How could one egg be two strikingly different colours on each hemisphere? One of my young chickens has just produced an unusually coloured egg. The egg on the left is more typical. I know eggshell colour is variable, even in eggs laid by the same hen on different days, but how did one egg undergo such a sudden and distinct colour change?
鈥� The answer probably lies in the fact that, until shortly before they are laid, hens鈥� eggs are white. The brown pigmentation associated with breeds such as the Rhode Island Red and the Maran is a last-minute addition during egg formation and, like a fresh coat of paint, can come off surprisingly easily.
More than 90 per cent of the shell of a hen鈥檚 egg comprises calcium carbonate crystals bound in a protein matrix. The shell starts to form after the egg has reached the uterus, where it stays for around 20 hours prior to being laid.
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During this time, glands secrete the shell around the membranes that hold the yolk and albumen. In brown-egg-laying breeds, the cells lining the shell glands release pigmentation during the last 3 to 4 hours of shell formation. Most of the pigment is transferred to the cuticle, a waterproof membrane that surrounds the porous eggshell.
Several factors can disturb the cuticle formation process and thus pigmentation, such as ageing, viral infections 鈥� including that perennial chicken farmer鈥檚 nemesis, bronchitis 鈥� and drugs such as , which has been widely fed to poultry to combat a disease caused by a type of protozoa. Possibly the most significant factor affecting egg pigmentation is exposure to stress during the formation of the egg.
If a flock of hens is disturbed by a fox during the night, for example, they might well lay paler eggs in the morning. The adrenaline the hens release puts egg-laying on hold and shuts down shell formation. The egg鈥檚 pigmentation will be affected if the cuticle doesn鈥檛 form properly.
鈥淚f a flock of hens is disturbed by a fox, they might well lay paler eggs in the morning鈥�
Even if the pigmentation is laid down, there is no guarantee that it will last, as Morris Steggerda and Willard F. Hollander found in 1944 while they were studying eggs from a flock of Rhode Island Reds in the US. When they cleaned the eggs, the brown pigment occasionally came away; the harder the eggs were rubbed, the more pigment was removed. Only those shells with a glossy sheen retained their colour, suggesting their cuticles had been fully formed, with a protective layer that acted rather like the varnish on an oil painting.
As for the egg photographed by your questioner, the bird was probably disturbed while the cuticle was being formed and so the pigment, inadequately protected, was rubbed off the larger, rounder end of the egg as it was forced out.
The issue may have some significance for public health. The waterproof cuticle is the egg鈥檚 defence against bacteria. As shell colour is affected by how well the cuticle forms, it also provides a visual test of how free of harmful bacteria an egg may be.
Hadrian Jeffs, Norwich, Norfolk, UK
鈥� Before an egg is laid, the hen鈥檚 shell gland secretes pigment into the fluid bathing the egg鈥檚 surface. The fluid smears readily, and any disturbance while the egg dries can create marks. Farmers are therefore fussy about the kind of bedding they use in nesting boxes.
Eggs usually are laid big end first. The hen that laid the egg in question may have resorted to using friction to release the partly laid egg from its cloaca, possibly by rubbing the egg against the bedding it was sitting on.
Alternatively, the hen may have paused halfway through laying, perhaps disturbed or exhausted, with the egg half-protruding from its cloaca. The part of the egg still within the cloaca had time to achieve a deep colour before the hen relaxed again, and this accounts for the sharp boundary in colouration seen in the photo. Such a scenario, though unusual, is not highly unlikely.
Jon Richfield, Somerset West, South Africa