The Avian Egg
The egg is a life support system for the baby bird. It has everything the baby needs except a heating system. The yolk is a store of food for the growing chick. This is surrounded by egg white, a thick sticky fluid which cushions the yolk and which will also be eaten by the chick.
Forming eggs is demanding for the mother so only one egg is released at a time. It is immediately attached to a yolk and begins its journey down a long passage called the oviduct. It is fertilised along the way, the white protective cushion is added and finally the shell. Cell division begins as the egg progresses and the early embryo begins its development. The entire process, from release to laying, takes a day in small birds, two days in larger ones.
The early embryo, on top of the yolk, can be seen as a white spot about 4 mm in extent on top of the yolk. Cell division stops after laying and won't resume until incubation begins. The egg is dormant at this stage and it does not need heat to survive.
Most small birds won't start incubating until the entire clutch has been laid. The weight of a clutch of blue tit eggs is greater than the weight of the mother. It would make too great a demand on the bird to gather enough resources to form eggs and to sit and incubate at the same time. Some species, however, manage to do both. To incubate her eggs, a bird must raise their temperature almost to that of her own body.
Eggs are fragile and their rich store of nutrient attracts rats, cats and magpies, so birds must nest in inaccessible places. Most species build a protective structure, a further demand on a bird's time and resources. Nests are built therefore well in advance of the release of eggs.
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