Pigeons and doves constitute the animal family Columbidae and the order Columbiformes, which includes about 42 genera and 310 species. The related word "columbine" refers to pigeons and doves. They are stout-bodied birds with short necks, and short slender bills that, in some species, feature fleshy ceres. They primarily feed on seeds, fruits, and plants. This family occurs worldwide, but the greatest variety is in the Indomalaya and Australasia ecozones. Pigeons and doves are likely the most common birds in the world. The distinction between "doves" and "pigeons" is not consistent. In modern everyday speech, as opposed to scientific usage or formal usage, "dove" frequently indicates a pigeon that is white or nearly white. However, some people use the terms "dove" and "pigeon" interchangeably. In contrast, in scientific and ornithological practice, "dove" tends to be used for smaller species and "pigeon" for larger ones, but this is in no way consistently applied. Historically, the common names for these birds involve a great deal of variation between the terms. The species most commonly referred to as "pigeon" is the species known by scientists as the rock dove, one subspecies of which, the domestic pigeon, is common in many cities as the feral pigeon. Pigeon is a French word that derives from the Latin pipio, for a "peeping" chick, while dove is a Germanic word that refers to the bird's diving flight. The English archaism "culver" appears to derive from Latin columba, pigeon, dove. Doves and pigeons build relatively flimsy nests – often using sticks and other debris – which may be placed on trees, ledges, or the ground, depending on species. They lay one or two eggs at a time, and both parents care for the young, which leave the nest after 7–28 days. Unlike most birds, both sexes of doves and pigeons produce "crop milk" to feed to their young, secreted by a sloughing of fluid-filled cells from the lining of the crop. Young doves and pigeons are called "squabs".
Pigeons and doves exhibit considerable variation in size, ranging in length from 15 to 75 centimetres (5.9 to 29.5 in), and in weight from 30 g (0.066 lb) to above 2,000 g (4.4 lb). The largest species is the crowned pigeon of New Guinea, which is nearly turkey-sized, at a weight of 2–4 kg (4.4–8.8 lb). The smallest is the New World ground dove of the genus Columbina, which is the same size as a house sparrow, weighing as little as 22 g (0.049 lb). With a total length of more than 50 cm (20 in) and weight of almost 1 kg (2.2 lb), the largest arboreal species is the Marquesan imperial pigeon, while the dwarf fruit dove, which may measure as little as 13 cm (5.1 in), has a marginally smaller total length than any other species from this family. Overall, the Columbidae tend to have short legs, short bills with a fleshy cere, and small heads on large, compact bodies. In a series of experiments in 1975 by Dr. Mark B. Friedman, using doves, their characteristic head bobbing was shown to be due to their natural desire to keep their vision constant. It was shown yet again in a 1978 experiment by Dr. Barrie J. Frost, in which pigeons were placed on treadmills; it was observed that they did not bob their heads, as their surroundings were constant. The wings are large, and have eleven primary feathers, and low wing loading; pigeons have strong wing muscles (wing muscles comprise 31–44% of their body weight) and are among the strongest fliers of all birds. They are also highly manoeuvrable in flight. Columbidae have unique body feathers, with the shaft being generally broad, strong, and flattened, tapering to a fine point, abruptly. In general, the aftershaft is absent; however, small ones on some tail and wing feathers may be present . Body feathers have very dense, fluffy bases, are attached loosely into the skin, and drop out easily. Possibly serving as a predator avoidance mechanism, large numbers of feathers come out in the attacker's mouth, if the bird is grabbed by a predator, facilitating the bird's escape. The plumage of the family is variable. Granivorous species tend to have dull plumage, with a few exceptions, whereas the frugivorous species have brightly coloured plumage. The Ptilinopus (fruit doves) are some of the brightest-coloured pigeons, with the three endemic species of Fiji and the Indian Ocean Alectroenas being the brightest. Pigeons and doves may be sexually monochromatic or dichromatic. In addition to bright colours, pigeons may sport crests or other ornamentation. Like some other birds, the Columbidae have no gall bladders. Some medieval naturalists concluded they have no bile (gall), which in the medieval theory of the four humours explained the allegedly sweet disposition of doves. In fact, however, they do have gall (as Aristotle had earlier realised), which is secreted directly into the gut.