There is no doubt that Pigeons and Doves must be regarded as forming an Order by themselves. That they are more nearly allied to the Sandgrouse than to any other birds is generally acknowledged, but the differences between the two groups are quite as great as those between some of the other orders here admitted. In some recent classifications the Pigeons have been placed near the Passerine birds chiefly on account of the newly-hatched young being helpless and naked, but this character by itself is of secondary importance, and the anatomy of the Columba) as a whole, as Huxley has shown, resembles that of Gallinaceous birds, though differing in many respects and showing certain affinities to the Owls and Vultures (P. Z. S. 1867, p. 460).
The principal external characters of the order are the following :—The upper mandible consists of two parts ; the tip, which is swollen, hard, and convex, and the basal portion, which is soft, being covered by skin in which the longitudinal slit-like nostrils open. Toes always four in number, and on the same level; no web between the toes, but the sole is considerably expanded in the more typically arboreal forms, and much narrower in those that seek their food on the ground. Aftershaft rudimentary or wanting. Spinal feather-tract well defined on the neck and forked in the interscapular region. Primaries 11; fifth secondary wanting (aquincubital). The number of tail-fathers varies.
The palate is schizognathous, the nasals schizorhinal ; basi-pterygoid processes present (except in the extinct Dodo). Cervical vertebra} 15. The sternum has generally two deep notches on each side of the posterior margin. Furcula U-shaped.
Deep plantar tendons as in Gallinae. The ambiens muscle, as in Parrots, is sometimes present, sometimes absent; the femoro-caudal, semitendinosus and accessory semitendinosus are always present, and the accessory femoro-caudal in all except the Australian genus Lopholoemus. The oil-gland is nude or wanting; caeca and gall-bladder are present in some genera, absent in others. Both carotids are present.
All Pigeons are phytophagous, the majority living on fruit or seeds. They are monogamous and pair for life. The majority make a nest on trees, a few on rocks or in holes; the nest is a platform of twigs or grass, without lining, simple in structure and very loosely put together. The eggs in the great majority of species are two in number. Some genera, as Carpophaga, Caloenas, and Alsocomus, lay a single egg. The eggs are white, oval, and usually glossy. The young emerge from the egg naked and unable to run, and they do not go through a downy stage; they remain in the nest for a long time, and are fed by the parents with a secretion from the crop.
An excellent Catalogue of the Pigeons by Count T. Salvadori has just been published by the British Museum; from this work the majority of the above details are taken. The classification of the order is, however, an extremely difficult subject; the anatomical data are not satisfactory and often conflict with the external characters. The order is divided into two suborders, of which one (Didi) is now extinct; the other is divided by Salvadori, external characters only being regarded, into five families, three of which have Indian representatives. I am, however, unwilling to accord the rank of families to groups only distinguished by details of plumage and small differences in the shape of the foot; and I shall accordingly leave all Indian Pigeons and Doves in a single family.