[chris (OP)]

What is the mechanism through which birds make and lay eggs? How is the shell formed?

[chris (OP)]

bump

[Don_1]

In  most birds, though they begin life with 2 ovaries, only the left one will develop to become functional.

The ovary becomes enlarged during the breeding season.
The ovum are picked up by the ostium, just as in mammalian reproduction, and cillary currents in the oviduct transport the ovum to the magnum section of the oviduct, where the ovum (yolk) is coated with albumen.

The ovum and it's coating of albumen then passes into the final part of the oviduct, the isthmus, where it is coated in shell membrane. This is that thin membrane you'll find between the white and the shell when you crack open a hard boiled egg.

From here the egg passes out of the oviduct into the Shell Gland, or Uterus. Here a calcareous shell and pigments coat the membrane. The gland secrets a shell mixture of Calcium carbonate, calcium bicarbonate, phosphorous, fatty acids and vitamins together with other bio-chemicals including sodium potassium and sulphur salts which, so far as I can gather, act in a way that causes the calcareous components of the shell to adhere to the shell membrane.

This laying on of the egg shell, the colour, patterning, size, shape, strength and porosity of the shell, is a highly complex business. this gives a little insight into the complexity of shell formation.

Once the shell hardens, it passes to the vagina and out through the cloaca. In the case of some reptiles, such as marine turtles, the shell is soft when laid.