Shell shock

Egg colour changes in chickens

source: New Scientist vol 207 no 2770, 24 July 2010 p65

Chickens produce white eggs, though some breeds add a brown coating just before they lay. Shells form over some 20 hours in the hen's uterus, with brown pigment secreted in the last three or four hours. The eggshell is porous, with a waterproof cuticle that can be affected by ageing, infection, or stress in the hen. Stress leads to a release of adrenaline, which halts shell formation. This can affect the cuticle, and so pigmentation. Lack of a cuticle can also mean unprotected pigmentation, so colour can rub off. Pigmentation that does not rub off shows a properly formed cuticle, which helps prevent bacteria invading eggs, and eggs with cuticles are safer for humans to eat.