Sphere-shaped body with a protective chalky covering (shell), produced by female birds and containing the germ of a new individual together with nutritious supplements (the yolk) and surrounding layers. The avian egg is well-structured for optimum growth and protection of the developing embryo. The yolk is enclosed in a thin elastic membrane and is a light yellow to orange viscous fluid. At the surface of the yolk body is a spot from which the embryo develops (germinal spot). The albumen or egg-white forms the rest of the egg complement and exists in two forms in the egg; a viscous (outside) and a gelatinous (inside, next to yolk) liquid. The albumen is separated from the shell by two interconnected layers of membrane which function in calcium exchange and protection. Another part of the egg is the air-cell or -chamber at the broad end of the egg and situated between the inner and outer membrane. It does not form until the egg is laid and increases in size as the embryo develops. It probably has a function in thermo-regulation. Last but not least, the eggshell consists of various layers of which the inner one connects to the outer membrane. It is mainly made from calcium carbonate and a little bit of protein. Through the shell traverse many minute channels or pores which facilitate gas exchange for respiration between the egg (or embryo) and its surrounding. Considerable variation can exist in shell characteristics (thickness, pore numbers, composition, etc.) from one end to the other. The pores show some variation in structure between species and can e.g. be multi-branched or forked. The shell pigments that give the egg its colour and/or specific pattern are situated in the shell itself.
The eggs of the various avian species can be distinguished from each other by the external differences; no two eggs are exactly alike. The most important differential factors are size, shape, surface and coloration. The shape of eggs varies greatly between species, from round to conical or elliptical (see picture). The principal forms are oval (ovate), elliptical, pointed (conical or pyriform) and rounded (sub-elliptical). Size is related to the bird being nidifugous (young leave the nest soon, see 'immature') or nidicolous (young incapable of leaving nest soon); the eggs of nidifugous birds being relatively large. Also the clutch size seems to have some influence on the egg-size. The most important relationship is of course the one between the size of the parent bird and the egg-size, although the relative egg-weight diminishes as the body weight increases. In a large number of birds this relation varies between 1/9th and 1/15th of body weight, but can be as low as 1/50th in a few of the larger birds. The largest egg is that of the Ostrich (17 by 13.5 cm, 1.4 kg) and the smallest that of a hummingbird (1.3 by 0.8 cm, 0.5 g). The surface of most species' eggs is smooth, varying from an even highly polished look to a dull look. Intermediates, such as matt (finely pitted) are most common. The highly coloured eggs of Tinamidae resemble balls of glazed porcelain. The surface can also look chalky (powdery), oily or soapy (e.g. of ducks for water-proof reasons), or can be ridged (emus). In many ratite (flightless running) birds the surface is granulated and pitted. With respect to coloration, there is a great tendency to similarity of the eggs of species of the same group. Especially egg coloration, in conjunction with other features, can be of taxonomic value (often between genera). Eggs of one species can differ greatly on the other hand, as in the case of the Guillemot. Coloration is sometimes greatly adaptive (cryptic) to minimize losses. Also protection from solar rays often plays a role; when completely sheltered during breeding, many eggs are white. The colours of eggshells are due to two main pigments; one red-brown and one blue-green, giving the entire range of hues. The pigments are most plentiful in the superficial layers of the shell. Sometimes the colour pigment is really superficial and can be rubbed off. Often there is a ground colour (ranging from white to almost black) on which additional markings are superimposed. The variety in the markings is almost infinite; e.g. the mottling in varying density of many species or the marbling on the eggs of nightjars. Clutch size is dependent initially on the degree of protection afforded to the eggs in the nest, and also greatly affected by food supply (the better the more eggs). It is further influenced by whether the species is altricial (poorly developed young at hatching) or precocial (well developed young at hatching). The number of eggs in a clutch, which can range from one to a few dozen, is more or less the same within each species and there is also a tendency in many genera or families for the constituent species to lay about the same number of eggs. Many wild species lay only one clutch, but loss of a clutch will stimulate the laying of another one, up to several times.

Alternative forms for egg : altricial, biconical, clutch, conical, eggs, eggshell, elliptical, oval, ovate, pointed, precocial, pyriform, rounded, yolk.