EGGS In birds, as in all other vertebrates, the gonads are paired organs on the dorsal wall of the body-cavity. In most birds the right ovary disappears early in life, leaving only the left to grow and mature. This rule, though usual, is not invariable, since in many hawks, particularly the harriers (Circus), and the small bird eating forms (Accipiter) usually, but not invariably, we find both ovaries present and functional. The ovum, or yolk, in birds is a relatively enormous cell which passes from the ovary into the mouth of a convoluted tube, the oviduct. As the yolk moves down the oviduct it receives first a deposit of gelatinous albumen, the "white," next a membranous sheath, and then is enclosed in a hard, calcareous shell, and is deposited as an egg (q.v.).
Eggs are ordinarily oval in shape, though this is not invariable, as the eggs of swifts are usually elliptical, and those of owls are nearly round. Eggs of birds that breed on rock ledges without constructing nests are much pointed, which allows them to roll about in a relatively small circle and so lessens the danger of a fall over the edge. Eggs of many shore-birds, which are relatively large in relation to the size of the birds, are also strongly pointed, which permits them f o pack closely with the pointed ends toward the centre, thus bringing the whole into a compass that permits the parent to cover them.
Eggs of owls, kingfishers and woodpeckers, which are laid usually in holes or cavities (though some owls occupy open nests) are white, and whitish eggs are found in grebes, albatrosses and petrels. Most eggs have, however, a coloured shell, or spots and blotches of colour spread over a lighter background. Where there is a pattern of markings present there is usually a wreath of heavy colour about the large end of the eggs. Herons' eggs are pale blue or bluish-green, and plain green or blue eggs are found in various perching birds. The tinamous of the New World, a primitive group, have very striking eggs, with smooth and highly polished shells, varying from green or pink to deep brown, their lustre suggesting porcelain.
The number of eggs produced annually varies widely in different groups, being adjusted to the requirements of each species, to enable maintenance of its normal numbers. The migratory species that covers a considerable range encounters more dangers than one that is sedentary and, therefore, must produce more of its kind.
This statement, however, has many exceptions. Migratory wild ducks lay from five to 12 eggs, while the bobwhite, which is sedentary, may have as many as 21, though usually not more than a dozen. Among small birds, on the average more eggs are de posited annually by the migratory species of the temperate zones than by the sedentary forms of the Tropics. On the other hand, the non-migratory titmice and wrens lay many more eggs than the migratory warblers (Sylviidae). Most perching birds of tem
perate regions have from three to five eggs, rarely six, in a set ting, but may nest two or three times each season. Most sand pipers deposit four eggs, though in some the number is reduced to three. Many gulls, hummingbirds, loons, and some species of pigeons have two eggs regularly. Other kinds of pigeons, with auks, petrels and albatrosses, lay but one egg. Boobies lay two eggs, but never seem to rear more than one young.
Adjustment of birds to their environment as regards the number of eggs deposited, to ensure continuance of each species, was made before the rise of man, who has been an active factor in the life of the earth for a relatively brief period compared to other ani mals. In some cases, where man has kept in check enemies of birds, these species have increased. In other instances, where man has been an active enemy, he has brought about reduction in numbers or actual extermination, because the birds concerned had become adjusted to a certain annual drain on their numbers from natural enemies and were unable to change to counteract added destruction by man. The great auk laid only one egg, and as it nested in colonies was quickly exterminated. The passenger pigeon of North America, which existed in colonial days in in numerable hosts, also produced but one young each season, which did not enable it to withstand the drain of hunting, so that it has become extinct. Usually, only those birds which rear a number of young each year can be maintained as game, and these must have protective regulation to enable them to hold their own. Incubation.—Af ter the egg is deposited it requires a certain definite temperature to develop to the point of hatching. This is normally accomplished by incubation on the part of one of the parents, usually the female, a period during which the bird remains closely on the nest, except for brief intervals required for a hasty search for food. The eggs are warmed by being brought into close contact with the breast and abdomen of the brooding bird, there being usually a sloughing of down and other feathers over a part of this area, to permit close contact between the skin and the eggs, and at the same time an increase in the blood supply to the skin, to bring a more even heat to bear. The actual degree of heat required to develop the embryo in the egg is known for only a few forms of birds. In the case of the domestic fowl the average incubation temperature is about 103° F. In charts published by Baldwin and Kendeigh, showing by use of a thermocouple fluctu ating temperature in the nest of a house-wren (Troglodytes aedon), the incubating temperature is shown to vary from 39° to 41° C, or 102° to io6° F, the average being about 104° F. Though, ordinarily, care of the eggs involves their being heated from the body of the incubating bird, in species that nest in the open, in warm climates, where the sun is torrid, brooding may be required to shade the eggs from too powerful sun-rays. The body of the bird here acts as a shelter and an equalizer of heat.