LORDS OF THE AIR
Although Archaeopteryx most probably could fly for short distances, its body was heavy and not well adapted for it. Birds had to greatly reduce their weight to make them able flyers. Today, birds lost the heavy reptilian tail and they have replaced a heavy jaw and teeth with a light but strong beak. Many of their bones are hollow and almost paper-thin, but they are reinforced by internal struts for extra strength. The lower vertebra of their back and the hip girdle are fused together for strength and act as an anchor for the muscles that control their tail feathers. They all have a keeled breastbone for attachment of the strong pectoral muscles that produce the downstroke in flight. The small breast muscles raise the wings by a “rope and pulley” attachment at the top of the arm bone. The collar bones are fused (wishbone) to give stability at their shoulders. (1, 2)
The position of their organs is also optimal for flight, with the lungs being high up in their trunk and the heavier organs like the heart, digestive organs and the pectoral muscles being positioned ventrally. This arrangement also places the centre of gravity in an ideal position, preventing the bird from flipping over in flight. (2) Even the reproductive organs have been modified by shrinking to a fraction of their functional weight out of breeding season. (1)
Because flight can demand high levels of energy (for the flight muscles can consume huge amounts of oxygen), a bird’s heart is big relative to its body. Its respiratory system is also unique. Birds have in addition to their lungs special air sacs that even extend into the bones. When they inhale, the air flows through the lungs and into the air sacs and when they exhale the air passed back through the lungs, giving the bloodstream another chance to take up still more oxygen and get rid of carbon dioxide. These special air sacs also function as a cooling system in times of intense exertion. (1)
Birds also need to take in high-calorie foods to provide them with the energy they need. They mostly eat foods like seeds, insects, fruit, nectar or small mammals and avoid food that is high in bulk and low in calories. Their metabolism is high, due to their relative high body temperature, which they maintain through their excellent insulating properties of their feathers. Their high metabolism rate can also be seen as a weight-saving advantage, as food is quickly digested and waste products soon excreted. (1)
Ultimately, it is the feather that has made the birds lords of the air. Their feathers can weigh twice as much as their skeleton, but individually they are “as light as a feather”. Apart from the downy underlayers, feathers have some 600 barbs on either side of the main shaft (quill), which forms the feather’s vanes. Each barb again has a main stem with about 800 barbules on either side. The barbules pointing towards the tip of the feather have little hooks, while those pointing to the base have a little ridge at the top onto which the hooks fit. This forms an almost air tight surface, which is essential for flying. It also has the advantage that a “torn” vane can be re-zipped by being stroked lengthwise several times. This is what actually happens when birds are preening themselves. They are repairing their feathers, keeping them in good condition. (Preening also entails waterproofing the feathers by spreading oil from a special oil gland just above their tail.) (1)
A bird’s wing shape differs according to their specific function and thus the bird’s lifestyle. Large seabirds have long narrow wings, which allow them to glide effortlessly which is very energy efficient. Soaring birds, like raptors have long broad wings with prominent primary feathers for manoeuvrability and stability in turbulent air. They make use of thermals to conserve energy. Ground feeding birds have short broad wings with arched bones that allow them to take off fast when threatened. Swept-backed wings that taper to slender tips make swifts very agile for catching flying insects, without having to flap their wings. A hummingbird has a unique wing structure. They are able to hover best of all birds and can even fly backwards. Their “arms” are very short, but their “hands” long with ten long and powerful primary feathers. Their elbow and wrist joints are rigid, but their shoulder joints are so mobile, that their wings can practically move in any direction, giving them their extraordinary flying abilities. But it comes at a cost, as it consumes huge amounts of energy. (1)
The birds are the most widely travelled and species rich of all vertebrates. Through flight they have mastered the air, but it is the uniqueness of their feathers that has given them this edge.
1. Reader’s Digest Editors. 1974. Animal Families. Marvels and mysteries of animal behaviour. Pages 250-261 in Animals in Action. Hong Kong: Reader’s Digest Association Far East Limited.
2. Wikipedia contributors. Bird skeleton [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 10, 23:12 UTC [cited 2006 Apr 30]. Available from: http://en.wikipedia.org/w/index.php?title=Bird_skeleton&oldid=43210309.
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