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Bird eggs are laid by 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.
Nonpasserines typically have white eggs, except in some ground-nesting groups, such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos, which lay eggs that often closely match the passerine host's egg. To varying degrees of success, host passerine species have combined speckles, blotches and other egg markings with visual pattern-recognition abilities to help them reject cuckoos’ counterfeit eggs. Most other passerines, in contrast, lay colored eggs, even if there is no need of cryptic colors.
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 sex determining 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.
In species such as the common guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed.
Bird eggshells are diverse. For example:
Tiny pores in a bird eggshell allow the embryo to breathe. The domestic hen's egg has around 7500 pores.
Most bird eggs have an oval shape, with one end rounded and the other more pointed. This shape results from the egg being forced through the oviduct. Muscles contract the oviduct behind the egg, pushing it forward. The egg's wall is still malleable, and the pointy end develops at the back side. Cliff-nesting birds often have highly conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly spherical eggs.
The shape has biological significance. A pointed egg will tend to sit on its side, with the big end tipped upward. The big end contains the air sac and its shell is pierced by a higher density of pores than the more pointed end. Tipping the big end upwards improves oxygen flow to the large head, with the physiologically demanding eyes and brain, that develops in the big end while the tail develops at the more pointed end.
Classic oval bird egg (Vanellus spinosus)
Almost spherical bird egg (Senegal parrot)
Long, "elliptical" bird egg (Loon)
Egg size tends to be proportional to the size of the adult bird, from the half gram egg of the bee hummingbird to the 1.5 kg egg of the ostrich. Kiwis have disproportionately large eggs, up to 20% of the female's body weight. This evolutionary trait results in kiwi chicks that can emerge from the nest days after hatching, ready to start foraging for food.
Many animals feed on eggs. For example, the principal predators of the black oystercatcher's eggs include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes. The stoat (Mustela erminea) and long-tailed weasel (M. frenata) steal ducks' eggs. Snakes of the genera Dasypeltis and Elachistodon specialize in eating eggs. Humans have a long history of both eating wild bird eggs and raising birds for farmed eggs for consumption.
Brood parasitism occurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include the cowbirds and many Old World cuckoos.
Smart, I. H. M., 1991. Egg-shape in birds. Pages 101-106 in D. C. Deeming and M. W. J. Ferguson (eds.). Egg incubation: its effect on embryonic development in birds and reptiles. Cambridge University Press, UK. 448pp.