All about bird eggs

Every year our gardens are home to beautiful processes of nature; processes we take for granted: everything from complex, ritualistic mating displays, nest building and song to aggression, territorial disputes and in-fighting. But there is one thing in particular that is, quite literally, a world unto itself, a true wonder. It is the egg.

The bird egg is a true phenomenon. It’s complex and intricate machinery isn’t so obvious at first, but take a deeper look at the sheer number of interplaying layers and you will soon appreciate its outstanding biological beauty. It is a factory for growing a bird, with ancient origins.

Which came first: the chicken or the egg?

Before we look any further, let’s ask and settle an argument: which came first, the chicken (i.e. bird) or the egg?

We’ve all been there, and it’s akin to asking about the beginning of a circle: a circle is circular; it has no beginning, and the exercise of asking about the first cause of a circular argument is completely futile. But, thankfully, this mother of all cause and effect quandaries can be answered, and it is in fact deeply rooted in evolutionary theory. Luckily for us, there is a simple explanation.

Which came first, the chicken or the egg?

Put simply, if we are referring specifically to a chicken egg, then of course the answer is the egg. Neil DeGrasse Tyson, the famous American Astrophysicist, wrote: “Which came first, the chicken or the egg? It was the egg – laid by a bird that was not a chicken.” This bird has been referred to as a “proto-chicken”, an intermediary between the present day chicken, and a preceding common ancestor. The same can be said of a Robin, a Blackbird or any other of our wonderful garden birds.

All birds in existence today evolved from a group of theropod dinosaurs which lived in the Mesozoic era, approximately 225 million years ago; Dinosaurs were reptiles, egg-laying creatures that were the first to evolve the ability to create an outer shell for protecting the egg from water, or other moist environments. Modern day birds have simply continued that reptilian tradition, with the added benefit of some design enhancements with regards to colour, shape and texture.

The egg, therefore, most certainly came before the chicken.

What’s in a bird egg?

Interestingly, all bird eggs have the same basic components and physiology. The exterior part of a bird egg is comprised of three layers: the cuticle, which is the soft, thin outer part surrounding the shell, acts as a protective barrier, preventing water evaporation and infection from penetrating the surface; the hardened shell, which is made primarily of calcium carbonate, is a semi-permeable layer, allowing air and moisture to pass through its pores; the final part is two inner layers, which are essential protein layers.

The anatomy of a bird egg

Within the egg, there are two key parts: the albumen and the yolk. The albumen, which is 90% water, functions to protect the yolk and provides added nutrition to the embryo. The yolk, of course, is there as a primary source of food for the development of the embryo. It is considerably rich in protein; in fact, the darker yellow the yolk is, the more protein is contained within it.

Interestingly, the size of a bird’s egg is relative to the size of its yolk. And, of course, the size of the yolk is relative to the size of the species.

Bird egg size vs bird size

*This chart shows the correlation between the size of common garden birds the size of their eggs.

Egg Colour

Many common garden bird eggs have different colours, although because they are all made of calcium carbonate, the default colour is white. What has been noted is that those eggs that are of a blue or greenish colour are generally laid by birds that nest in trees and shrubs (Blackbirds, Starlings and Song thrushes), an obvious way to camouflage for survival.

Birds that nest in holes produce white and very pale blue eggs; while birds that lay on open ground tend to produce brown or very speckled eggs.

The colour and pigmentation of a bird egg is determined by the mother’s DNA.

Sometimes you may find the odd egg shell on the floor in your garden, possibly under a nest; if you do, try using our bird egg identifier to see if you can work out the species of garden bird.

Click to see our bird egg identifier