Bird eggs

Posted: Updated:

An egg is an external womb that provides nourishment and protection for the growth and development of the embryo inside.

Birds' eggs, like the birds themselves, vary enormously in size. The largest egg from a living bird belongs to the ostrich. It is more than 2,000 times larger than the smallest egg produced by a hummingbird. Ostrich eggs are about 7 inches-long and 5.5 inches wide and weigh 2.5 pounds. Hummingbird eggs are 1/2 inch-long and .3 inches wide and they weigh only .01 oz.

Although large birds lay larger eggs than do small birds, small birds actually have proportionately larger eggs. It takes 60 ostrich eggs to equal the weight of one ostrich but only nine hummingbird eggs to equal the weight of the Calliope Hummingbird. Within a species, egg size may differ. For example, younger birds of a species tend to lay smaller eggs than older birds of the same species. Even eggs laid by a particular bird may vary in size, both within and between clutches. Food availability is a controlling factor. Environmental niche is also a factor in egg size. Typically, altricial nestlings (blind, helpless and wholly dependent on parents) hatch from smaller eggs than nestlings that are precocial (active soon after hatching). Birds that live and nest on the ground tend to produce precocial young.

Eggs are not all the same shape or color. Some eggs are more pointed and some more spherical. Egg shape is determined by the internal structure of the hen. Her oviduct, distribution of internal organs and shape of her pelvic bones all affect egg shape. Probably the most striking and variable feature of eggs is their coloration. Although many eggs are white, eggs representing almost every color of the rainbow are known. Color is added to the eggshell from pigments secreted by cells in the oviduct wall. The timing of pigment deposition affects color. Pigment deposited as the egg enters the oviduct results in the base color of the egg. Pigments added right before the egg is laid form the patterns or markings on its surface. If the egg remains still while pigments are applied, spots appear. If the egg is moving, lines or scrawls appear. Egg coloring is controlled to a large extent by genetics. Usually, egg coloring is typical of a species. All robins, for example, lay blue eggs without markings. However, egg coloring or marking may be extremely variable among females of a species.

Egg colors and markings have strong adaptive values. Eggs that are laid on the ground or in open nests in trees, rather than in cavities, often exhibit cryptic coloration. The eggs blend in with their surroundings and are much less visible to potential predators. In addition, research shows that birds nesting in areas of the woods where the soil is known to be low in calcium produce eggs that are more heavily speckled. It was also shown that speckled areas of shells are significantly thinner than un-pigmented patches and that heavily speckled eggs are lighter, and therefore thinner, than their less spotty counterparts laid in the same broods. Researchers speculates when pigmentation is deposited and the cells are short of calcium at a particular part of the shell surface, pigment may be added to make up for the deficiency.

Eggshells vary widely in surface texture. Most eggs are smooth but some, like those of the Cormorant, can be quite rough. As well, eggs of cormorants are chalky, rather than shiny, as in tinamous. Duck eggs are oily and waterproof, and cassowary eggs are very heavily pitted.

Regardless of how smooth an egg may feel, all eggshells have tiny holes or pores. The domestic hen's egg, for example, may have 7,500 pores. Most of these pores are at the blunt end of the egg. Pores connect the egg with its surroundings. Respiratory gases as well as water vapor travel through these channels and enable the egg to breathe.

We normally think of eggshells as being very fragile. However, eggshells are remarkably strong and durable. They provide the embryo with good protection from predators as well as from soil invertebrates and harmful bacteria. Eggshells are made of calcium and magnesium salts within a fibrous network. Calcite is the main salt used as building material for the shell. As the embryo develops, calcium is transferred from the shell to the growing bones of the embryo. After a chick hatches, the mother sometimes eats the shell to recover some of the calcium lost in the making of an egg. The outer surface of the eggshell is covered by the cuticle. The cuticle is a thin layer made of proteins. The cuticle adds strength to the shell, gives the shell its texture and provides a barrier against bacteria.

Are you interested in a particular bird's egg? Check out to see what your favorite bird's eggs looks like.