Birds produce the most complex reproductive cells of all vertebrates. Bird eggs, like those of reptiles, are encased in multi-layered membranes reinforced with a calcium carbonate, shell providing protection for the developing embryo. After it is laid, the egg is completely self-contained and mostly waterproof, while still allowing oxygen and carbon dioxide to pass across the shell. The developing embryo survives on nutrients stored in the yolk and water from the egg albumen (white). The albumen also provides an insulating cushion that protects the embryo from rapid changes in temperature or movement.
The thickness of the shell is determined by genetics and by environmental factors, especially the availability of calcium carbonate. It must be thick enough so that the egg does not crack under the weight of the brooding adults, but it also must be thin enough so that the chick can peck its way out during hatching. Environmental pollutants, especially chlorinated hydrocarbon pesticides can interfere with the deposition of calcium and cause birds to produce thinner eggs.
The shape of the egg is determined partly by the size and shape of the oviduct. Spherical eggs are stronger and are better at conserving heat and water. They also pack together better in enclosed spaces. Owls and woodpeckers that nest in tree cavities produce spherical eggs. Terns and other shorebirds produce eggs that are more pointed at one end. These birds typically use little if any nesting material and eggs are often laid in a very shallow depression. The conical shape ensures that the egg will not roll away; when moved, such asymmetrical eggs rotate in a small arc around the tip. Shorebird eggs are also relatively large. When placed inward, the pointed ends also fit very close together, minimizing the area that the female needs to cover during incubation. (Drent 1975)
In some species, such as the Yellow Rail, the entire clutch of eggs weighs 25% more than the female that laid them.
Egg and Clutch Size
In general, larger birds lay larger eggs. Among Wisconsin birds the Sandhill Crane has the largest eggs, roughly 95 x 60 mm (3.75" by 2.33"), whereas the Ruby-throated Hummingbird has the smallest, only the size of a pea. However, small birds tend to lay larger eggs relative to their body mass. For example, the House Wren's egg is close to 1/7th of the bird's total mass, while a Cormorant's egg is only 1/25th of the bird's total mass. Smaller birds also tend to lay more eggs than large birds. Young birds, nesting for the first time, often produce fewer and slightly smaller eggs.
Among birds of similar size, egg size can vary a great deal. The Common Grackle and Killdeer are similar in size, but the Killdeer lays a much larger egg. This is because Killdeer chicks are "precocial"- that is they hatch ready to go, covered with down, eyes open and able to run around. Such advanced development requires more food for the embryo and thus a larger egg. Grackles on the other hand build substantial nests and their "altricial" young hatch naked with their eyes closed, incapable of locomotion. They must stay in the nest for a long time and depend on the parents for shelter and food.
Comparison of eggs of the Common Grackle (left) and Killdeer (right)
Birds hasten development of the embryo by incubating the egg with their own body heat. Most birds develop featherless brood patches on their abdomens to increase the transfer of heat to the eggs. But some, such as cormorants and pelicans, use their feet for incubation. The number of eggs laid in one period is limited by the availability of nutrients and minerals and by how many can be successfully incubated. Some birds rely on fat reserves augmented by daily food intake, while others must consume food on a daily basis to provision their eggs. Most wild birds are considered "determinate" layers, which mean they lay a definite number of eggs per clutch. Some birds, including many species in the Galliformes, are "indeterminate". These continue laying past the normal clutch size when eggs are removed from the nest. Grouse, quail, and the domesticated chicken are good examples.