Bird Anatomy: How do birds fly?

Almost every part of a bird’s anatomy has evolved in some part to enhance flight. Birds must be lightweight to fly, so have evolved very lightweight hollow bones. The structure of their bones resembles honeycomb, making them very strong but also very light. For example, frigate birds have a wingspan of over two metres, but the skeleton weighs 113g. Birds also have fewer organs (e.g. only one ovary) and no teeth. Birds use a digestive organ called a gizzard to grind up food.

Birds maintain higher body temperatures than mammals, about 40 degrees Celsius. This enables cells in their muscles to work around 2.2 times faster, and allows muscles to relax more rapidly. The higher body temperature is enabled through the insulating properties of feathers, and in some species, a layer of fat.

Birds have highly efficient respiratory and circulatory systems which keep their tissues well supplied with oxygen and nutrients, supporting a high metabolic rate. Bird lungs are full of elastic air sacs that help to dissipate heat and reduce the density of their bodies. The eyesight of birds is said to be the best of all vertebrates. Excellent eyesight and coordination helps birds to fly safely.

Despite the popular expression “bird-brained” being used to describe somebody who lacks intelligence, birds’ brains are proportionately larger than those of reptiles and amphibians (their closest living relatives), and research has shown that birds are capable of very complex behaviour. Indeed, some birds migrate over 20,000kms per year, without any of the satellite navigation devices we use to find our way.

The most obvious adaptations for flight are bird’s wings. To flap their wings and provide power for flight, birds contract their large pectoral (breast) muscles which are anchored to a keel on their sternum (breastbone).  Many birds, such as birds of prey use air currents to soar and glide, whereas birds such as hummingbirds must flap continuously to hover while feeding. Some birds have evolved into flightless birds, in the absence of natural predators or in the case of penguins, to enable them to swim. In all birds capable of flight, it is the shape and arrangement of feathers which enables them to create lift with their wings.  

Feathers are remarkably light and strong. They are composed of keratin, which is the same protein that makes up our hair and nails, as well as the scales of reptiles. A feather consists of a central hollow shaft, rather like a tree trunk. The shaft has vanes which radiate from it like the branches of a tree. The vanes are known as Barbs, and from these radiate barbules, similar to the smaller branches of a tree. Barbules have hooks which cling to barbules on neighbouring barbs, like a tree with entangled smaller branches. When birds preen, they run the length of the feather through their bills, which engages the hooks and shapes the barbs into a vane. The arrangement of the preened feathers enables the birds’ wings to be airfoils, “structures whose shape creates lift by altering air currents” (Campbell and Reece, 2002). As a result of this shape, the air pressure pushing down on the wing is less than the air pressure pushing upward on the bottom of the wing. The difference in pressure provides the “lift” for flight. 

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