The Vertebrate Skeleton

by Sidney H. Reynolds

In most birds the pennae are not uniformly distributed over the whole surface of the body, but are confined to certain tracts, the pterylae; while the intervening spaces or apteria are either bare or covered only with down feathers. In some birds, however, such as the Ratitae and the Penguins, pennae are evenly distributed over the whole body.

In many birds the calamus or quill bears two vexilla or vanes, the second of which, called the aftershaft or hyporachis, is generally much the smaller, and is attached to the under surface of the main vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult bird are nearly equal in size; though in the nestling Emeu one is much longer than the other. The aftershaft is very small in most Passeres and gallinaceous birds, but is comparatively large in Parrots, Gulls, Herons and most birds of prey. It is absent or extremely small in the Ostrich, Apteryx, Rhea, Pigeons, Owls, Anseres, and others.

The quill feathers include two groups, the remiges or wing quills, and the rectrices or tail quills. In most birds the primary remiges, or those which are attached to the bones of the manus, are ten or eleven in number, and are set in grooves in the bones, being firmly attached to them. In the Ostrich however the primaries are little specialised in character and are as many as sixteen in number. They are also less definitely attached to the bones; as their ends do not lie in grooves in the bones, but project beyond them.

The secondary quills or those attached to the ulna vary much in number according to the length of the bone. The large dark quills in the wings of Cassowaries are the secondaries.

The wing of Penguins is very little differentiated. It is covered at the margin by overlapping scales which gradually merge into scale-like feathers at the proximal end. The wing of the Penguin has nothing comparable to the remiges of other birds.

In some birds, such as Herons (Ardea), there occur in places plumulae of a peculiar kind, which grow persistently and whose summits break off into fine powder as fast as they are formed. These feathers are known as powder-down feathers. They occur also in some Parrots and are then scattered indiscriminately all over the body.

Other exoskeletal structures besides feathers are commonly well developed. Thus the extremities of the jaws are sheathed in horny beaks whose form varies enormously according to the special mode of life.

In ducks and geese the beak with the exception of the anterior end is soft, and its edges are raised into lamellae, while in the Mergansers these lamellae become pointed processes supported by bony outgrowths. These lamellae act as strainers. In Parrots and Hawks, on the other hand, nearly the whole of the beak is hard.

The toes and tarso-metatarsus are usually featherless and are covered either with granular structures or with well-formed scales. The toes are nearly always provided with claws, and these vary in correlation with the character of the beak. Claws[105] also sometimes occur on the manus. Thus Archaeopteryx and some Ostriches and Rheas have claws on all three digits. Most Ostriches and Rheas, and many Anseres and birds of prey, have them on the first two digits, while the Secretary bird (Gypogeranus) and many fowls, ducks, and birds of prey, especially kestrels, have a claw only on the pollex. In the Cassowary, Emeu, Apteryx and some Ostriches and Rheas only the second digit is clawed.

Claws should not be confounded with spurs, which are conical horny structures developed on bony outgrowths of the radial side of the carpus, metacarpus, or metatarsus. They occur in a number of birds, but are most commonly developed in gallinaceous birds, by which they are used for fighting. A single spur occurs on the metacarpus in Megapodius, in Palamedea, in Parra jacana and in Hoplopterus spinosus, the Spur-winged plover. The Derbian Screamer, Chauna derbiana, has two metacarpal spurs, borne on the first and second metacarpals. The Spur-winged goose, Plectropterus gambensis, has a carpal spur borne on the radial carpal. Metatarsal spurs are quite common.