# Olive Egger Chickens: Genetics, Egg Color and Care An Olive Egger chicken is not a breed. It is a crossbred hen that inherits a dominant blue eggshell gene from one parent and a heavy brown pigment deposit from the other. The blue shell pigment shows through from the inside while the brown pigment coats the outside, and the two combine into shades of olive and green. That single genetic trick is the entire reason these birds exist, and it is also why no two Olive Egger flocks look or lay exactly alike. This article explains where the olive egg color comes from at the level of genes and pigments, how the choice of parents changes the shade, why some offspring lay blue or brown eggs instead of olive, and why the color of a single hen's eggs fades as the laying season goes on. It then covers what owners actually need to know about keeping these birds: appearance, temperament, housing, feeding, predator protection, and vaccination. The genetics are the interesting part. The husbandry is the part that keeps the hen alive and laying. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Key Takeaways - Olive Egger chickens are crossbred hens, not a standardized breed, that lay olive or green eggs by combining the dominant blue eggshell gene, oocyan, with heavy brown pigment from a dark-egg parent. - The blue shell color comes from biliverdin deposited throughout the shell, while brown comes from protoporphyrin IX deposited mainly on the outer shell and cuticle, and the two layers together read as olive. - A hen must inherit both the blue eggshell gene and the brown-depositing trait to lay olive eggs; chicks inheriting only one trait lay blue or brown eggs instead. - Parent choice determines shade: a deep blue layer crossed with a very dark brown layer such as Black Copper Marans trends toward dark olive, while moderate parents give lighter sage or khaki green. - The brown pigment half fades with hen age and laying cycle, so a hen's first eggs of the season are usually the darkest and color lightens as the season continues. ## At a Glance | Trait | Olive Egger | |--|--| | Status | Crossbred, not a standardized breed or recognized variety | | Primary purpose | Colored egg production | | Egg color | Olive to green, with individual and seasonal variation | | Egg color mechanism | Blue biliverdin through the shell plus brown protoporphyrin coating | | Comb type | Pea comb typical when blue-egg parent is Ameraucana or Araucana; Cream Legbar crosses usually have single combs | | Facial features | Muffs and beard possible, inherited from Ameraucana or Easter Egger parent lines | | Temperament | Variable, depends on parent stock | | Hardiness | Variable, depends on parent stock | | Lifespan | Varies with parent stock and care, similar to other backyard hens | | Weight and height | Varies with the parent breeds | | Grooming needs | Routine poultry care | | Good with children | Depends on individual bird temperament | Olive Eggers are produced by many different hatcheries and breeders using many different parent combinations, so a single weight, height, or lifespan number would be misleading. ## What an Olive Egger Actually Is The term Olive Egger describes a hen that lays olive or green eggs because of a specific combination of two independently inherited traits. It does not describe a breed with a closed gene pool, a written standard, or a registry. Two Olive Eggers from different hatcheries can be as genetically different from each other as two unrelated chickens. The practical definition has three parts. First, the hen must carry the dominant blue eggshell gene, called oocyan. This gene is found in Ameraucana, Araucana, Cream Legbar, and other blue-egg lines [1]. Second, the hen must also inherit the ability to deposit a heavy brown pigment coating on the shell. That trait comes from dark brown egg layers such as Marans, Welsummer, and Barnevelder [2][3]. Third, both traits must express in the same hen. A hen with only the blue gene lays blue eggs. A hen with only the brown trait lays brown eggs. A hen with both lays olive or green eggs. Because the blue gene is dominant and the brown deposit is a separate, additive trait, the olive result is not guaranteed in every chick from an Olive Egger mating. It is a predictable outcome of a specific cross, not a fixed property of a "breed." The blue eggshell trait itself is autosomal dominant, meaning a single copy of the gene is enough to produce a blue shell [1]. That is why a first-generation cross between a blue-egg parent carrying two copies of the gene and a brown-egg parent produces blue or olive eggs in essentially all daughters, while later generations become less predictable. ## The Genetics of Olive Egg Color ### The Blue Eggshell Gene and the Oocyan Allele The blue eggshell trait, called oocyan, was traced to an endogenous avian retroviral insertion, designated EAV-HP, near the SLCO1B3 gene [1]. Endogenous means the viral sequence is built into the bird's own genome and is inherited like any other gene. The insertion acts as a genetic switch that drives over-expression of SLCO1B3 in the shell gland and oviduct, the tissues that build the eggshell [1]. That over-expression is the mechanism behind blue shells. The SLCO1B3 gene encodes a transporter protein that moves biliverdin, a blue-green bile pigment, into the shell gland where the shell is formed. When the retroviral insertion boosts SLCO1B3 activity, more biliverdin reaches the shell, and the egg comes out blue or blue-green [1]. ### The Brown Pigment Layer Brown eggshell color comes from protoporphyrin IX. It is the major pigment in the shells of brown-egg laying hens, with only traces of biliverdin and its zinc chelates present [4]. The pigment is synthesized in the shell gland and deposited onto all shell layers, including the shell membranes, but most of it concentrates in the outermost layer of the calcareous shell and in the cuticle [4]. The extent of brown pigment deposition is not fixed. It is influenced by the housing system, hen age, hen strain, diet, stressors, and certain diseases such as infectious bronchitis [4]. This is a key point for Olive Egger owners, because it means the brown half of the olive equation is the variable half. The blue half is genetic and stable. The brown half responds to the hen's condition. Protoporphyrin is made through the same pathway birds use to make heme, and the activity of several genes in that pathway influences how brown the shell becomes [5]. ### How Blue Plus Brown Becomes Olive The olive color is a visual result of two pigments in two different places. Biliverdin is deposited throughout the shell structure, giving the shell a blue-green base. Protoporphyrin is deposited mainly on the outside, giving it a brown overlay. Blue underneath plus brown on top reads to the human eye as olive or green. The pigment chemistry supports this. In a study of blue-shelled egg subtypes ranging from light blue to olive, darker, more brown-toned shells carried more protoporphyrin [6]. In plain terms, more brown pigment pushes the color toward olive and green, while more biliverdin alone keeps it blue. The two pigments are laid down together as the shell is built, not painted on afterward, with pigment deposition fastest in the outermost shell layer [7]. A separate study of Changshun blue-shell chickens compared dark blue, light blue, greenish-brown, and brown shell types. The brown and greenish-brown groups had significantly higher protoporphyrin than the blue group, while there was no significant difference in biliverdin concentration between the brown and blue groups [8]. That finding is important. It means the shift from blue to greenish-brown is driven mainly by adding protoporphyrin, not by removing biliverdin. The blue is still there. The brown is what changes the final color. ### Why F1 and Later Generations Vary A first-generation (F1) Olive Egger comes from crossing a blue-egg parent with a dark brown-egg parent. Because the blue gene is dominant, F1 daughters that inherit it will lay blue-based eggs, and because they also inherit brown-depositing genetics from the other parent, those eggs tend toward olive [2][3]. Later generations behave differently. Once you breed Olive Eggers to each other, the traits segregate. Some chicks inherit the blue gene and the brown trait and lay olive. Some inherit the blue gene without strong brown deposition and lay blue or blue-green. Some inherit strong brown deposition without the blue gene and lay brown. The more generations you go, the wider the spread of outcomes. This is why hatcheries that sell Olive Eggers usually sell F1 or specific F2 crosses rather than promising a fixed result. The parent combination determines the odds, and the odds are never 100 percent. ### Why Parent Choice Changes the Shade The shade of an Olive Egger's egg depends on two variables: how blue the blue parent's eggs are, and how dark the brown parent's eggs are. A hen from a line selected for deep blue shells contributes more biliverdin. A hen from a dark brown line such as a Black Copper Marans contributes more protoporphyrin. Combine a deep blue layer with a very dark brown layer and the result trends toward a darker, muddier olive. Combine a lighter blue layer with a moderate brown layer and the result trends toward a lighter sage or khaki green. The genetics of blue shell depth are heritable and selectable. In a three-generation selection experiment on a blue-eggshell line, breeders were able to shift shell color toward either blue or green [9]. This confirms that eggshell color in blue-egg lines is a trait breeders can push in a chosen direction. It also confirms that the color is not a single on-off switch. It is a quantitative trait with a range. ### Why Some Offspring Lay Blue or Brown Eggs The blue eggshell gene is dominant, but it is not the only gene in play. A chick that inherits the blue gene but not the brown-depositing genetics will lay blue or blue-green eggs. A chick that inherits the brown-depositing genetics but not the blue gene will lay brown eggs. Only the chicks that inherit both express the olive phenotype. This is a common source of disappointment for new Olive Egger owners who hatch their own chicks. If you breed an Olive Egger hen to a non-blue rooster, roughly half the daughters may lack the blue gene entirely and lay brown eggs. If you breed two Olive Eggers together, the outcomes scatter across blue, green, olive, and brown. ### Why Shade Fades During the Laying Season The brown half of the olive equation fades as the hen ages and as the laying season progresses. This is a well-documented pattern in brown-egg layers. A study of 62-week-old Hy-Line brown-egg laying hens compared hens laying darker shells with hens laying lighter shells. Egg weight, eggshell weight, protoporphyrin IX content of the eggshell and shell gland, and biliverdin content of the shell gland were all significantly lower in the light-shell group [10]. The practical translation is straightforward. As a hen ages and as she moves through a long laying cycle, her shell gland produces less protoporphyrin. The blue biliverdin base is still there, so the egg does not turn blue. It turns a paler, less saturated olive or green. Many owners notice that a hen's first eggs of the season are the darkest and that the color lightens as the season continues. That pattern is consistent with the pigment biology. The brown pigment review reaches the same conclusion from a different angle. The extent of pigment deposition is influenced by hen age, hen strain, diet, stressors, and disease [4]. Anything that stresses the hen or disrupts her shell gland can lighten the brown overlay and therefore lighten the olive. ## Common Parent Crosses and Expected Egg Shade The table below lists parent combinations that backyard breeders commonly use. The expected shade column describes the general direction, not a guarantee for any individual chick. | Blue-egg parent | Brown-egg parent | Expected egg shade in daughters | |--|--|--| | Ameraucana | Black Copper Marans | Dark olive to deep green | | Ameraucana | Welsummer | Medium olive to khaki | | Ameraucana | Barnevelder | Medium olive to brownish green | | Araucana | Marans | Dark olive to deep green | | Cream Legbar | Marans | Dark olive, often with strong green tone | | Cream Legbar | Welsummer | Medium olive to sage green | | Blue-egg line selected for deep blue | Dark brown line | Darker, more saturated olive | | Blue-egg line selected for light blue | Moderate brown line | Lighter sage or pale olive | Two rules govern the table. The darker the brown parent's eggs, the darker the olive. The deeper the blue parent's shells, the more saturated the green. When both parents are at the extreme end of their respective traits, the daughters tend toward dark olive. When either parent is moderate, the daughters tend toward lighter, more variable shades. The table also carries a warning. These are F1 expectations. If you breed the resulting Olive Eggers to each other, the next generation will not follow the table. It will scatter. ## Appearance and Variability Because Olive Eggers are crosses, their appearance varies with the parent stock. There is no standard. Two Olive Eggers from different sources may share almost no visible features. The one feature that is fairly consistent is the pea comb, and only when the blue-egg parent is an Ameraucana or Araucana. The pea comb is a trait of those blue-egg lines, and it is inherited along with the blue egg gene in many crosses. Cream Legbars have single combs, so Legbar crosses usually do too. A pea comb is a low, triple-rowed comb rather than a single upright blade. Muffs and a beard are also possible. These are the fluffy cheek feathers and the tuft of feathers under the beak that give Ameraucana birds their distinctive face. Araucanas are clean-faced but may carry ear tufts instead. Whether an Olive Egger has them depends on whether the blue-egg parent carried those genes and whether they were passed on. Body type, size, plumage color, leg color, and skin color all follow the parent stock. A Marans-cross Olive Egger may inherit feathered legs. A Legbar-cross Olive Egger may inherit the Legbar's crested head. A Welsummer-cross Olive Egger may inherit the Welsummer's partridge pattern. Plumage color is the most visible variable. Olive Eggers can be black, blue, splash, wheaten, partridge, cuckoo, or any combination the parent lines produce. There is no "correct" Olive Egger color. ## Temperament Temperament in Olive Eggers is inherited, not fixed by the cross itself. A bird from calm, handleable parent lines tends to be calm and handleable. A bird from flighty or aggressive parent lines tends to be flighty or aggressive. The olive egg trait has no relationship to temperament. This is worth stating plainly because some hatchery descriptions imply that all Olive Eggers share a personality. They do not. The temperament of any individual bird is a product of its genetics, its handling as a chick, and its environment. Owners who want a calm backyard hen should ask the breeder about the temperament of the parent birds. That is more informative than any breed-level description. ## Hardiness Hardiness follows the same rule as temperament. It depends on the parent stock. A cross between two hardy parent lines tends to produce hardy offspring. A cross involving a less robust parent line may produce less robust offspring. Hybrid vigor can work in the bird's favor in a first-generation cross, but it is not guaranteed, and it is reduced in later generations. The blue egg gene itself does not make a bird more or less hardy. It is a shell pigment trait. It affects the color of the egg, not the health of the hen. ## Egg Production Numbers Egg production numbers for Olive Eggers are variable, and any specific number should be treated with caution. The reason is the same as everywhere else in this article. Olive Eggers are not a breed, so there is no breed standard for annual egg production. A hatchery that crosses two high-production parent lines may get a hen that lays steadily through the season. A breeder who crosses a show-quality Marans with a show-quality Ameraucana may get a hen that lays fewer eggs of exceptional color. There is no clear evidence that the blue egg gene or the brown pigment trait by itself reduces production. Both are shell traits. Production is determined by the parent lines. Owners who care most about egg numbers should ask the breeder about the laying performance of the parent stock. Owners who care most about egg color should ask about the shell color of the parent stock. The two goals can be pursued together, but they are separate questions. ## Housing Olive Eggers need the same housing as any other backyard chicken. The requirements are not breed-specific. The essentials are a coop that keeps the birds dry, ventilated, and protected from drafts, with enough roost space for every bird to perch without crowding. Floor space should be adequate for the flock size so that birds are not forced into constant contact, which promotes feather pecking and stress. Stress matters more in Olive Eggers than in some other birds, because stress affects the brown pigment deposition that gives the egg its olive tone [4]. A hen that is crowded, bullied, or chronically uncomfortable may lay paler eggs. Good housing is not just a welfare issue. It is an egg color issue. Nesting boxes should be dark, quiet, and lined with clean bedding. Hens prefer to lay in a secluded spot, and providing one reduces the chance of eggs being laid on the floor where they can crack or be eaten. ## Feeding and Nutrition Feeding an Olive Egger is straightforward. A complete layer feed formulated for backyard hens provides the base nutrition the bird needs. The key nutritional consideration for laying hens is calcium, because the shell is made largely of calcium carbonate and a hen mobilizes a large amount of calcium every time she lays. Oyster shell offered free-choice is the standard way to let a hen top up her calcium intake. Hens that need more calcium will eat it. Hens that do not need it will leave it. This is particularly relevant for high-producing hens and for older hens, whose shell quality can decline. The brown pigment review notes that diet influences the extent of pigment deposition [4]. That does not mean a specific feed will make eggs darker. It means a hen on a poor or unbalanced diet may lay paler eggs than the same hen on a complete diet. The reliable approach is a complete layer ration plus free-choice oyster shell, with treats kept to a small fraction of total intake. Fresh, clean water should be available at all times. Water intake affects feed intake, and feed intake affects egg production and shell quality. ## Predator Protection Predator protection is a housing requirement, not an optional extra. Backyard flocks are vulnerable to raccoons, foxes, hawks, owls, snakes, rats, opossums, and domestic dogs and cats. The defense is a secure coop with hardware cloth rather than chicken wire on openings, a predator-proof latch on the door, and a covered run. Hardware cloth resists chewing and pulling in a way that chicken wire does not. Buried aprons or buried wire skirts around the run perimeter deter digging predators. A common failure is a coop that is secure during the day but left open at night. Many predators hunt after dark. A coop that is closed and latched at dusk and opened at dawn prevents many losses. ## Health and Marek's Vaccination Marek's disease is the one health topic that deserves specific attention for anyone buying Olive Egger chicks. Marek's disease is caused by a herpesvirus that infects chickens and can cause tumors, paralysis, and death. It is widespread in the environment, and it is one of the most economically significant diseases of poultry. Vaccination is the standard control measure. The practical question for owners is whether the chicks they buy have been vaccinated. Hatcheries that sell Olive Eggers typically state whether their chicks are vaccinated for Marek's disease, and buyers should ask if it is not stated. Chicks are usually vaccinated at the hatchery on the day of hatch, which is the appropriate time because the vaccine is most effective before the chick is exposed to the virus. Marek's vaccination does not prevent infection. It greatly reduces the risk of disease. Vaccinated birds can still carry and shed the virus, but they are far less likely to develop tumors and paralysis. Beyond Marek's, Olive Eggers face the same health risks as any backyard chicken. Respiratory disease, internal and external parasites, and reproductive problems such as egg binding are common in backyard flocks. The brown pigment review specifically notes that infectious bronchitis can affect pigment deposition, which means a respiratory infection in an Olive Egger may show up as a change in egg color [4]. Owners should establish a relationship with a veterinarian who treats poultry before a problem arises. Many small animal practices do not see chickens, so finding a poultry-experienced veterinarian in advance avoids a scramble during an emergency. ## Recommended Screening and Preventive Care Routine preventive care for a backyard flock includes regular visual inspection of the birds for signs of illness, parasites, or injury, and periodic examination of the coop for red mites and other external parasites. A veterinarian can perform a physical examination and, where indicated, fecal testing for internal parasites. There is no breed-specific screening panel for Olive Eggers because there is no breed-specific disease. Screening is based on the individual bird's signs and history. Owners should monitor egg production and egg quality as a general health indicator. A sudden drop in production, a sudden change in shell quality, or a sudden change in shell color can be an early sign of illness or stress. Because Olive Eggers already have variable egg color, owners should learn what is normal for each individual hen so that a genuine change stands out. ## Lifespan There is no fixed lifespan figure for Olive Eggers, because lifespan follows the parent stock and the quality of care. What can be said is that a hen's productive laying life and her total lifespan are different things. A hen may lay well for a few years and then continue to live for several more with reduced or no production. Owners who keep hens as pets should plan for the retirement phase, which requires the same housing and feeding as the laying phase. ## Finding a Responsible Breeder or Rescue Because Olive Eggers are crosses, the breeder matters more than the label. A responsible breeder can tell you exactly which parent lines produced your chicks and what egg color to expect. Ask three questions. What are the parent breeds? What color eggs do the parent hens lay? Are the chicks vaccinated for Marek's disease? A breeder who can answer all three is selling you a bird you can plan around. A breeder who cannot answer them is selling you a bird whose egg color is a guess. Rescue is also an option. Backyard flocks sometimes need rehoming when owners move or lose interest, and poultry rescues and farm animal sanctuaries place birds. A rescued Olive Egger may be of unknown parentage, which means the egg color is unknown until she lays. For owners who care more about giving a bird a home than about egg color, that is not a drawback. ## Who Should Not Get an Olive Egger Olive Eggers are not the right choice for every household. They are a poor fit for anyone who needs guaranteed egg color. The olive egg is a probability, not a promise, and later-generation birds scatter widely. An owner who wants every hen to lay a specific shade should buy from a breeder who can document the parent lines and should accept that even then the result varies. They are a poor fit for anyone who wants a standardized show bird. Olive Eggers are not a recognized breed and cannot be shown as one. They are a poor fit for anyone unwilling to provide secure housing and daily care. Like all chickens, they depend on their owners for protection from predators and for clean water and complete feed. They are a poor fit for anyone in a community that restricts poultry. Local ordinances vary, and owners should confirm that chickens are permitted before acquiring birds. ## Limitations and When to Contact a Veterinarian This article describes the genetics and general care of Olive Eggers. It cannot diagnose or treat a specific bird. Individual hens vary in health, temperament, and production, and a veterinarian who examines the bird is the only reliable source of advice for a medical problem. Contact a veterinarian promptly if a hen stops eating or drinking, becomes lethargic or reluctant to move, shows labored breathing, has a swollen or fluid-filled abdomen, strains without producing an egg, passes bloody or abnormal droppings, or develops a sudden drop in egg production or a sudden change in shell quality. Sudden paralysis or an inability to stand warrants immediate attention, particularly in an unvaccinated bird, because those signs are consistent with Marek's disease. Contact a veterinarian if a bird is being bullied or is losing weight despite access to feed, because chronic stress affects both welfare and egg quality. Contact a veterinarian if you see external parasites on the bird or in the coop, or if you suspect internal parasites. For flock-level concerns such as a respiratory infection moving through the coop, a veterinarian can advise on treatment and on protecting the rest of the flock. Infectious bronchitis is one example of a disease that can affect eggshell pigment, so a change in egg color during a respiratory outbreak is worth reporting [4]. ## Frequently Asked Questions ### Is the Olive Egger a breed? No. The Olive Egger is a crossbred chicken, not a recognized breed or standardized variety. It is defined by the combination of a dominant blue eggshell gene and a brown pigment deposit, not by a breed standard. ### Why do Olive Eggers lay olive eggs? The blue eggshell gene deposits biliverdin, a blue-green pigment, throughout the shell. The brown-egg parent contributes protoporphyrin, which coats the outside of the shell. Blue underneath plus brown on top reads as olive or green. ### Will every chick from an Olive Egger mating lay olive eggs? No. Some daughters inherit the blue gene without strong brown deposition and lay blue or blue-green eggs. Some inherit the brown trait without the blue gene and lay brown eggs. Only those that inherit both lay olive. ### Why do my Olive Egger's eggs get lighter as the season goes on? Brown pigment deposition declines as a hen ages and as the laying cycle progresses. The blue base remains, so the egg shifts from a saturated olive toward a paler green rather than turning blue. ### Do Olive Eggers have pea combs? Often, yes, when the blue-egg parent is an Ameraucana or Araucana. The pea comb is inherited from those lines, while Cream Legbar crosses usually have single combs. Crosses using other blue-egg parents may produce different comb types. ### Are Olive Eggers good backyard chickens? They can be, but temperament and hardiness depend on the parent lines, not on the olive egg trait. Ask the breeder about the parent birds before buying. ### Should Olive Egger chicks be vaccinated for Marek's disease? Marek's vaccination is the standard control measure for the disease and is typically given at the hatchery on the day of hatch. Ask the seller whether the chicks have been vaccinated. ### How many eggs does an Olive Egger lay? There is no fixed number, because Olive Eggers are crosses rather than a breed. Production depends on the parent lines. Ask the breeder about the laying performance of the parent stock. ## Related Articles - [Raising Chickens for Eggs: Breed Selection, Care, and Egg Production](/knowledge/animal-farming/poultry/raising-chickens-for-eggs-breed-selection-care-and-egg-production) - [Chicken Breeds for Beginners: Eggs, Housing, Climate, and Flock Welfare](/knowledge/veterinary-medicine/backyard-poultry/chicken-breeds-beginners) - [Grit and Oyster Shell: Essential Supplements for Chicken Digestion and Eggs](/knowledge/animal-farming/poultry/grit-oyster-shell-supplements-chicken-digestion-eggs) - [Small-Scale Chicken Farming: Breeds, Housing, and Flock Management](/knowledge/animal-farming/poultry/small-scale-chicken-farming-breeds-housing-and-flock-management) - [Indigenous and Specialty Chicken Breeds: Farming Kadaknath, Kuroiler, and Native Breeds](/knowledge/animal-farming/poultry/indigenous-specialty-chicken-breeds-farming-kadaknath-kuroiler-native) - [Egg Bound Chicken: Recognizing Symptoms and Emergency Warm Bath Protocol](/knowledge/veterinary-medicine/backyard-poultry/egg-bound-chicken-recognizing-symptoms-and-emergency-warm-bath-protocol) - [Cage-Free Chicken Farm Management: Housing, Welfare, and Egg Production](/knowledge/animal-farming/poultry/cage-free-chicken-farm-management-housing-welfare-egg-production) - [Australorp Chickens: Egg Laying, Temperament and Care](/knowledge/veterinary-medicine/backyard-poultry/australorp-chicken-breed-guide) - [White Leghorn Chickens: Egg Production, Temperament and Health](/knowledge/veterinary-medicine/backyard-poultry/white-leghorn-chicken-breed-guide) - [How Long Do Chickens Live? Lifespan by Breed and Care](/knowledge/veterinary-medicine/backyard-poultry/how-long-do-chickens-live) ## Sources 1. [Endogenous retrovirus EAV-HP linked to blue egg phenotype in Mapuche fowl.](https://pubmed.ncbi.nlm.nih.gov/23990950/) 2. [Getting to Know Olive Egger Chickens - Backyard Poultry (backyardpoultry.iamcountryside.com)](https://backyardpoultry.iamcountryside.com/chickens-101/getting-to-know-olive-egger-chickens/) 3. [How to Breed Olive Egger Chickens: 6 Questions Answered - Cackle Hatchery (cacklehatchery.com)](https://www.cacklehatchery.com/how-to-breed-olive-egger-chickens-6-questions-answered/) 4. [Eggshell color in brown-egg laying hens - a review.](https://pubmed.ncbi.nlm.nih.gov/26240390/) 5. [Quantitative expression of candidate genes affecting eggshell color.](https://pubmed.ncbi.nlm.nih.gov/24612318/) 6. [The pigments in eggshell with different colour and the pigment regulatory gene expression in corresponding chicken's shell gland.](https://pubmed.ncbi.nlm.nih.gov/37043933/) 7. [Study of the deposition process of eggshell pigments using an improved dissolution method.](https://pubmed.ncbi.nlm.nih.gov/17878455/) 8. [Pigment concentrations in eggshell and their related gene expressions in uterus of Changshun blue eggshell chickens.](https://pubmed.ncbi.nlm.nih.gov/34585996/) 9. [Genetics and breeding of a black-bone and blue eggshell chicken line. 4. Responses to 3 cycles of selection for blue or green eggshell.](https://pubmed.ncbi.nlm.nih.gov/41529493/) 10. [Mitochondrial transcription factor A induces the declined mitochondrial biogenesis correlative with depigmentation of brown eggshell in aged laying hens.](https://pubmed.ncbi.nlm.nih.gov/33518349/)