A bird is designed for flight. The combination of light weight, strength and shape, as well as precision control, is largely responsible for giving birds their special ability for sustained flight. Every part gives maximum power with a minimum of weight. The heavier the animal, the bigger its wings need to be. The bigger the wings, the more muscle is needed to move them.
Birds’ feathers are essential for flight.
Birds’ feathers are light, strong and flexible
Birds’ feathers are designed to be light but very strong, flexible but very tough. Although it looks like feathers grow all over a bird, they actually grow in specific areas called feather tracks. In between the feather tracks are down feathers. This keeps the body weight down.
Feathers are made of a tough and flexible material called keratin. The spine down the middle, called the shaft, is hollow. The vanes are on the two halves of the feather. They are made of thousands of branches called barbs. Because there are many spaces between these barbs, a feather has as much air as matter.
The contour feathers of a bird are the outside feathers – the ones that you can see. They provide the colour and the shape of the bird. The contour feathers tend to overlap each other, much like tiles on a roof. The feathers tend to shed rain, keeping the body dry and well insulated.
Each contour feather can be controlled by a set of specialised muscles, which controls the position of the feathers, allowing the bird to keep the feathers in a smooth and neat condition. The smooth and streamlined surface is achieved because the feathers’ barbs are joined together with barbules (branches on the barbs). The barbules have hooks that lock the barbs together. If the barbules are disrupted, the bird passes its bill though the feather to link them again.
The contour feathers used for flight are known as remiges (wing feathers) and rectrices (tail feathers).
Remiges (wing flight feathers)
These feathers are strong and stiff, supporting the bird during flight. They can be divided into three groups:
- Primary feathers: These are the largest of the flight feathers and propel the bird through the air. They are the farthest away from the body, attached to the skin of the wing on the ‘hand’ of the bird. In most bird species, there are 10 primary feathers on each wing. If these flight feathers are damaged or lost, a bird cannot fly.
- Secondary feathers: These run along the ‘arm’ of the wing and sustain the bird in the air, giving it lift. The number of secondary feathers varies with different species. Experiments have shown that, if half of the secondaries are removed, a bird will still be able to fly, but some control will be lost.
- Tertiary feathers: These are on the ‘upper arm’ of the bird. They are the short, innermost flight feathers on the rear edge of a wing, close to the body of the bird. They are not as important for flight as the primary and secondary feathers.
Rectrices (tail flight feathers)
The rectrices or tail flight feathers are mainly concerned with stability and control. They are used as a rudder, helping to steer and balance the bird and allow the bird to twist and turn in flight. These feathers also act as a brake for landing.
Bordering and overlaying the edges of the remiges and the rectrices are rows of feathers called coverts. These help streamline the shape of the wings and tail (minimising drag) while providing the bird with insulation.