Among living birds it is rare indeed to find within the limits of a single species a combination of characters which entitles it at once to generic, family, and sub-ordinal rank, but such is the distinction of that most curious of birds, the Hoactzin (Opisthocomus hoazin), which is the sole tenant of the family Opisthocomidce and suborder Opisthocomi. It is a small, quite Pheasant-like bird in appearance, about twenty-three inches in length, with a long, loose crest of rather stiff-shafted feathers, relatively stout feet and legs, long, rounded wings, and a long, slightly rounded tail composed of ten feathers; the eyes are set in the midst of a patch of bare skin and provided with bristly lashes. The coloration above is dark brown glossed with olive and slightly varied with white, the wing-quills being chestnut and the under parts pale buff, shading into chestnut on the sides and abdomen, while the tail is broadly tipped with yellowish white; the sexes are alike. The skeleton and soft parts of the Hoactzin exhibit a number of remarkable features which are quite unknown in other birds. Thus the breast-bone has two lateral edges parallel for two thirds the length and then divergent, so that it is actually wider behind than in front. The keel is only developed on the posterior part of the sternum, the front portion being aborted or cut away, a condition which has been apparently brought about by the pressure of the enormous crop, which has also excavated a deep cavity in the surface of the great pectoral muscles. ” Furthermore,” says Newton, ” this crop is extremely muscular, so as more to resemble a gizzard, and consists of two portions divided by a partial constriction, after a fashion of which no other example is known among birds.” In addition to these peculiarities, the bones of the shoulder girdle are completely fused to one another and to the breast-bone.
The home of the Hoactzin is in the northern and western portions of South America, from Colombia to Bolivia, but especially in the country watered by the Amazon. It is met with mainly on low trees and bushes along the shores of streams and lakes, in flocks usually of from ten to twenty individuals, and, as might be premised from the above enumeration of skeletal characters, it enjoys but weak powers of flight. “In the early morning or in the late afternoon,” says Quelch, ” they will be seen sitting in numbers on the plants, while toward the middle of the day, as the fierce heat of the sun increases, they betake them-selves to shelter, either in the dense recesses of the growth, or among the individual trees of denser foliage, or among the tangled masses of creeping and climbing vines, along the very edge of the water. Late in the evening, after feeding, they will be seen settling themselves down in suitable places for the night.” They feed almost entirely upon leaves, of which they consume great quantities, especially of an arum, which imparts to the flesh a very strong, disagreeable odor, whence the bird is often called the “Stink-bird” or “Stinking Pheasant.” The nest of the Hoactzin is a rude affair of sticks loosely laid together and placed in bushes near the water, while the eggs, usually two or three in number, are oval, yellowish white profusely blotched and spotted with reddish. The young are hatched almost naked and have the thumb and index finger provided with claws by means of which they are enabled soon after birth to clamber about among the branches, thus making the nearest approach in appearance to a quadruped found among existing birds. In a valuable contribution on the weapons and wings of birds, Mr. F. A. Lucas says: “Soon after the hatching of the eggs the nestlings begin to crawl about by means of their wings and legs, the well-developed claws on the pollux and index being constantly in use for holding and hooking to surrounding objects. If they are drawn from the nest by means of their legs, they hold on firmly to the twigs both with bill and wings; and if the nest be upset by means of a rod pushed up from below, they hold on to all objects with which they come in contact by means of bill, feet, and wings, making considerable use of the bill, not only to reach objects above them, but also, with the help of the clawed wings, to raise themselves to a higher level. When the parent bird is driven from the nest, owing to the close approach of a boat, the young birds, unless they are quite recently hatched, crawl out of the nest on all fours and rapidly try to hide in the thick brush beyond.” When upset into the water, the young are also able to swim rapidly, and even to dive, in order to avoid capture. In the adult birds the claws on the wings have entirely disappeared, and even the thumb is so poorly developed that no one would suppose its presence in the nestlings.
From the peculiar characters above pointed out it is little wonder that there has been much speculation as to the relationships of the Hoactzin. The mature bird, as already stated, resembles a game bird, while the nestling has certain characters possessed only by the fossil Archceopteryx, the oldest known bird. The eggs resemble closely the eggs of Rails, but the abundant skeletal characters separate it not only from the Rails but from most other birds as well. Huxley says that it “resembles the ordinary gallinaceous birds and pigeons more than it does any others, and where it diverges from them, it is either sui generis, or approaches the Plantain-Eaters (Musophagidce).” Therefore, while ornithologists are perfectly agreed that the Hoactzin constitutes a distinct order or sub-order, they are by no means of one opinion as to where it shall stand. By some it is placed between the game birds and the Rails, by others, as Dr. Sharpe, between the Pigeons and Rails, while Dr. Gadow refers it as a suborder to the Galliformes, but locating it at the end of that order and therefore near to the Rails.