A bird egg is a mechanical structure strong enough to hold the chick securely during development, yet weak enough to break out of. The shell must let oxygen in and carbon dioxide out, yet be sufficiently impermeable to water to keep the contents from drying out.

Bird eggs

Eggs are interesting structures. They are beautifully designed, self-contained life-support systems for the developing bird. All the nutrients, minerals, and water needed during incubation, as well as the necessary energy supply, are present in the freshly laid egg. This well-designed microcosmos contains everything needed for the growth and production of the hatchling chick, with one crucial exception: Oxygen. Furthermore, the shell of the avian egg is a simple physical system that is exceptionally well suited to considerations of scaling.

A hummingbird egg may weigh less than 0.3 g, and an ostrich egg over 1 kg, a 3000-fold range. The birds that lay these eggs range in size from 3-g hummingbirds to 100-kg ostriches, a 30000-fold range. The largest bird that has ever lived, the elephant bird (Aepyornis) from Madagascar, was a sizable animal, standing perhaps 3 m tall and weighing over 500 kg (Feduccia, 1980). Its giant egg weighed about 10 kg, 10 times as much as an ostrich egg and 30000 times as much as a hummingbird egg.