Although externally resembling the Herons and Storks with which, indeed, they were formerly united, the Cranes really constitute a very well marked group of birds, no one of which is likely to be confused with any other when once their distinctive characters are understood. They are all birds of large size and frequent grassy plains as well as marshes. They have long legs and neck, generally a long bill, which is equal to or longer than the head, and long wings in which the inner secondaries are rather longer than the primaries, and are generally composed of drooping plumes with more or less dissociated webs. The tail is rather short and composed of twelve feathers. The toes are short, especially the posterior toe, which is so much elevated above the other three that its claw scarcely touches the ground. The powder-down patches, so characteristic of the Herons, are absent, except in the anomalous Kagu and the Sun-Bitterns. In the structure of the skeleton and the soft parts the Cranes depart widely from the Herons and Storks, the most important difference being the split (schizognathous) instead of the band form of the palate, and the slit-like (schizorhinal) form of the nostrils. A further structural feature of much interest, which is present in most Cranes, is afforded by the great length and peculiar convolution of the trachea or windpipe. In the young chick, as it is about to emerge from the egg, the windpipe is said to be perfectly simple, but with advancing age this organ elongates and coils upon itself, after the manner of a French horn, within the keel of the breast-bone. The extent of this coiling appears to reach its maximum development in the Whooping Crane, when a length of no less than twenty-eight inches of the windpipe is packed away within the hollowed keel before it passes into the lungs. The entire length of the windpipe in this bird is about five feet, which is nearly or quite the total length of the bird itself. It is apparently upon this great length of the trachea that the powerful, resonant, and trumpet-like voice of the Cranes depends, as on a similar organ does the voice of the Trumpeter Swan. It has been said on high authority that a striking point of difference between the Whooping and Sandhill Cranes is the absence of convolutions in the trachea of the latter, but this is incorrect, for while not convoluted to as great an extent in the former it is nevertheless distinctly folded. Beautiful preparations of the trachea of all three American species are in the United States National Museum, and all are distinctly convoluted.
Another distinguishing mark is afforded by the condition of the nestlings, these being covered with down and able to run about within a few hours after they are hatched. The feather-tracts and their disposition are also quite peculiar.
The Cranes number about nineteen species and are distributed over the whole of North America as far south as Mexico and Cuba, and the greater part of Europe and northern Asia, whence they extend into northeast Africa, Lower Egypt, northwest India, and the Yangtse basin in China. In the more northern parts of their range they are strictly migratory, while in the more southern portions they are less or not at all so. By some students they are referred to only one or two genera, while by others they are divided, mainly on external characters, into nine or more. It is clearly a very ancient group, since no less than seventeen fossil forms have been described, the oldest of which comes apparently from the Eocene of England with half a dozen or more nominal species found in the Eocene and later horizons of North America.