Egg (bird)

Bird eggs are laid by the females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one (as in condors) to about 17 (the grey partridge). Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs, which are sometimes called wind-eggs.

The default color of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce colored eggs. The pigments biliverdin and its zinc chelate give a green or blue ground color, and protoporphyrin produces reds and browns as a ground color or as spotting.

However, the photographic markings on passerine eggs have been suggested to reduce brittleness by acting as a solid state lubricant. If insufficient calcium is available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil.

For the same reason, later eggs in a clutch are more spotted than early ones, as the female's store of calcium is depleted.

Birds which build in trees generally have blue or greenish eggs, either spotted or unspotted, while birds that build in bushes or near or on the ground are likely to lay speckled eggs.

The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting the gene responsible for pigmentation is on the female-specific W chromosome (female birds are WZ, males are ZZ).

Color was once thought to be applied to the shell immediately before laying, but this research shows coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral.

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Wikipedia