White eggshells:
A study identified that it is notable that the protoporphyrin content which creates brown eggshell color in the uterine tissue of white-egg laying birds is not dissimilar to that of brown-egg laying birds. White-shelled eggs are not necessarily devoid of pigment. (Sparks, 2011).
A study identified that the presence of biliverdin and/or protoporphyrin IX does not inevitably mean that an egg is colored. Both pigments were identified as being widespread. When found in eggshells, they can cause brown or blue–green coloration, but many white avian eggs contain the pigments in low levels. An extensive survey determined that pigment-free white eggs are rare. An egg containing these pigments could thus either be colored or not, depending on the concentration of pigments. (Wiemann et al., 2019).
Brown eggshells:
A study by Samiullah et al found that the major pigment in eggshells of brown-egg laying hens is protoporphyrin IX, but traces of biliverdin and its zinc chelates were also present. The pigment appears to be created in the shell gland. This study reflected that the protoporphyrin IX synthetic pathway is identified, but precisely where and how it is synthesized in the shell gland of the brown-egg laying hen is unclear. The pigment was found to be deposited onto all shell layers including the shell membranes, but most of it was concentrated in the outermost layer of the calcareous shell and in the cuticle. The extent of pigment deposition was impacted by the housing system, hen age, hen strain, diet, stressors, and certain diseases such as infectious bronchitis. (Samiullah et al., 2015).
A study analyzed three different types of hens that generated white, pink or brown eggshells. Their data revealed three ways in which eggshell color was genetically influenced. First, high‐level expression of CPOX generated more protoporphyrinogen and a brown eggshell color. In contrast, high expression of FECH likely converted more protoporphyrinogen into heme, reduced protoporphyrinogen levels within the eggshell and generated a light color. Second, heme transporters also affected eggshell color. High‐level expression of BCRP, HRG1 and FLVCR were associated with brown, white and generally lighter eggshell colors, respectively. Finally, protoporphyrin precipitation also affected eggshell color, high expression of both SLCO1A2 and SLCO1C1 were associated with brown eggshell color. They thus identified seven genes in which expression levels in different tissues were associated with eggshell color. (Jianfei et al., 2020).
A study found a negative association between plasma and feces protoporphyrin concentrations during egg laying and eggshell brown chroma. This indicates that an increased creation of protoporphyrin in the liver resulting in elevated plasma and feces protoporphyrin concentrations could inhibit eggshell protoporphyrin pigmentation by way of affecting enzymatic activities. They identified that circulating pigment levels may influence shell gland pigment synthesis, therefore linking the physiological level of the female to eggshell coloration. (Hargitai et al., 2017).
Blue eggshells:
A study reflected that green and blue‐green shells contain biliverdin, an oxidative ring opening product of PPIX with broad absorption in the blue and orange‐red region. The study found that small differences in the absorption width or maximum induce the change of the color impression from more green to more blue. The green shells typically contain PPIX in addition to BV, resulting in a dark color. The study identified that the color pigment protoporphyrin IX (PPIX) is embedded in the protein phase of all four shell types as highly fluorescent monomers, in the white and light green shells additionally as non‐fluorescent dimers, and in the brown and dark green shells mainly as non‐fluorescent poly‐aggregates. Green shell colors were identified as being formed from an approximately equimolar mixture of PPIX and biliverdin. (Ostertag et al., 2019).
A study in 2021 narrowed down SNPs to a region of ~ 653.819 Kb on GGA21 that harbors five candidate genes associated with eggshell color: AJAP1, TNFRSF9, C1ORF174, CAMTA1, and CEP104. The shell gland of chickens laying dark and light blue eggshell were examined for detection of mRNA expression of the five candidate genes. The results showed differential expression levels of these genes in the two groups. The specific function of these genes has not yet been defined clearly in chickens and further in-depth studies are needed to explore the new functional role in chicken eggshell blueness. (Ming-Yuan et al., 2021).
A study found that the insertion site in the blue-shelled chickens from Araucana is different from that in Chinese breeds, which reflects independent integration events in the blue-shelled chickens from the two continents. (Wang et. al, 2013).
A genome wide association study (GWAS) was conducted in Chinese Dongxiang blue-shelled chicken underlying four traits of blue eggshell pigments: quantity of biliverdin (QB), quantity of protoporphyrin (QP), quantity of total pigment (QT), and color density trait (CD). (Darwish et al., 2019).
Miscellaneous:
A study indicated that phylogenetic dependence must be considered in further explanations of the functional significance of avian eggshell color. (Wragg et. al., 2013).
A study identified that organic anion transporting polypeptide family members (including solute carrier organic anion transporter family, SLCO1C1, SLCO1A2, SLCO1B3 and LOC418189) may affect pigment transport within eggshells. (Chuanwei et al., 2014).
A study reflected that a female’s ability to handle oxidative stress influences pigment deposition in the eggshell. They found a positive relationship between eggshell pigment distribution and maternal and yolk antioxidant protection, indicating that eggshell pigmentation is a signal of female (and offspring) quality. (Giordano et al., 2015).
The effect of parasites on eggshell color:
A study found a correlation between spot darkness and laying order that weakened under high parasite load. Their study suggests that anemia in females due to parasites led to diminished protoporphyrin from disintegrated red blood cells and hence a decreased deposition of protoporphyrin. (De Coster et al., 2012).
Eggshell color as affected by carotenoids:
A study completed found that biliverdin, the pigment responsible for the blue‐green color of a eggshell, is a strong antioxidant and that only females with high antioxidant capacity can deposit higher concentrations of biliverdin as eggshell pigment. Antioxidants such as carotenoids and vitamins are also abundant in the egg yolk, which serve as nutrient reserves for the developing embryo. Eggshell color was positively related to the concentration and amount of carotenoids and vitamin E in the yolk. (Hargitai et al., 2013). Another study also found similar results and reflected that eggshell color intensity was positively related to the concentration and amount of carotenoids and vitamin E in the yolk. (Navarro, 2011).
A study found that throughout the laying sequence, biliverdin concentration increased while eggshell thickness, yolk carotenoid concentration, and lysozyme concentration in the albumen all decreased. The biliverdin concentration explained, at most 46% of the variation for all eggshell coloration metrics and suggest biliverdin concentration is a better predictor of egg quality than egg coloration in the birds studied. (Butler et al., 2015).
Morales et al completed a study in 2011 and found that there was a decline in eggshell color with laying order, indicating pigment limitation for females. However, carotenoid-supplemented females had increased second egg coloration compared to controls. Their results suggest that biliverdin-based eggshell coloration is costly to create and can be eased by carotenoid accessibility. (Morales et al., 2011).
Eggshell color as affected by temporal factors:
Eggshell color was studied on over 400 eggs at the moment of oviposition, at the onset of incubation, and at the end of incubation. Results showed that egg color faded significantly during the study period. This study indicates that egg coloration studies should account for temporal changes in egg coloration. (Moreno et. al, 2011).
A study identified that blue‐green chroma and brightness of the same eggs varied significantly between years. This implies the need for future research into the amount of chemical and physical deterioration of eggshell appearance even during relatively short‐term storage. (Cassey et al., 2011).
A study found high correlations between the shell color at 42 wk of age and subsequent ages (50, 60, and 70 wk) of laying hens, indicating that the intensity of eggshell color is more stable after egg-laying peaks. They also found that the intensity of brown eggshell color fluctuates greatly among the whole laying cycle. (Huijuan et al., 2018).
Eggshell color as affected by environmental conditions:
A study showed that eggshell pigmentation is not only affected by female body condition but is also shaped by the female’s early life experience such as stress exposure. Eggshell color was influenced by breeding conditions, which stresses the complexity of the relationship between eggshell pigment concentrations and environmental conditions. (Duval, 2014).
A study found that females who experience a decline in antioxidant capacity due to food limitation experience a reduction in blue‐green eggshell color. Food-restricted females laid eggs with significantly higher eggshell brown chroma, spot intensity, and protoporphyrin concentration. The results suggest that limitation in nutrient availability increases deposition of protoporphyrin into the eggshell. (Hargitai et. al, 2017).
Another study found that eggs with lower yolk antioxidant concentration had higher average eggshell brown chroma. Results from the study indicated that eggshell color is affected by female antioxidant and nutrient accessibility. (Hargitai et al., 2016).
A study identified that the blue‐green color of eggshells can disclose an inherent aspect of females' physiological condition, with only high‐quality females having adequate antioxidant capacity to pigment their eggs with large amounts of biliverdin. Subsequent work has argued instead that eggshell color might signal condition‐dependent traits based on diet. The study reflected that individual females contrasted strongly and consistently from one another, despite having been reared under uniform circumstances. (Dearborn et al., 2012).
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