What Are Eggshells Made Of?
A bird’s eggshell is mostly formed from calcium carbonate covered with thousands of tiny pores. It is a semipermeable membrane, which means air and moisture can pass through it. Covering the shell is a thin coating called the cuticle made of dried mucus laid down by the uterus that protects the embryo from bacterial infection.
Contained within the shell is a protein matrix which stabilizes and strengthens the calcium carbonate crystals. Without the protein the shell would be too brittle to hold its shape.
The different colours of the shells of birds’ eggs have fascinated us for centuries. In 350 BC Aristotle wrote an essay devoted to birds’ eggs and noted:
“Some eggs are white, as those of the pigeon and of the partridge; others are yellowish, as the eggs of marsh birds; in some cases the eggs are mottled, as the eggs of the guinea-fowl and the pheasant; while the eggs of the kestrel are red, like vermillion”.
In the 1970s it was discovered that just two pigments are responsible for the colours of eggshells – a reddish-brown colour and a bluish-green colour. By mixing these pigments in different quantities a diverse range of patterns can be made which cover birds biological needs such as camouflage, recognition, and even protection from solar radiation.
Research has also shown that the coloration is part of the development process with the same protein that stabilizes the structure of the egg responsible for the different markings.
Larger birds tend to lay eggs with thicker shells. For example the ostrich lays eggs which have shells about 30 mm thick while hummingbirds’ eggs have very thin shells, although they are some exceptions and chickens’ and ducks’ eggs have relatively thick shells.
Scientists believe the discrepancy is because species that are laid in thin-shelled eggs do not absorb as much calcium from the shell and get the calcium they need once they are hatched from the food they eat. Birds laid in eggs with thick shells are born with stronger skeletons as they have absorbed more calcium from the shell.