COLUMBIFORM

any member of an order of birds (Columbiformes) that includes pigeons, doves, the extinct dodoes, and sandgrouse. Pigeons and doves (family Columbidae) are distributed worldwide, with the exception of the polar regions and some oceanic islands. The terms pigeon and dove are interchangeable and imply no biological distinction. A few species have become extinct within the last few centuries. The best known of these is the passenger pigeon (Ectopistes migratorius) of North America. These gregarious birds traveled in flocks of hundreds of thousands and were shot indiscriminately during the 19th century; the last passenger pigeon died in the Cincinnati, Ohio, zoo in 1914. The dodo (Raphus cucullatus), a native of Mauritius, was exterminated by 1681, through the combined efforts of man and his domestic animals. The related solitaires on Runion and Rodrigues islands (Raphus solitarius and Pezophaps solitaria, respectively) were extinct by 1791. Sandgrouse are found in arid areas of Asia and Africa. The earliest known sandgrouse are from the Eocene Epoch in France (about 40 million years ago); they were members of the living genus Pterocles. The first pigeon in the fossil record is Gerandia calcaria from the Early Miocene Epoch (23.7 to 16.6 million years ago) in France. The living members of the order are relatively short birds with small heads and long, pointed wings. Species adapted to ground feeding, such as the quail doves (Gallicolumba), tend to have longer legs and shorter, rounder wings than their more generalized relatives. Bill shape is related to diet. Slender bills are associated with seed eaters, whereas fruit eaters (such as Treron) have a bill that is larger and sharply hooked. The serrated edge of the large bill in the tooth-billed pigeon (Didunculus strigirostris) is used to rip leaves, fruits, and the like into small pieces before swallowing. The seed-eating pigeons typically have long intestines and powerful gizzards; the fruit eaters have shorter intestines and do not need strong gizzards. Pigeons have evolved the crop, a pouch on the esophagus, that serves as a storage organ for food, thereby allowing the Columbidae to live as efficient grazers. A unique characteristic of the Columbidae is their production of crop milk, or pigeon's milk, a secretion used to feed the young. This nutritive substance is secreted from the crop wall (in the bird's gullet) when the hormone prolactin is secreted from the pituitary gland. This pigeon's milk has a composition quite similar to mammalian milk. Another characteristic limited to the Columbidae is their unbirdlike manner of drinking. All other birds take a sip of water and then raise their heads to swallow it; pigeons, however, suck up water using muscular contractions of the esophagus, in a mammal-like fashion. Sandgrouse are said to share this habit, but they must tip up the head to swallow. The Australasian region supports a large number of species of fruit-eating pigeons, including the green fruit pigeons (Treron) and the fruit doves Ptilinopus and Ducula. Their success in this part of the world is probably enhanced by the absence of monkeys, which in Africa and South America compete for the niche of arboreal fruit-eater. Most of the fruit doves are at least partially green in colour. In the breeding season males of most species set up a nesting territory, which is defended from other males of the same species. The territory is protected by threat displays, followed if necessary by attack. The male may attract a female to his territory, or he may just court the first female to enter his territory. After various displays, a pair bond forms and is strengthened as the two birds preen each other's head and neck feathers. Pigeons frequently nest several times a year, but no pigeon lays more than two eggs in any clutch, and many fruit doves lay only one; the sandgrouse lays three eggs in a depression in the ground. Pigeons usually build simple twig nests, either in trees or on rock faces. Incubation and parenting is shared by the pair in many species. Crop milk secretion is stimulated by the process of incubation. The young, called squabs, are brooded continuously at first. The squab feeds by prompting the parent to regurgitate food when the squab inserts its bill into the corner of the parent's mouth. The squab obtains crop milk by poking its bill down the parent's throat. (order Columbiformes), any member of the group that comprises the pigeons, doves, dodoes, and sand grouse. The suborder Columbae embraces the extinct dodo and solitaires in the family Raphidae and the extinct and living pigeons and doves in the family Columbidae. The names pigeon and dove are synonymous and imply no biological distinction. The sand grouse are given a distinct suborder, Pterocletes. The pigeon family is a natural and homogeneous assemblage of about 285 species of readily defined birds, unique in producing, for feeding their young, a nutritive secretion from the crop wall. Pigeon's milk is similar in composition to mammalian milk and is also induced by the secretion of the hormone prolactin from the pituitary gland. Pigeons also are distinctive in their unusual manner of drinking, in which water is sucked in as a continuous draft, the process being assisted by muscular contractions of the esophagus, whereas other birds take a sip of water and then tip back the head to swallow. It is frequently stated that the sand grouse share this typically pigeon habit; in fact they suck and then tip back the head to swallow, repeating the process four to 10 times during any drinking session. Additional reading Accounts of the natural history of columbiform birds are available in Derek Goodwin, Pigeons and Doves of the World, 3rd ed. (1983); Ronald Keir Murton, The Wood Pigeon (1965), on the biology of an abundant European species; Wendell Mitchell Levi, The Pigeon, rev. ed. (1957, reissued 1977), a complete discussion of the care and breeding of domestic pigeons; and A.W. Schorger, The Passenger Pigeon (1955, reissued 1973), an exceptionally thorough account of the natural history, economics, and demise of this once-abundant North American pigeon. In addition to Goodwin's book above, the taxonomy of pigeons is discussed in R.F. Johnston, Taxonomy of Pigeons, Condor, 64:6974 (1962); Goodwin's Taxonomy of the Genus Columba, Bulletin of the British Museum (Natural History), Zoology, no. 6, pp. 123 (1959); and E.W. Gifford, Taxonomy and Habits of Pigeons, Auk, 58:239245 (1941). Ronald K. Murton The Editors of the Encyclopdia Britannica