Graham Butcher, of Tawa, asks :-
I recently cracked an egg and found it had three yolks, two small and one large. How rare is this?
Ben Bell, a zoologist at Victoria University of Wellington, responded.
I cannot do any better than quote Wikipedia's entry. Double-yolk eggs occur when ovulation occurs too rapidly, or when one yolk becomes joined with another yolk. These eggs may be the result of a young hen's reproductive cycle not yet being synchronised. Some hybrid breeds of hens also produce double-yolk eggs by default. Such eggs are produced in India. Eastern states known for that are West Bengal and Andhra Pradesh.
Some hens will rarely lay double-yolked eggs as the result of unsynchronized production cycles. Although heredity causes some hens to have a higher propensity to lay double-yolked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay. Usually, a double-yolked egg will be longer and thinner than an ordinary single-yolk egg. Double-yolked eggs usually lead to observed successful hatchlings only under human intervention, as the chickens interfere with each other's hatching process and die.
Rarely, higher-order yolks occur, although heavier poultry breeds such as the Buff Orpington have been known to lay triple-yolk eggs in their lifetimes.