The fact that birds fly is one of the most amazing things about them. The sight of crows or ravens playing in the wind, of seagulls gliding effortlessly along seashore or of buzzards soaring silently about the countryside always inspires me.
|There is no doubt that if I had the ability to change into another animal, something other than a human being I would be a bird simply because of the incredible wonder of flight on a sunny day.|
|Birds are the ultimate flying machines, though insects have been doing it for longer and in many cases are more versatile; their small size makes the physics of it much easier. Birds fly far better than any machinery mankind has yet made and like our machines they use their wings and tails in a variety of ways to achieve their expertise.
Bird flight is dependent on the shape of the birds wings and the way they use them. Generally, bird flight can be divided into two modes of functioning, i.e. gliding or soaring flight, and flapping or powered flight.
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.
This is not the end of the story though, because 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: