The full complement of eggs laid by a bird is known as a set or clutch. The number varies greatly with different species. The Leach’s Petrel, Murre, and some other sea birds, have but one egg. The Turkey Vulture, Mourning Dove, Hummingbird, Whip-poor-will, and Night-hawk lay two. Various Thrushes, such as the Robin, Veery, and Wood Thrush, deposit from three to five, four being the most usual number. Wild Ducks, Turkeys, and Grouse range from eight to a dozen or more; while Quails sometimes lay as many as eighteen.
Eggs are variously coloured, and some are so marked that the blending of their colours with those of their surroundings renders them inconspicuous. Thus those of the Killdeer, Sandpiper, and Night-hawk, for example, are not easily distinguished from the ground on which they lie.
Many eggs that are laid in holes or other dark places are white without markings of any kind, as illustrated by those of the Chimney Swift, Belted Kingfisher, and all Woodpeckers. In such instances Nature shows no disposition to be lavish with her colouring matter where it is not needed.