4 - Bird feathers: what, where and why

Most of a bird’s body surface is covered with feathers, which serve a number of purposes, including:
– heat insulation.
– buoyancy and steering ability.
– aerodynamic qualities.
– colour and signalling.
– protection against rain and water.

There are different types of feathers because the functions cannot be performed by a single type of feather. The feathers with closed barbs are the largest. They enable flight and give the bird its shape and colour. Underneath are smaller feathers without real barbs. They provide the necessary warmth and also serve as a powder box. We can divide the feathers into two groups, namely:
– contour feathers.
– the down feathers.

The contour feathers include the flight feathers, the tail feathers and the coverts. These can be divided into large and small contour feathers. The large ones include the wing feathers (Fig. 1, large and small flight feathers) and the tail feathers. They cover by far the largest part of the bird. The coverts, which cover the rest of the bird’s body, are among the smallest contour feathers. They are the most numerous. All contour feathers are basically constructed in the same way. The lower part is connected to the bird’s body and is called the quill. This consists of horn (comparable to our nails). The spindle is implanted in the feather sac, a deep depression in the skin. At the bottom of the spindle is a small hole through which blood vessels ran during growth to supply the cells with food and oxygen. An adult feather therefore consists of dead tissue.

The extension of the quill, the part to which the flag is attached, is called the shaft (Fig. 2). This part appears to be solid, but when opened, it turns out to consist of air-filled chambers (the so-called pith). It is sturdy yet flexible and provides the bird with considerable weight savings. Some down often grows between the quill and the shaft (at the bottom of the shaft). This down only occurs on the coverts, which is how you can recognise them. It provides complete coverage of the body and thickens the plumage (an insulating function). Flight feathers and often also the outer tail feathers are asymmetrical in structure, with the outer part of the feather being narrow and sturdy and the inner part being wide and flexible. In order to optimally support and steer the bird’s body, the feathers of the contour feathers must be completely closed. In addition, a feather is so flexible that it is not immediately irreparably damaged by wind and weather. This is possible thanks to a mechanism that is somewhat similar to a zip fastener. Sometimes part of a feather splits open, but the birds can close it again by rubbing it.

The down feathers in adult birds are almost always located under the coverts. Their structure is the same as that of the contour feathers, except that they do not have a zip fastener. This means that the barbs do not interlock. On the contrary, they maintain the greatest possible distance between each other. All down hairs are negatively charged, causing the barbs to repel each other. There are transitions between contour feathers and down feathers. They do have a shaft, but the hooks are missing. They cannot form a sturdy flag and are therefore classified as down feathers. They are usually for decoration or to aid insulation. Real down feathers do not have a shaft. The barbs are implanted directly onto the coil. Together, they form a dense, air-filled layer under the cover feathers. Down feathers play an important role in maintaining body temperature. The expansion of the down feathers traps more air, which increases their insulating effect. This is why birds look a little plump in winter and during cold weather. However, many birds also have powder down. These are down feathers that break down into small horn-like particles at the ends. This powder is used to dust the contour feathers. They remain supple and are thus probably also protected against moisture.

Like scales, nails, hair and hooves, feathers are keratinised outgrowths of the epidermis. They start to grow while the young bird is still in the egg. However, some young birds are born naked, such as kingfishers, woodpeckers and cuckoos. First, the down feathers form. In principle, there is no difference between the growth of contour feathers and down feathers. When the feather parts are fully grown and keratinised, most of the nourishing connective tissue recedes. The material can be reused when the feather needs to be replaced, for example during moulting. The feather papilla remains alive, sinking into the skin. An adult bird feather consists of dead tissue. There is no longer a living connection to the body, so the feather cannot be partially renewed if damage occurs. A feather that is not yet fully grown can be recognised by its shaft, which is coloured red or blue due to the presence of blood. Fully grown feathers are white or horn-coloured.

A feather does not last a bird’s entire life, which is why adult birds moult at set times to replace split feathers or to acquire special plumage. This makes them more attractive to a mate and plays a role in courtship. Moulting can take place before, during and after the breeding season. Changing plumage should not have too great an impact on flying ability and heat regulation. That is why it is generally a slow and gradual process. This is especially true for species that depend on flying to obtain food. In birds of prey, moulting can take up to 1 to 2 years. To prevent problems, not all feathers on a particular part of the body are replaced at the same time. The flight and steering feathers are replaced in pairs, always one on the left and one on the right, in order to maintain flying ability. The papilla in the feather sac is activated, and a new feather grows in the skin. Even if the feather is lost, a new one grows in its place.

Some birds lose all their flight feathers at once, such as ducks and geese. Between 3,000 and 5,000 feathers per bird must then be replaced. They moult after the breeding season, because this event requires a great deal of energy. The animals therefore experience a period during which they are unable to fly. Ducks become shy and tend to hide in the reeds. After all, their most important means of defence, flying away, is unavailable. Before moulting, they migrate to an area where there is sufficient food and where they are safe from disturbance and predators. General moulting also offers the opportunity to change colour, as feathers acquire pigments during their growth. In summer, for example, the magnificent plumage of strikingly coloured males is replaced by grey feathers, making them look very similar to females. Females also moult, but this is less noticeable because all their feathers are roughly the same. Moulting can last from 1 to 6 months. Larger birds generally moult for the longest period of time.

Of all vertebrates, birds are undoubtedly the most colourful. We can divide the colours into two main groups, namely pigment colours and structural colours. The most important colourants in birds (pigments) are melanin and carotene. Melanin is the most common. They vary from ochre yellow to brown to black. The pigments are produced in special cells in the skin and transferred to the growing feathers. Carotene is absorbed with food. These pigments give the colours yellow, orange and red. Although the colourants can change during digestion, they usually end up in the feathers of the skin without too many changes. The structural colours are not caused by the pigments, but are due to the fine structures of the feathers. Certain parts of the colour spectrum are transmitted and others are reflected. For example, blue is never caused by a colouring agent. Such a colouring agent simply does not exist. The barbs and barbules reflect the blue from the spectrum, and the rest of the colours are transmitted and absorbed. A bird’s colours have quite a few functions. Camouflage colours are common. A female blackbird, for example, sits exposed on the nest, and her camouflage suit comes in handy. Birds that fly a lot often have a dark upper body. This makes them less noticeable from above to birds of prey flying higher. The same applies to penguins swimming in the sea with their black upper parts and white bellies. They are inconspicuous both from above and from below. Colours serve for communication between conspecifics. They can deter rivals and attract the opposite sex. Colours promote group formation, serve as warnings or give other signals.

Coloured or not, feathers need to be maintained. Most birds have a gland on the upper side of their tail, called the uropygial gland. This gland produces an oily substance, which is absorbed by the bird’s beak when it preens its feathers. The oil is spread over the feathers. This keeps the feathers in place and makes them water-repellent.