A bird's feathers are light in weight, but strong enough to support the bird, once in flight. According to Michael Corral, author of "The World of Birds: A Layman's Guide to Ornithology," some authorities believe that since the bird evolved from small dinosaurs during a warm period of the earth's history, it has used its feathers for additional temperature control. Since that time, bird feathers have evolved to help them adjust to their environment.
How a Feather Emerges
According to Corral, bird feathers are protein outgrowths of cells that develop within the skin follicles. Much like hair or scales, the feathers arise from the dermal layers beneath the bird's skin surface, and as the cells proliferate, they erupt on the surface. The cells produce a cone-shaped sheath, which enfolds a tightly rolled feather. Once the sheath splits, a feather unfurls and displays the structure. Blood to the external structure is then cut off, leaving the feathers embedded within the skin.
Structure of a Feather
A bird feather is constructed alongside a shaft. The vane of the shaft, which attaches to the rachis, is rectangular. According to Corral, smaller structures called barbs, which are parallel to one another, are projected off either side of the rachis. The longest barbs appear at the centre of the rachis. The inner edge of a barb, contains finer structures of barbules that fold to form teeth or spines or tiny hooks, when located on the outer edge of a barb. Since the barbs run parallel to each other, the two types of barbules interlock, forming the surface of a vane. Below the rachis in the cross section, lies the oval-shaped calamus, which is the portion of the feather that becomes embedded in the skin.
How Feathers Help Flight
Once a bird's wing is pulled down, the individual flight feathers overlap to form a continuous area, according to John Gooders, author of "The Practical Ornithologist: What to Look for, How and When to Look for it and How to Record What you See." The feathers, on the upbeat, twist and open to allow air to pass through them. The tail feathers help provide the bird lift when in flight, as well as acting as rubber and brake.
The contour feathers are the external covering of a bird. They provide the largest amount of feathers, which give the bird its shape and colour. The colour of a bird's contour feathers offers it a level of defence against physical objects, sunlight, wind and rain. The bird's contour feathers, which provide a streamlined shape, include flight feathers of the wing and tail. Contour feathers are the same for all birds with minor modifications.
Down feathers, which are found underneath the contour feathers, provide insulation. This is a result of the down feather's ability to trap large volumes of air between barbs and barbules, according to Corral.
Semiplume feathers are also located beneath the contour feathers. These feathers are abundant on waterfowl, and are usually along the sides of the birds abdomen--to provide insulation and buoyancy.
Filoplumes and Bristle Feathers
Filoplumes are mostly made up of a shaft with very few barbs. These feathers provide birds with a sense of touch. The filoplume feathers are most abundant around the flight feathers, with tactile nerve endings in the follicles that help the bird keep its feathers in order. When flight feathers are in use, and they press against filoplume feathers, it allows the bird to know the position of its feathers. The bristle feathers form eyelashes on some bird species, along with filtering dust from the air entering their nasal opening. Bristle feathers are most common in fly catching species.