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Flight

Wing Shape

The dynamics of bird flight like all physical actions are governed by the laws of physics. In its simplest expression, flying is a balance between two sets of forces. lift and weight, and thrust and drag. Weight is the result of gravity and is reduced as much as possible in birds (see anatomy). Lift is generated by the flow of air over the wings.
The red line is the leading edge, the air first hits the wing here. It travels up over the green line and then down the back of the wing where it leaves the wing at the blue line, the trailing edge.

Basically, birds wings are not flat but are shaped like an aerofoil - concave. Air passes over or under the wing as the bird moves forward, or as the wind blows. The air that moves over the top of the wing has further to travel to get across the wing, thus it speeds up. This causes the pressure to drop because the same amount of air is exerting its pressure over a greater area. Therefore, any given point experiences less pressure. This effectively sucks the wing up. Meanwhile the air going below the wing experiences the opposite effect. It slows down, generates more pressure and effectively pushes the wing up. Hence a bird with air moving over its wings, is pulled up from above and pushed up from below. The more curved the aerofoil the greater the lift providing the degree of curve does not impede the flow of air.

This causes the pressure to drop because the same amount of air is exerting its pressure over a greater area. Therefore, any given point experiences less pressure. This effectively sucks the wing up. Meanwhile the air going below the wing experiences the opposite effect. It slows down, generates more pressure and effectively pushes the wing up. Hence a bird with air moving over its wings is pulled up from above and pushed up from below. The more curved the aerofoil the greater the lift providing the degree of curve does not impede the flow of air. The air passing over the wings and the rest of the body creates drag. This is the resistance the air gives to anything passing through it. The faster you move the more drag you experience because you come into contact with more air per second (or other unit of time). Thirdly, because nature does tend to even things out, the low pressure air on top of the wings represents a sink that the high pressure air under the wing seeks to move towards, a bit like water running down hill. This happens most along the thin trailing edges of the wing and causes a spiralling vortex of disturbance at the wing tip. These spiralling vortices increase drag, therefore, the most efficient wings are those which supply lift while reducing drag. In practice this means the crescent shaped wings of swallows and swifts. However, birds use flight in different ways, some are on the wing most of the time, while others make only short flights from one perch to another. Also birds live in different habitats which generate different aerodynamic problems. It is not surprising then that birds of different species have different shaped wings.

Changing the shape of a wing gives it different aerodynamic properties. One way to assess these properties is to measure what is called the 'aspect ratio'. This is the ratio of wing area2 divided by wing breadth. Long wings are better for gliding but harder to flap quickly and are therefore not much good at quick acceleration. Another way is to look at flight capabilities is to look at 'Wing Loading', this can show the differences between birds with similar wing shapes but different sizes. Wing loading is the relationship between total body mass and total wing area, it is expressed as grams of body mass over centimetres squared of wing area. Thus the Long-tailed Hornbill (Tockus albocristatus) which weighs 297gram, has an aspect ratio of 4.65 and a wing loading of only 0.175 has light buoyant flight while the Yellow-casqued Wattled Hornbill (Ceratogymna elata) which weighs 2100grams, has an aspect ratio of 4.53 but a wing loading of 0.709 has much heavier and more laboured flight.

Four different basic wing shapes include:

1) resident passerine or pheasant - wings like this have a low aspect ratio of around 3.0 to 6.0 and allow their owners to explode into flight suddenly and are quite adequate for relatively slow powered flight, but not good for gliding;

2) Waders have medium length wings with an aspect ratio of around 12.5, they also tend to be pointed and directed backwards after the first half. These wings are shower to take off, but allow for a faster top speed and a little gliding. They are good for long distance migrants;

3) Eagles and Vultures have broad, long wings with an aspect ratio of around 9.3 and the feathers at the ends separate out into fingers which help with minute controls (like aerofoils) while the birds are gliding. These are basically terrestrial birds riding high above the ground using a variety of updrafts to avoid flapping;

4) Albatrosses have long, thin wings with an aspect ratio of around 13.8 and higher and no fingers. These are good for gliding over the sea, close to the surface, using small changes in wind direction to maximum advantage. These four examples pinpoint extremes and among the 9703 species of known birds, wings with many similar but small variations can be found.

Most information on this page was contributed by EarthLife.

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Last updated: 01 January 2003