SPECIAL FEATURES OF BIRDS (5): The wings

The wing is a forelimb and an adaptation to flight that is unique to class Aves. The bones of the wing reveal their reptilian ancestry. From the shoulder to the elbow (the humerus) and from elbow to wrist (radius to ulna) the bones of all vertebrates look quite similar, including those of birds. Only the hand shows a distinctly different adaptation in birds. In vertebrates with paws or hands, the metacarpal bones are shaped into fingers. In birds, they are fused and taper off into one bone (the end part is called manus, Latin for ‘hand’). Unlike the ‘hands’ of reptiles and the paws of many mammals, the bird-wing ‘hand’ cannot be moved up and down but has lateral rotation for wing beating.

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There is a large but invisible difference between mammalian bones and avian bones—a bird’s bones are not filled with marrow but with air. Mammalian bones are heavy which would impede flight or make it impossible. The bones of birds are hollow and this gives them a lightness that is a very important adaptation to flight.

The wings of birds are covered in specialised feathers that help to carry the birds in flight. Feathers are part of the entire covering (called the ‘integument’) that divides the skin from the surrounding air. In fact, feathers are the main part of the bird’s integument, although scales on the legs and horny protrusions on the head also form a part. How feathers evolved is an interesting study, one that is not yet agreed upon. The architecture of the feathers is not the same in all birds and not all birds possess the same number of feathers. A small songbird may have 1000 feathers while a large bird, such as a swan, can have more than 25000. The function of feathers, however, is the same in all bird species.

In addition to their role in flight, they provide insulation against cold, heat and rain. They protect the skin and they are replaceable. To fulfil these various functions, birds have several different types of feathers on their body. The flight feathers on the upper part of the wing (manus) are called primaries. In the ulna area, there are the secondary feathers. There are also downy feathers for insulation and feathers specialised in shape and colour for displaying during courtship (such as the tail feathers of the peacock) or for threat displays (as in the yellow bittern, for instance). Some feather types are age-dependent. Altricial species—that is, birds that are immature at hatching, grow up in a nest and are fed by parents—have no tail feathers or primary feathers before fledging. Adults have no downy feathers covering all of their bodies, as nestlings do, although some remain and some of the different feather types are indicated by a colour change from chick to adult.

Perhaps the most ingenious aspect of feather structure is how the feathers of the wing can withstand immense air pressure during flight without being torn apart. Each feather is a separate unit but gaps may also occur between strands of a single feather. It is possible to prise the single strands of a feather apart, yet they hold firmly when in flight. This is because each strand of a feather has small protrusions, called barbs, which intermesh with the barbs and smaller branches of the next strand of the feather. While light in weight, these hooked cross-connections cling together so strongly that they prevent air from ripping them apart in flight.

Since wings are needed for flight, landing and even balance, the feathers are replaced (a process called ‘moulting’) at regular intervals but never all at once. A few feathers can be shed without disabling flight. The new feathers start to form underneath the existing feathers and slowly push out the old ones. The shape of the wings also helps to maintain a moulting bird’s ability to fly. Wings are always concave in shape and this is important for aerodynamics.

The form of the wings is also shaped according to the environment in which the bird lives. The albatross, gliding most of the time across oceans, has long, slender wings whereas birds negotiating between tree branches have short, broad wings. Some species have lost the ability to fly; they may have lived on islands where they have faced no predators. These flightless birds retain only vestigial wings that are rarely used, or they may be adapted for swimming instead of flying. A penguin, for example, ‘flies’ in the water. (To be continue)

A Part of Introduction The Amazing Book of Gisela Kaplan & Lesley J. Rogers “BIRDS, THEIR HABITS AND SKILLS.

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