Original by Dr Margaret Rowe, 2002
Last revised by the Department of the Environment and Heritage, December 2004
ISBN 0 6425 4820 X
The skeleton of a typical bird is lightweight and strong–well suited to flight. The bones of birds are not solid; they contain air and are supported internally by struts. The bill, too, is light and strong. Bones form a strong and rigid frame for the chest cavity so it will not collapse during flight.The strong flight muscles are attached to the large keel of the breast bone (sternum). Birds, such as emus and penguins, that do not fly, have heavier bones.
The wing contains bones equivalent to those in the forelimbs of other vertebrates and to the human arm. In birds, the sizes and shapes of the bones and the structure of the joints are suited to flight and to folding the wing neatly against the body while resting.The bones that are equivalent to the human hand and finger are joined and shaped in a way that gives strength to the outer wing.
If you have chicken wings to eat, have a look at the bones
Figure 4. Skeleton of a bird showing large breast bone, where wing muscles are attached.
Birds have two pairs of main flight muscles connecting the wing bones to the bones of the chest. One pair of muscles lifts the wings and the other group, the stronger group, pulls the wings down. It is the downstroke that propels the bird forward; the upstroke then lifts the wing into position for the next downstroke.
The muscles contain a mixture of red and white muscle fibres. The red muscle fibres, which use oxygen, provide endurance, enabling the bird to fly long distances.The white fibres, which obtain energy without using oxygen, are capable of a few rapid, powerful movements, but tire quickly.The bird relies on these white fibres to provide the explosive power for take off and fast turns.
Birds of different species, adapted to different ways of life, vary in the mix of red and white fibres. They vary in their ability to endure long flights and in their ability to move rapidly over short distances.
Compare a robin, or other bush bird, with a shorebird.
- Which one would be best at making quick turns?
- Name another bird that would be good at making quick manoeuvres?
- Which would be best at enduring long flights?
- Name another bird that would be good at enduring long flights.
Like boomerangs and the wings of aeroplanes, the wings of birds are aerofoils. They are shaped in a way that provides lift when air flows over them. If the wing is tilted too strongly, the effect of the aerofoil is lost and the bird slows and stops–this is used in landing.
The upward force (lift) of an aerofoil is created by the rounded shape of the upper surface, and the flatter shape of the lower surface.The upper surface has a larger area than the lower surface.
Air that passes over the top of the wing spreads out more than the air that passes across the flatter underneath surface. Because of this, the air on the top of the aerofoil has a lower pressure than the air underneath it. Overall, this creates an upward lift that supports the weight of the bird.
The lift is provided only when an airstream is moving over the wing. For this reason, birds usually take off by facing into the wind, or by running or jumping so that air moves over their wings and creates lift.
To demonstrate that the shape of the wing provides lift fold a sheet of paper as shown in Fig.5 and tape it in place. Make sure that the underneath surface is flat and the top surface is curved. Without damaging the curved shape of the "wing", punch two holes and thread a length of fishing line through the paper as shown. Face this model wing into a gentle airstream from a fan, or create an airstream by walking as you hold the model.
Figure 5 The wing as an aerofoil
Figure 6 One the downstroke the feathers at the tips of the wings act like propellers
Once they are in the air, birds move forwards by flapping their wings.The stroke down and towards the back pushes them through the air. The feathers on the outer half of the wing (the primaries) move vertically through the air on the downstroke.They have the effect of little propellers, pushing the bird forward. The effect of the airstream over the upper and lower wing surfaces holds the bird up.
Most birds fly at 30 to 60 kilometres per hour. Birds use more power when flying very slowly or very rapidly than they do at medium speeds.
Birds manoeuvre through the air, using about fifty different muscles to control the positioning of each wing and its feathers. Friction between the birds' feathers and the air results in drag which has to be overcome by the birds' efforts. When flying in flocks, birds avoid flying in the turbulent airstream directly behind another bird. They sometimes fly in a V formation. Birds have difficulty flying if the wind is very strong.
Birds differ in wing loading. Wing loading is the weight of the bird compared with the area of its wings. Birds that are heavy, compared to the area of their wings, have difficulty in taking off and have to run to become airborne.
Long narrow wings provide more lift and less drag than short broad ones. Long narrow wings are suited to gliding and high speed flight. Shorebirds and terns have fairly long pointed wings. Long narrow wings are an advantage to birds that migrate over long distances. Birds that live in dense vegetation usually fly in short bursts and have shorter, rounded wings that allow the bird to manoeuvre within confined spaces.
Name three birds that are heavy compared with their wing size and three that are light compared with their wing size.
(Hint: the heavy ones would have big bodies and make good food for humans).
Figure 7 Wing shapes are adapted to the bird's way of life
Look at the illustrations of wing shapes.
- Compare the shape of the shorebird's wing with the others. What sort of flight is typical of shorebirds? Is the wing suited to these types of flight?
- Compare the shape of the shorebird's wing with the others. What sort of flight is typical of robins? Is the wing suited to these types of flight?