Speckled eggs: water-loss and incubation behaviour in the great tit Parus major

Many small passerine birds worldwide lay white eggs speckled with red, brown and black protoporphyrin pigment spots (maculation). Unlike some patterns of avian eggshell pigmentation which clearly serve a crypsis or signalling function, the ubiquity of maculation among passerines suggests that its origins lie in another function, not specific to any particular ecological or behavioural group. Elsewhere, we have presented evidence that protoporphyrin pigments serve a structural function related to eggshell thickness and calcium availability: eggshell maculation in the great tit Parus major increases with decreasing soil calcium levels, pigments demarcate thinner areas of shell, and both the pigment intensity and distribution are related to shell thickness. Here we show that maculation also affects the rate of water loss from the egg during incubation (≈ Mass Loss per Day or MLD, which is critical to egg viability), but not that of unincubated eggs. We also demonstrate, both by observation and experiment, that the effect of female incubation behaviour on MLD compensates in some way for variation in egg characteristics, and that differences between females in the degree of such compensation are related to differences in clutch maculation. Our results suggest that, while a principal function of maculation in this species may be to strengthen the eggshell, it may also reduce eggshell permeability when large amounts of pigment are used, and that this necessitates a behavioural adjustment from the female during incubation. We discuss these findings and make further testable predictions from our model.     

Avian eggshell pigments are not consistently correlated with colour measurements or egg constituents in two Turdus thrushes

The colourful appearance of avian eggshells is a prominent aspect of maternal reproductive effort in birds. Some differences in eggshell coloration have been reported to co‐vary with various measures of maternal condition and these patterns support the hypothesis that, in some bird species, several aspects of eggshell colour (i.e. primary chroma and brightness) function as a signal of maternal and offspring quality to induce greater paternal investment. We directly quantified eggshell pigment concentrations of blackbird Turdus merula and song thrush T. philomelos eggshells and tested how the two key pigments (protoporphyrin IX and biliverdin) co‐varied with other eggshell traits and egg constituents as measures of maternal reproductive investment, including total yolk carotenoid concentration, total lipid concentration, yolk mass, and shell thickness. Contrary to predictions, we detected few statistical patterns overall. We found that protoporphyrin IX concentration was negatively associated with blue‐green chroma in blackbirds but not in song thrush. The concentration of protoporphyrin IX was significantly greater in blackbirds and also showed different patterns of association with total yolk lipids and yolk carotenoid concentrations between these two species (significant species interaction terms). Our results reveal that it is not appropriate to simply assume in these two avian species that reflectance‐based eggshell colour measures are a suitable proxy for eggshell pigment concentrations or can be used as consistent predictors of maternal reproductive investment. These results highlight the need to assess and validate the strength and direction of the statistical relationships between eggshell colour measures, pigment concentrations, and maternal resource deposition in the egg for other species of birds.     

Review: an embryo's eye view of avian eggshell pigmentation

Current research into the evolution and adaptive function of avian eggshell pigmentation, including maculation, has focused mostly on signalling‐based and structural function hypotheses but ignored the potential consequences of shell pigmentation for the developing avian embryo, especially in moderating the embryo's interaction with its light environment. The exposure of the eggs to sunlight that frequently accompanies avian incubation behaviour is one of the major evolutionary steps setting apart birds and reptiles, and coincides with the appearance of eggshell pigmentation. This suggests that shell pigments could play a major role in ensuring the successful development of the avian embryo. We propose that the effects of shell pigments on the egg contents should be considered in addition to established hypotheses of shell pigmentation such as crypsis, egg recognition or a possible structural function. This approach has the potential to identify trade‐offs between different pigment functions and to resolve some of the long standing paradoxa in the evolution of eggshell colour, such as the occurrence of conspicuous blue eggs in passerines or the secondary evolution of white eggshells in cavity nesters. In particular, we identify seven hypotheses, which address how the interaction of eggshell pigments and the light environment may influence embryonic development. These hypotheses are the: thermo‐regulation; UV‐B protection; photo‐acceleration; lateralization; circadian rhythm; photo‐reactivation; and antimicrobial defence. We believe that the understanding of eggshell pigmentation will greatly benefit from taking these hypotheses into consideration when studying the functional significance of eggshell pigmentation and suggest a number of promising directions for future experimental and comparative research.     

An evaluation of different methods for assessing eggshell pigmentation and pigment concentration using great tit eggs

The striking variation in colour and maculation of bird eggs has fascinated biologists since centuries, and many hypotheses based on mechanical, physiological or signalling functions have been proposed for its evolution by natural and sexual selection. Protoporphyrin is the main eggshell pigment found in brown maculated eggs, and is assumed to function as a mediator of these selection processes. It is a precursor of heme with pro‐oxidant properties, and hence a link between brown maculation and female condition has been proposed and tested in a number of studies, albeit with contrasting results. A variety of different visual methods have been used to quantify outer eggshell pigmentation, which has been assumed to correspond to overall quantity of protoporphyrin in the shell. Yet, this relationship has rarely been tested. The aim of this study was to apply four commonly used methods to assess pigmentation in great tit eggs with protoporphyrin as the predominant eggshell pigment, and to compare the results of these methods. Specifically, we 1) ranked eggshell pigmentation by human naked eye, 2) applied a granularity approach and 3) measured spectrophotometric reflectance of eggshell pigments. Second, we estimated the relationship between outer eggshell pigmentation (i.e. estimated by three different methods above) and true protoporphyrin concentration deposited in the entire shell measured by HPLC. Among‐method estimates were significantly correlated for the traits describing pigment ‘darkness’ only. While the model including scores based on human naked eyes explained 16% of the variance of pigment concentration in the entire shell, spectrometry explained 27%, and the granularity approach explained 40%. Thus, the estimation of true pigment concentration in the entire shell from the visible outer side of the shell is most reliable with the granularity approach. It is relevant for studies where the maintenance of the integrity of the eggs is essential.     

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 pigmentation pattern in relation to breeding performance of blue tits Cyanistes caeruleus

1. We test the consequences, in terms of breeding success and parental effort, of eggshell pigmentation pattern in a hole-nesting bird, the blue tit Cyanistes caeruleus that lays eggs asymmetrically speckled with reddish spots (maculated eggs). 2. We assess the effect of distribution of spots (pigment 'spread') and spot size and pigment intensity (pigment 'darkness') on eggshell physical properties and breeding parameters concerning nestling condition, investment of parents in offspring care and reproductive output in two different habitat types: a deciduous oakwoodland and an evergreen forest. 3. Blue tit clutches with more widely distributed spots showed a thicker eggshell, a shorter incubation period, a lesser amount of mass loss per day and a higher hatching probability than those with spots forming a 'corona' ring. While eggs with larger and darker (more pigment intensity) spots showed a thicker eggshell and a shorter incubation period. In the light of 'signal function hypothesis', these egg traits may reflect female health status and, consequently, this could affect male parental effort. 4. Here we show supports for some of the necessary assumptions of this hypothesis. We found a positive relationship between egg pigment 'spread' and male but not female provisioning rates per day. On the other hand, pigment 'darkness' of blue tits' clutches was positively related to female tarsus length, while pigment 'spread' was positively related to clutch size, male body mass and nestling tarsus length. Our study shows that eggshell pigment 'spread' can be used as an indicator of clutch quality. Further investigations are needed to understand the role of calcium availability as possible causal agent of deviant eggs and its relation to the maculation phenomenon.     

Copulatory behaviors and body condition predict post-mating female hormone concentrations, fertilization success, and primary sex ratios in Japanese quail

Environmental cues and social interactions are known to influence reproductive physiology and behavior in vertebrates. In female birds, male courtship displays can result in the growth of ovarian follicles, the production of reproductive hormones, and stimulation of oviduct development, all of which have the potential to influence maternal investment. Male Japanese quail follow a typical sequence of copulatory behaviors during a mating interaction and often force copulations with unreceptive females. We hypothesized that female Japanese quail could adjust maternal investment in response to male copulatory behaviors during a single mating interaction. We investigated the relationships between 1) male copulatory behaviors and post-mating concentrations of steroids in the female, 2) female steroid concentrations and fertilization success of inseminations and 3) female steroid concentrations and the offspring sex ratio. We found that male condition and copulatory behaviors predicted female steroid concentrations and maternal investment in eggs laid after a mating trial. The body condition of one or both mates was a significant predictor of the changes in female corticosterone and testosterone concentrations after mating, whereas specific male copulatory behaviors significantly predicted changes in female progesterone concentrations. Male and female body condition, male neck grabs and post-mating concentrations of female corticosterone, progesterone, and testosterone were all significant predictors of egg fertilization rates. Female body condition, male copulation efficiency, and female testosterone concentrations were significant predictors of offspring sex ratios. Our results show that phenotypic and behavioral characteristics of male Japanese quail modulate female steroid concentrations and result in changes in maternal investment.     

Pigment limitation and female reproductive characteristics influence egg shell spottiness and ground colour variation in the house sparrow (Passer domesticus)

One of the hypotheses to explain egg colour variation in birds lays in the context of sexual selection, where egg colour may signal the female’s physical condition and antioxidant capacity. We tested one of the assumptions following from this hypothesis, that eggshell pigment deposition should be limited for females. The study was conducted in a captive house sparrow (Passer domesticus) population over several years under constant environmental conditions. This multi-brooded species lays eggs which vary in ground colour (biliverdin pigment) and in the intensity and distribution of brownish-red spots (protoporphyrin pigment). Spot darkness, spread and ground colour diminished along the laying sequence, suggesting that the deposition of both pigments was limiting for females over the short term. Also, the proportion of eggs with biliverdin diminished in consecutive clutches laid by the same female over the breeding season, suggesting a long-term cost of biliverdin deposition. On the other hand, spots were darker at the end of the breeding season, indicating that protoporphyrin deposition was probably not limited over the long term. This result could indicate a lower capacity for calcium deposition over the long term, which was compensated for by darker spots. Female age also significantly affected the proportion of bluish eggs and spot patterns. Egg pigmentation decreased with age, indicating that senescing passerine females lay less pigmented eggs. Clutch size was positively related to the proportion of bluish eggs and to spot patterns (darker and more evenly spotted). These results are in accordance with assumptions for the sexual selection hypothesis.     

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.     

Effects of experimental calcium availability,egg parameters and laying order on Great Tit Parus major eggshell pigmentation patterns

Many bird species lay eggs speckled with protoporphyrin‐based spots, however, for most of them the function of eggshell spotting is unknown. A plausible hypothesis is that protoporphyrin might have a structural function in strengthening the eggshell and is therefore deposited when calcium is scarce. In this study, we experimentally provided Great Tit Parus major females with supplemental calcium to examine its effect on the protoporphyrin‐based maculation of their eggs. In addition, we studied variation in eggshell pigmentation patterns in relation to other egg parameters and laying order. Calcium‐supplemented females laid larger eggs but shell thickness was not significantly affected by the treatment. Calcium supplementation may reduce the time and energy females devote to searching for calcium‐rich material, so that they can collect more nutrients and so lay larger eggs. Furthermore, pigment darkness was associated with egg volume and shape, which suggests that female quality and environmental food availability may also influence the shell pigmentation pattern. Within clutches, later‐laid eggs had larger and darker spots that were distributed more unevenly on the shell surface. This within‐clutch pattern could be explained by the increase in egg volume and egg shape and a decline in shell thickness with egg‐laying order, which characteristics were all related to shell‐spotting pattern. Eggs with a coronal ring had thinner shells, but pigment intensity and spot size were not related to shell thickness. Thus, our results suggest that concentrated spotting distribution may have a mechanical function, supporting the structural‐function hypothesis.     

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.     

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.     

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.     

Are eggshell spottiness and colour indicators of health and condition in blue tits Cyanistes caeruleus?

We explore the relationship between general eggshell colour (background and spots) and proportion of surface covered by spots on the eggs of blue tits Cyanistes caeruleus and several variables reflecting female health and condition. Females laying more spotted eggs showed a poorer body condition, higher cellular concentration of the stress protein HSP70 and marginally lower total immunoglobulin levels in blood. In addition, these females were paired with males with higher levels of HSP70 and lower concentrations of immunoglobulins. We interpret these results based on the properties of the eggshell pigment protoporphyrin, which is known as an inductor of oxidative stress in females. Although observational, this study presents, for the first time, clear evidence of eggshell spottiness and general colour as indicators of general condition and stress in avian populations.     

More colourful eggs induce a higher relative paternal investment in the pied flycatcher Ficedula hypoleuca: a cross-fostering experiment

The signalling hypothesis of eggshell colouration in birds posits that females of species with blue-green eggs signal their phenotypic quality or the quality of their eggs to their mates through deposition of the antioxidant biliverdin as pigment. Males respond by investing more in the offspring. Through a cross-fostering experiment where we have exchanged whole clutches between pairs of pied flycatcher Ficedula hypoleuca nests, we managed to break potential associations between female quality and clutch chromaticity. We show that males respond to incubated clutches with more variable and higher peak values in blue-green chroma through a higher proportional investment in nestling provisioning on day 4 of the nestling period, when males invest more heavily than females in provisioning. More variable clutches show higher peak chroma values. We also show that egg colour during the two-week incubation period has a significant effect, which is not found for the colour of eggs during the laying period. Finally, the proportion of male provisioning visits affects negatively female brooding effort and nestling mortality, and thereby has positive effects on female fitness. Blue-green chroma in the pied flycatcher functions as a signal of female or clutch quality to males which respond by adjusting their relative investment with respect to total pair effort.     

Avian eggshell coloration: new perspectives on adaptive explanations

Recent work suggests that the evolution of egg coloration may have been constrained in three important ways that have not yet been critically synthesized in any review. First, on account of birds being able to see in the ultraviolet spectrum, the interaction between the properties of avian vision and the light environment of nests imply different perceptions of egg coloration from those experienced by humans. Second, a new hypothesis to explain blue–green egg coloration interprets it as a sexually selected signal to males of the laying female's genetic quality. Third, evidence from taxa as divergent as sparrowhawks and great tits indicates that protoporphyrin pigments responsible for maculation (spotting patterns) have a structural function in compensating for eggshell thinning, as caused by calcium stress, and, more recently, dichlorodiphenyltrichloroethane. We consider this to be the most convincing explanation for the primary function of spotting, although an important secondary function might arise through the fact that individual patterns of maculation may allow birds to identify their own eggs, effectively serving as signatures in the face of inter‐ or intra‐specific brood parasitism. These constraints or hypotheses are not mutually exclusive, and should not be taken to imply that one, but not other, agents of selection might apply to any one species. However, the sexually‐selected eggshell coloration hypothesis is least plausible for hole‐nesting birds because of the poor light quality available, although such species have been the focus of research in this area, and only a single experimental study has shown a link between egg coloration and male provisioning. Furthermore, the observed relationships between female phenotypic quality and egg traits do not necessarily imply that they have signalling functions. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 753–762.     

Egg colour reflects the amount of yolk maternal antibodies and fledging success in a songbird

Blue-green colours in avian eggs have been proposed as post-mating signals of female phenotypic quality to their mates. Egg colour may not only indicate female value, but also the quality of the eggs themselves and of resulting offspring. To date, there has been no demonstration of an association between egg colour and egg or offspring immune quality. We here show that the intensity of blue-green colour of pied flycatcher Ficedula hypoleuca eggs reflects the amount of maternal antibodies in the yolk, a form of passive immunity crucial for offspring performance. Moreover, egg colour predicts fledging success. Also, incubating females in better condition lay more colourful eggs. The immunoglobulin level in incubating females is positively associated with that in the eggs. These results support the signalling hypothesis of eggshell coloration, underlining its role as an indicator of trans-generational transmission of immune defences in birds.     

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.     

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 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.