Reptiles were the first animals to produce eggs in shells so that the egg would not dry out when laid out of water. Birds, being descendants of the reptiles (see Evolution), have evolved the egg to a miraculous life support system that allows them to lay eggs where reptiles cannot.
Unlike most animals, the female bird has only one ovary and oviduct. The fertilised ovum leaves the female's ovary and travels down the oviduct. While passing along the oviduct a layer of albumen (the egg white) is applied around the ovum. The shell membranes are then deposited as it moves further along the oviduct towards the uterus. In the uterus, the outer shell is formed and pigmentation applied - the movement of the egg within the uterus determines how the pigments are applied and hence the pattern on the eggshell.
The chick, or embryo, develops from the germinal spot, which appears as a white spot on the yolk. The yolk is a fatty food store for the developing embryo.
The albumen is the embryo's water supply, but also assists the movement of gases from the embryo to the shell as well as providing some shock protection.
The chalaza is a twisted string-like structure that suspends the developing embryo in the albumen and also ensures that it remains above the yolk.
In addition to the yolk and albumen, the embryo requires only oxygen and warmth from the outside world.
The oxygen diffuses through the porous shell, which is a complex mesh of protein fibres and calcium carbonate (chalk). Likewise, the waste products of respiration - carbon dioxide and water - pass out through the pores.
The air sac increases in size during incubation and provides the mature chick with its first breath of air.
Most eggs are egg-shaped, but owls' eggs tend to be spherical, and some species, such as cliff nesting sea birds, lay very pointed eggs that roll around in a circle instead of over the cliff-edge.
Some species, such as the Blue Tit, feed their young almost exclusively on insect larvae, which are most abundant in early summer. Consequently, they generally put "all their eggs in one basket" by laying a single, large clutch of eggs.
Other species, for example Blackbird, that have a more reliable, steady food supply will lay two or three clutches of a several eggs.
In both these examples, all the eggs of a clutch are laid at daily intervals and the parent does not start incubating until the last egg has been laid. Consequently, all the young hatch at about the same time and compete for food on equal terms.
Birds of prey, such as Tawny Owls, and some other species begin incubation as soon as the first egg is laid and will continue to lay eggs at two or three day intervals. When all the young have hatched they will be different ages and sizes. If food is in short supply, the smallest and youngest will not be able to compete with the elder nestlings and so will die, and may even be eaten by their older siblings. This seems harsh but if they were all the same size and age then the entire brood would be lost.
The eggs of hole-nesting birds are generally white or pale blue so that the parent birds can locate them and avoid breaking them, and camouflage is less important because the nest itself is usually quite well hidden and camouflaged.
Birds that lay their eggs in the open and on the ground, such as plovers, rely heavily on their eggs being very well camouflaged.
Cuckoos lay eggs that are very variable in colour and pattern, but are also able to mimic the appearance of the host bird's eggs.
The female of most species is the parent that incubates the eggs. For this purpose she has a special adaptation - a brood patch - that usually forms just before the breeding season and disappears afterwards.
The brood patch is an area of the belly where there are few feathers and a dense network of blood vessels that increases the warmth passed to the eggs. The bird is often able to regulate the temperature (to about 37°C) by increasing or decreasing its metabolic rate. The parent will also periodically turn the egg so that it is warmed evenly.
In some species, the male bird shares equally the work of incubation and he will have a brood patch, otherwise if the male does incubate the eggs it is usually only for a short time while the female takes a rest.
A day or so before hatching, the chick may be able to communicate with the parent by making vocal sounds and responding to the parents' calls.
The chick's bill has a hard tip to the upper mandible called the egg-tooth. While laying on its back, inside the egg, the chick will continually raise its head and push the egg-tooth against the eggshell. As the shell weakens, the chick attempts to straighten itself and so push the two halves apart.
In some species it may take a couple of days for the chick to hatch.
Generally, the birds with the longest incubation period will have more well-developed hatchlings.
For example, Blue Tits have relatively short incubation periods and the hatchlings are blind, naked and helpless (altricial or nidicolous).
Strictly speaking, nidicolous means "remaining in the nest", but some precocial nestlings are nidicolous.
The nestlings lose the egg-tooth after a few days, or a couple of weeks in some species.
As the nestling grows the flight feathers develop and once these are fully developed the bird becomes capable of flight and can leave the nest, it is called a fledgling at this time.
Over the next few weeks its feathers finish growing and it acquires its juvenile plumage, which can either look very much like the adult plumage of a mature bird or be quire different depending on the species. For example, the juvenile Wren looks almost identical to an adult, but the juvenile Robin with its spotted plumage is quite different to an adult.
Egg collecting is destructive, selfish and immoral. In Britain, egg collecting is against the Law and can result in fines and imprisonment.