Kiwi Skeleton with Egg
They are unusual among other birds in that along with some raptors, they have a functioning pair of ovaries. (In most birds and in , the right ovary never matures, so that only the left is functional.) Kiwi eggs can weigh up to one-quarter the weight of the female. Usually only one egg is laid per season. The kiwi lays the biggest egg in proportion to its size of any bird in the world, so even though the kiwi is about the size of a domestic chicken, it is able to lay eggs that are about six times the size of a chicken’s egg. The eggs are smooth in texture, and are ivory or greenish white. The male incubates the egg, except for the Great Spotted Kiwi, A. haastii, in which both parents are involved. The incubation period is 63–92 days. Producing the huge egg places a lot of demands on the female. For the thirty days it takes to grow the fully developed egg the female must eat three times her normal amount of food. Two to three days before the egg is laid there is little space left inside the female for her stomach and she is forced to fast.