue‐green chroma and yolk antioxidant concentration in both species, suggesting that eggshell colour reflects the antioxidant investment of the female into the eggs. Our results suggest that cuckoo females increase the size, growth rate and competitive ability of their young by providing them with more nutrients and more dietary antioxidants for embryonic development, and not through elevated yolk testosterone or antibody levels. In addition, increased porosity of cuckoo eggshells may allow embryos to develop more rapidly because of a greater capacity of gas exchange.     

Eggshell Spottiness Reflects Maternally Transferred Antibodies in Blue Tits

Blue-green and brown-spotted eggshells in birds have been proposed as sexual signals of female physiological condition and egg quality, reflecting maternal investment in the egg. Testing this hypothesis requires linking eggshell coloration to egg content, which is lacking for brown protoporphyrin-based pigmentation. As protoporphyrins can induce oxidative stress, and a large amount in eggshells should indicate either high female and egg quality if it reflects the female''s high oxidative tolerance, or conversely poor quality if it reflects female physiological stress. Different studies supported either predictions but are difficult to compare given the methodological differences in eggshell-spottiness measurements. Using the blue tit Cyanistes caeruleus as a model species, we aimed at disentangling both predictions in testing if brown-spotted eggshell could reflect the quality of maternal investment in antibodies and carotenoids in the egg, and at improving between-study comparisons in correlating several common measurements of eggshell coloration (spectral and digital measures, spotted surface, pigmentation indices). We found that these color variables were weakly correlated highlighting the need for comparable quantitative measurements between studies and for multivariate regressions incorporating several eggshell-color characteristics. When evaluating the potential signaling function of brown-spotted eggshells, we thus searched for the brown eggshell-color variables that best predicted the maternal transfer of antibodies and carotenoids to egg yolks. We also tested the effects of several parental traits and breeding parameters potentially affecting this transfer. While eggshell coloration did not relate to yolk carotenoids, the eggs with larger and less evenly-distributed spots had higher antibody concentrations, suggesting that both the quantity and distribution of brown pigments reflected the transfer of maternal immune compounds in egg yolks. As yolk antibody concentrations were also positively related to key proxies of maternal quality (egg volume, number, yellow feather brightness, tarsus length), eggshells with larger spots concentrated at their broad pole may indicate higher-quality eggs.     

Protoporphyrin‐based eggshell pigmentation predicts hatching success and offspring sex ratio in the barn swallow

Inter‐ and intraspecific variation in eggshell colouration has long fascinated evolutionary biologists. Among species, such variation may accomplish different functions, the most obvious of which is camouflage and background matching. Within species, it has been proposed that inter‐female variation in eggshell pigmentation patterns can reflect egg, maternal or paternal traits and hence may provide cues to conspecifics about egg, maternal or paternal phenotypic quality. However, the relationship between protoporphyrin‐based eggshell pigmentation and egg or maternal/paternal traits appears to be highly variable among species. We investigated patterns of intraspecific variation in Eurasian barn swallow Hirundo r. rustica protoporphyrin‐based eggshell pigmentation, and analysed its association with egg and clutch characteristics, maternal/paternal phenotypic traits and parental feeding effort. Eggshell pigmentation pattern significantly varied between breeding colonies, was significantly repeatable in first clutches laid by the same females in different years (intraclass correlation coefficient ranging between 0.56 and 0.63), but it was not significantly associated with egg traits, such as position in the laying sequence, egg mass, yolk testosterone concentration and antioxidant capacity. It was weakly or non‐significantly associated with female and male traits (sexual ornaments), but females laying darker (higher pigment intensity) first clutches had higher hatching success, suggesting that eggshell pigment intensity may predict fitness. Male nestling feeding effort was not predicted by eggshell pigmentation. In addition, females with darker breast plumage colouration (a melanin‐based trait related to fitness) laid highly protoporphyrin‐covered eggs, suggesting the presence of a previously unappreciated link between protoporphyrin biosynthesis and plumage melanisation. Moreover, the proportion of male offspring increased in clutches originating from highly protoporphyrin‐covered eggs, suggesting that parents could acquire visual cues about their future brood sex composition before egg hatching. Our results support the idea that intraspecific signalling via eggshell pigmentation is a species‐specific rather than a general feature of avian taxa.     

Parents, predators, parasites, and the evolution of eggshell colour in open nesting birds

The colourful surface of birds’ eggshells varies dramatically between species, but the selective pressures driving this variation remain poorly understood. We used a large comparative dataset to test several hypotheses proposed to explain the evolution of eggshell colouration. We tested the hypothesis that predation pressure might select for cryptic eggshells by examining the relationship between predation rate and egg colouration. We found that predation rates were positively related to eggshell brightness. The blackmail hypothesis suggests that females lay colourful eggshells to coerce males into providing additional care during incubation to keep colourful eggs covered. According to this hypothesis, conspicuous eggs should be found in situations with high risk of visual detection from predators or brood parasites. In support of this hypothesis, proportional blue-green chroma was positively related to parasitism risk, and eggs with higher proportional blue-green chroma or higher ultraviolet chroma received higher combined parental nest attendance during the incubation period. The sexual signalling hypothesis states that blue-green colour indicates female quality; however, we did not find that blue-green eggshell colour was greater in species where males participate in any form of parental care, and relative male provisioning was unrelated to blue-green eggshell chroma. We found some support for the hypothesis that brood parasitism may select for high inter-clutch variation in eggshell colour to facilitate egg recognition. In our dataset, parasitism risk was negatively related to inter-clutch repeatability of blue-green chroma. Our study highlights the diversity of selection pressures acting on the evolution of eggshell colour in birds and provides suggestions for novel areas of future key research direction.     

A nondestructive method for extracting maternally derived egg yolk carotenoids

ABSTRACT.   Maternally deposited carotenoids are a prominent component of egg yolk and are vital for the development and growth of the embryo. In most studies of avian yolk carotenoids, eggs are destructively sampled and this may limit both the number of clutches studied and the research questions addressed. We describe an empirical field trial for a nondestructive biopsy method to extract small samples (0.05 ml) of egg yolk for high-performance liquid chromatography (HPLC) analysis of yolk carotenoid concentrations. We sampled 180 clutches ( N = 44 biopsies) of two species of introduced thrushes (genus Turdus ) from agricultural habitats in central North Island, New Zealand. Once the protocol was established, all biopsied eggs from clutches that were not depredated or deserted before candling were found to be developing normally after 3–5 d of incubation ( N = 28) and all hatched. Biopsy samples (>0.02 g) produced concentrations of yolk carotenoids (and variances) that were statistically indistinguishable from whole yolk destructive samples. In addition, our samples (>0.02 g) confirmed previously reported differences in yolk carotenoid concentrations between the two thrush species and revealed a significant decline in yolk carotenoid concentration with laying order. Further examination of how variability in yolk carotenoid concentration and identity influences offspring sex, success, and survival or, later in life, reproductive success and ability to efficiently incorporate dietary carotenoids into both integument and immune tissues will require larger sample sizes. Studies to date have been restricted by the number of destructive samples that investigators are willing (or permitted) to obtain from wild species. Thus, we hope that our nondestructive method of sampling yolk will promote further examination of the links between carotenoid uptake from the environment and maternal investment in the avian yolk.     

Eggshell coloration and its importance in postmating sexual selection

Avian eggshell color seems to fulfill multiple functions, some of them being structural and others signaling. In this study, we tested whether or not eggshell coloration may play a role in sexual selection of Tree Sparrows (Passer montanus). According to the “Sexually selected eggshell coloration” hypothesis, eggshell coloration signals female, egg or chick quality and males adjust parental investment according to this signal. Eggs of this species are covered with brown spots and patches, and variation between clutches is high. We found that eggshell coloration correlates with both protoporphyrin and biliverdin, but protoporphyrin concentrations are ten times higher. Eggshell coloration reflects egg and offspring quality, but not female quality. Thus, eggshell coloration may signal female postmating investment in offspring rather than female quality. Furthermore, differential allocation in terms of maternal investment is supported by the fact that females lay more pigmented clutches when mated to males with bigger melanin‐based ornaments relative to their own. Moreover, males invested proportionally more to chicks that hatched from more pigmented clutches. Our correlative results thus seem to support a role of sexual selection in the evolution of eggshell coloration in birds laying brown eggs, pigmented mainly by protoporphyrin.     

Laying‐sequence variation in yolk carotenoids and egg characteristics in the red‐winged blackbird Agelaius phoeniceus

In many bird species with asynchronous hatching, smaller, later‐hatched nestlings are out‐competed for food by their larger, earlier‐hatched siblings and therefore suffer increased mortality via starvation. It is thought that female birds can either maintain or reduce the survival disadvantage of later‐hatched nestlings by differentially allocating maternal resources across the eggs of a clutch. Carotenoid pigments are an example of resources that female birds allocate differentially when producing a clutch, but laying sequence patterns for these pigments remain poorly studied in North American songbirds. We examined intraclutch variation in yolk carotenoids and egg metrics in 27 full clutches of red‐winged blackbird Agelaius phoeniceus eggs collected from eight wetlands in central Alberta, Canada. We predicted that carotenoids would decrease across the laying sequence, as in this species, later‐hatched, marginal nestlings suffer greater mortality than earlier‐hatched, core nestlings. We found nine carotenoid pigments in red‐winged blackbird egg yolks, including two that have never been described from avian yolks: α‐doradexanthin and adonirubin. As predicted, concentrations and amounts of most carotenoids decreased across the laying sequence, suggesting that female red‐winged blackbirds depleted their carotenoid resources as they laid more eggs. However, egg mass and yolk mass both increased across the laying sequence, suggesting that female red‐winged blackbirds may use other maternal resources to compensate for the size and survival disadvantage experienced by later‐hatched, marginal nestlings.     

An experimental test of the effect of food and antioxidants on blue‐green eggshell coloration in mountain bluebirds Sialia currucoides

Evolutionary biologists have long debated the functional relevance of the ornamental traits that occur in many animal taxa, and yet, female ornaments have received relatively little attention compared to those produced by males. A greater understanding of these traits, particularly those that are unique to females, may shed light on the potential for sexual selection to shape female phenotypes. Recently, blue‐green eggshell colour, derived from the antioxidant pigment biliverdin, has been proposed as a candidate trait that advertises female quality to males in species of birds with biparental care. However, studies have been equivocal in their support for blue‐green eggshell colour being an informative signal, and correlations between eggshell colour and other female characteristics have been inconsistent. We conducted a supplementation experiment to test if improving the access of female birds to food resources and micronutrients, thereby improving their condition prior to egg laying, would intensify the blue‐green colouration of the eggs they laid. We provided mountain bluebirds Sialia currucoides with food and carotenoids during nest building and egg laying in two breeding seasons, and assessed both within‐ and among‐clutch variation in colour. Supplementation did not affect patterns of within‐clutch variation, but did result in differences in colour among clutches. Specifically, we found that food, but not carotenoids, resulted in higher colour saturation, and decreased the brightness of blue‐green eggshells. Although this trend was observed in both years, the effect was statistically significant only in one year. Our results suggest that food supplementation influences eggshell colour, but that conditions, such as weather and natural food availability, which can vary annually, may also determine how female birds allocate pigment to eggshells.     

Eggshell biliverdin concentration does not sufficiently predict eggshell coloration

Avian eggshell coloration may have arisen due to selection on the biological, chemical, or physical properties of the pigments embedded within the eggshell, or due to selection on the coloration that emerges due to pigment deposition. Within both hypothetical frameworks, pigment‐based eggshell coloration would be related to metrics of egg quality; however, no one has evaluated the relative strength of coloration and pigment concentration in predicting egg quality. Here, we examined 66 European starling Sturnus vulgaris eggs and quantified eggshell biliverdin concentration (an antioxidant that produces the eggshell's blue coloration) and used 28 different coloration metrics derived from both photographic and spectrophotometric data. We also measured egg size, eggshell thickness, concentration of carotenoids in the yolk, and concentration of lysozyme in the albumen to capture variation in egg quality. We found that throughout the laying sequence, biliverdin concentration increased while eggshell thickness, yolk carotenoid concentration, and lysozyme concentration in the albumen all decreased, but this variation was not captured by any eggshell coloration metric. Both eggshell coloration and biliverdin concentration were negatively associated with yolk carotenoid concentration, but only eggshell biliverdin concentration was negatively associated with yolk mass. Lastly, biliverdin concentration explained, at most, only 46% of the variation for all eggshell coloration metrics. Our results suggest biliverdin concentration is a better predictor of egg quality than egg coloration in European starlings, supporting the hypothesis that eggshell pigment concentration per se may be the target of selection.     

Are brighter eggs better? Egg color and parental investment by House Wrens

ABSTRACT Recent work suggests that avian egg color could be a sexually selected signal to males that provides information about female condition, female genetic quality, or maternal investment in eggs. Theory predicts that egg color should influence male investment if it is an honest signal of the marginal fitness returns on paternal investment; a male should invest more in a colorful clutch if that investment increases offspring success more than an equivalent investment in a less colorful clutch. Some experimental support for this hypothesis has been found for species that lay blue eggs containing the pigment biliverdin, a potentially costly antioxidant. However, the brown eggshell pigment protoporphyrin, a pro‐oxidant associated with poor female condition, has received less attention as a potential predictor of female quality or investment. We performed a cross‐fostering experiment with House Wrens (Troglodytes aedon) in southwest Michigan in 2007 to test whether brown egg color was related to female condition or maternal investment, and whether male provisioning of nestlings was related to egg color. We swapped entire clutches between nests and measured egg characteristics and parental provisioning rates. We found that brighter eggs (i.e., those with less brown pigment) were heavier, and that nestlings that hatched from brighter eggs were fed at higher rates by their foster mothers, but not by their foster fathers. This pattern is consistent with the hypothesis that egg color is a potential signal of egg quality and female investment, but we found no evidence of a male response to this potential signal. This lack of a response could be the result of methodological limitations, a nonadaptive biological constraint, or adaptive indifference because chicks from brighter eggs do not actually yield increasing marginal returns on paternal investment. Clearly, additional study is needed to differentiate among these possibilities.     

Eggshell coloration in relation to male ornamentation, female condition and egg quality in the collared flycatcher Ficedula albicollis

The blue-green eggshell pigment biliverdin is an efficient antioxidant, the deposition of which may be costly and act as a handicap to the female's antioxidant capacity. Thus, it is hypothesized that there exists a positive correlation between the degree of eggshell coloration and the condition and health state of the mother. In this study, we measured the blue-green chroma of collared flycatcher Ficedula albicollis eggs, and investigated the associations between egg colour and parental characteristics, laying date, clutch size and laying order. We also analysed the possible correlations between egg size, yolk carotenoid and antibody levels and eggshell pigmentation. Our results revealed no significant relationships between egg colour and female age, morphological and serological variables. Blue-green colour intensity of eggs increased throughout the breeding season, which could be the result of either a proximate constraint or an adaptive strategy. We found a positive association between egg colour saturation and yolk lutein concentration. This relationship could be the result of the seasonal improvement of caterpillar supply, which could influence the deposition of both components into the egg. Furthermore, we found that females mated to lower-quality males produced eggs with higher biliverdin pigmentation. It is possible that deposition of biliverdin is correlated to some unmeasured aspect of egg quality that could enhance the viability of offspring of lower-quality fathers. We found that intensity of blue-green coloration increased with laying order between the first and fifth eggs.     

Egg colouration predicts brood size,telomere length and body condition of spotless starling fledglings

Understanding the impressive interspecific variation in avian eggshell colouration has attracted the attention of evolutionary ecologists for more than a century. Several functional explanations predict positive covariation between eggshell pigmentation and phenotypic quality of nestlings. We test this prediction in spotless starlings Sturnus unicolor by using biometric measurements and telomere length of hatchlings and fledglings as proxies of phenotypic quality. Female spotless starlings lay immaculate blue‐green eggs, a sexually selected signal directed to males. Pigmentation predicts positive associations with concentration of antioxidants and testosterone in the yolk and with paternal provisioning effort during nestling growth. Eggshell pigmentation (blue‐green chroma) is not associated with telomere length of hatchlings, which suggests weak maternal effects on this trait. However, we find negative associations of eggshell colouration with both body condition and telomere length of fledglings. Moreover, we find positive associations between eggshell colouration and clutch size, which suggests that sibling competition is higher in nests with more coloured eggshells. Previous works demonstrated that level of sibling competition is positively related to telomere erosion and, thus, the detected negative associations between eggshell colouration, body condition and telomere length of fledglings would reflect higher level of competition in nests with more coloured eggshells. We therefore speculate with the possibility that females that lay larger clutches also lay more coloured eggshells that elicit increased paternal provisioning effort and, thus, raise larger broods at the expense of telomere erosion of their offspring.     

The modelling of avian visual perception predicts behavioural rejection responses to foreign egg colours

How do birds tell the colours of their own and foreign eggs apart? We demonstrate that perceptual modelling of avian visual discrimination can predict behavioural rejection responses to foreign eggs in the nest of wild birds. We use a photoreceptor noise-limited colour opponent model of visual perception to evaluate its accuracy as a predictor of behavioural rates of experimental egg discrimination in the song thrush Turdus philomelos. The visual modelling of experimental and natural eggshell colours suggests that photon capture from the ultraviolet and short wavelength-sensitive cones elicits egg rejection decisions in song thrushes, while inter-clutch variation of egg coloration provides sufficient contrasts for detecting conspecific parasitism in this species. Biologically realistic sensory models provide an important tool for relating variability of behavioural responses to perceived phenotypic variation.     

Growth of yellow-headed blackbird Xanthocephalus xanthocephalus nestlings in relation to maternal body condition, egg mass, and yolk carotenoids concentrations

Condition‐dependent resource allocation to eggs can affect offspring growth and survival, with potentially different effects on male and female offspring, particularly in sexually dimorphic species. We investigated the influence of maternal body condition (i.e., mass‐tarsus residuals) and two measures of female resource allocation (i.e., egg mass, yolk carotenoid concentrations) on nestling mass and growth rates in the polygynous and highly size dimorphic yellow‐headed blackbird Xanthocephalus xanthocephalus. Egg characteristics and carotenoid concentrations were obtained from the third‐laid egg of each clutch and were correlated with the mass and growth rates of the first two asynchronously hatched nestlings. Maternal body condition was associated with the growth of first‐hatched, but not second‐hatched nestlings. Specifically, females in better body condition produced larger and faster growing first‐hatched nestlings than females in poorer body condition. As predicted for a polygynous, size‐dimorphic species, females that fledged first‐hatched sons were in better body condition than females that fledged first‐hatched daughters. Associations between egg mass, yolk carotenoid content, and nestling growth were also specific to hatching‐order. Egg mass was positively correlated with the mass and growth rates of second‐hatched nestlings, and yolk concentrations of β‐carotene were positively correlated with second‐hatched nestling mass. Surprisingly, the relationship between yolk lutein and hatchling growth differed between the sexes. Females with high concentrations of yolk lutein produced larger and faster growing first‐hatched sons, but smaller first‐hatched daughters than females with lower lutein concentrations. Mass and growth rates did not differ between first‐ and second‐hatched nestlings of the same sex, despite asynchronous hatching in the species. Results from this study suggest that maternal body condition and the allocation of resources to eggs have carotenoid‐, sex‐, and/or hatch‐order‐specific effects on yellow‐headed blackbird nestlings.