How do birds’ eggs form?

BBC Wildlife contributor Mike Toms answers your wild question.

Blue-footed booby eggs.

Blue-footed booby eggs © Lance McMillan


An egg starts as a cell, and development is initiated by hormonal changes within the female’s body.

First the cell increases in size, then, over a period of a few days, a recognisable yolk is laid down.

The growing egg passes down into the mouth of the oviduct (the tube leading from the ovary) where, assuming that viable sperm are present, it will be fertilised.

The journey down the oviduct then continues irrespective of whether or not fertilisation has occurred (an unfertilised egg will still be laid, though obviously the embryo does not develop).

At this stage, the egg membranes and albumen (egg-white proteins) are added, after which the mass is hydrated and plumped into the oval shape we all recognise.

Next, the shell forms, from heavily calcified deposits on the outer membrane. The thickness of the shell varies between species, as does the colour and texture of its surface.


Emu at nest with eggs © Auscape / UIG / Getty


Generally speaking, larger birds have eggs with proportionally thicker shells, reflecting the bigger structural size of the eggs they produce. Emu eggs, for example, are about a millimetre thick and weigh 700–900g.


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