Feathers

Feathers are one of the marvels of nature, they are light, strong and flexible. A bird has several different types of feathers, each is adapted for a specific purpose, whether it is flight, insulation, display or whiskers.

What are they made of?

Feathers are made from a tough, fibrous protein called keratin. This is similar to the keratin that our finger nails and hair, and reptilian scales are made from. Indeed, birds probably evolved from reptilian ancestors, see Evolution.

How are they made?

The flight feathers, such as the primary and secondary wing feathers, are called remiges. These feathers comprise an airfoil-like vane with a central shaft, which is hollow and filled with air for lightness.

The quill is the end of the shaft that is attached to the bird and it fits into a follicle in the flesh just like the roots of our hair.

The feather is asymmetrical with the shaft closer to the front (anterior) edge of the feather. When a bird raises its wings, the feathers open up and allow air to pass through. On the down stroke, the feathers close up and present a solid surface to the air and this generates the lift needed for the bird to remain airborne.

The vane is not solid but is made up of thousands of hair-like filaments that zip together.

Each hair-like filament is called a barb and under a microscope the barb can be seen to have lots of smaller barbs called barbules - rather like a tree has branches, and the branches have smaller branches. The barbules have hooklets that fasten over the barbules of adjacent barbules.

In this way, if a bird collides with a twig and disrupts the smooth, aerodynamic structure of its feathers it can quickly repair the damage by reconnecting the barbs while preening.

However, not all of the bird's feathers have this mesh-like construction.

The bristle feathers are found around the eyes, mouth and nostrils of birds. These bristles provide protection (like eye lashes) and a sense of touch like a cat's whiskers. The next time a Blackbird is in your garden, take a closer look with a pair of binoculars at the bristles near the base of its bill.

Downy feathers are adapted for insulation, to keep the bird warm, and are a haphazard tangle of barbs. These feathers, on an Eider, form one of the best insulating materials in existence - Eider down.

Powder down feathers have barbs that turn to dust, like talcum powder, and are usually found in birds like pigeons that do not have preen glands. The powder helps the birds to groom. This powder can be seen when a pigeon has flown into a window and leaves behind a ghostly impression of itself.

What gives them colour?

Feathers are coloured for several reasons:

1. Camouflage
   A plumage intended as a camouflage is often drab, for example, the dappled buff and brown colours of a Tawny Owl make it very difficult to see next to a tree trunk. These colours are created from the pigment melanin.
   
2. Mating Displays
   The feathers are usually brightly coloured or iridescent, for example, Goldfinches and Magpies. The bright colours, such as red and yellows, are based on pigments called carotenoids, which are derived from their diets. Blues and iridescence (i.e. metallic sheen) are not created from pigments, but from the shape of the barbs: miniature mirrors reflect light, and prisms split (or refract) light into its component "rainbow" colours.
   
3. UV Protection
   Melanin is the brown pigment in our skin that protects us from the harmful ultraviolet rays and so protects birds too.
   
4. Wear Resistance
   Feathers are naturally white and quite soft and so wear quickly. The addition of pigments makes them stronger and harder wearing. For example, the winter plumage of a Starling is dark and iridescent with white coloured spots. The spots are formed from white tips to the feathers. During the course of the winter the tips wear more quickly than the rest of the feather and by the end of the winter the Starling has lost its spots.

Once a feather is fully grown it becomes a dead part, unlike our finger nails and hair which grow continuously. Before the feather becomes too worn, discoloured and damaged, the bird usually replaces them in an annual moult.

Albinism, Leucism and Melanism

Pure albino birds lack pigmentation and because feathers are made from keratin, which is naturally whitish in colour, their plumage is white. The absence of pigmentation also affects eye, leg and bill colour - the eye and legs appear pink owing to the blood vessels showing through, and the bill will be whitish. As well as pure albinos there are partial albinos which simply have a few white patches on their plumage or have white plumage but retain their proper eye or leg colour.

Albinism is usually a genetic condition that causes the absence of pigment in plumage and eyes but may also be caused through malnutrition, parasites or injuries. A common belief was that too much white bread was the cause of albinism, but this is not the case. As well as white feathers, albinos have red eyes.

Leucism is a similar condition to albinism except that the normal plumage appears very pale as a result of weak pigmentation; their eyes are coloured normally.

Some birds, such as Blackbirds, may have a single feather or patches of feathers without pigment, but their eyes are coloured normally. This is popularly referred to as partial albinism but should be properly described as leucism.

The opposite condition to leucism is melanism. A melanistic bird has excessive pigmentation, giving them a darker appearance.