All birds have feathers and only birds have feathers even if the feathers are highly modified as on penguins.
While most feathers share a common overall structure, there are several different kinds of feathers adopted for specialized roles. Changes in feather structure provide the adaptations necessary for feathers to be used in many different ways.
Feathers also support the behavior of the bird within its environment and its lifestyle. Feathers that support the soaring flight of an eagle have a much different role than the feathers that protect an American Dipper, which spends much of its time in fast-flowing streams.
This page primarily covers feather types and feather topography. It is intended for the serious student.
A typical wing feather consists of a central, stiff shaft with the softer vanes on each side. The leading edge of the feather during flight is called the outer vane. The opposite vane is wider than the outer vane and is referred to as the inner vane.
In greater detail, feathers are broken down into the following structural elements.
The central shaft of a feather is divided into two regions.
The calamus is the part of the shaft closest to the bird's body. It is hollow and does not contain any vanes.
The distal end of the central shaft is referred to as the rachis. The rachis is solid and is defined as the area to which vanes are attached.
The vanes extend from each side of the feather. A series of parallel branches called barbs make up the vane.
Extending from the barbs are a series of short branchlets called barbules. Tiny hooklets tie the barbules, and ultimately the barbs, together. This somewhat complex arrangement creates the strong but light structure of the feather.
Feathers are not attached to birds in a random manner over the entire body of the bird. Instead they are usually found in often linear tracts celled pterylae. The spaces on the bird's body without feather tracts are referred to as apteria. The densest area for feathers is often on the bird's head and neck.
Types of feathers:
When you look at a bird the contour feathers are the outermost feathers, or the ones you see. They provide the color and the shape of the bird. The wing feathers are strong and stiff, supporting the bird during flight. The contour feathers tend to lie on top of each other, much like shingles on a roof. The feathers therefore tend to shed rain, keeping the body dry and well insulated.
Each contour feather can be controlled by a set of specialized muscles which control the position of the feathers, allowing the bird to keep the feathers in a smooth and neat condition.
The largest contour feathers are often the large flight feathers, which are collectively called the remiges. Since they are responsible for supporting the bird during flight, remiges are attached by ligaments or directly to the bone. The outer remiges are referred to as the primaries and are the largest and strongest of the flight feathers. They are attached to the skeletal equivalent of the "hand" of the bird.
The inner remiges are called the secondaries and are attached to the "forearm" of the bird. They are located between the body of the bird and the primaries. The secondaries provide lift in both soaring and flapping flight.
The tail feathers are used to provide stability and control. They are referred to collectively as rectrices. The rectrices are connected to each other by ligaments, with only the innermost feathers attaching directly to the tailbone.
Bordering and overlaying the edges of the remiges and the rectrices on both the lower side and upper side of the body are rows of feathers called coverts. The coverts help streamline the shape of the wings and tail while providing the bird with insulation.
Attached to the lower shaft of some contour feathers are the typically much smaller afterfeathers. The afterfeathers resemble the main feather and provide an extra layer of warmth.
In North American birds, the afterfeathers of grouse are especially well developed for their life in seasonally cold and arctic regions.
The flightless Emu of Australia has specially adapted afterfeathers that are as large as the main feather and provide protection as the bird moves through the thick brush of its natural habitat. Relatively recent additions to the Emu's normal range are four-foot high sheep fences topped with barbed wire. Seemingly unperturbed, the flightless Emu crosses these fences by running straight into them, resulting in a high-speed somersault over the fence. The tuft of feathers left behind is a sure indication of an Emu crossing and a testament to the amount of protection the feathers offer.
Highly specialized feathers, bristles are small contour feathers which lack barbs on the outermost part and have an especially stiff rachis.
Rictal bristles project from the beak of many insect-eating birds, including flycatchers, nightjars and even the American Robin. They are believed to provide protection for the bird's eyes as it consumes its wriggly prey. The bristles may also provide tactile feedback, like the whiskers on a dog or cat.
Up to the challenge of keeping birds warm are the down feathers. In down feathers, the rachis is either missing completely or substantially reduced in length. The barbules lack hooks, which combined with the lack of rachis, results in a very soft and fluffy feather. Without the hooks, the barbs and barbules create a puffy tangle of insulating air pockets.
Natal downs are present on some birds at the time they hatch. They are responsible for giving baby chicks and ducklings their fluffy appearance. Natal downs are typically found on well developed hatchlings that can almost immediately walk or swim independently of their parents.
The young of most Passerine species, such as Blue Jays, are totally helpless and virtually naked at birth. It is thought that the baby birds save energy by not producing down and are able to absorb the body heat of the parent bird more easily.
Semiplumes are found between other feathers, providing an additional layer of warmth and helping to maintain the smooth, streamlined shape of the bird. Semiplumes are a cross between down feathers and contour feathers. They do have a supportive rachis, as on contour feathers, but lack the hooks that hold the barbs together. The resulting feather does not form a vane and has a downy feather look.
The simplest feather is the filoplume. It consists primarily of the rachis with no barbs or only a few isolated barbs at the tip. These relatively stiff and hair-like feathers lack specific feather muscles but have sensory receptors next to the base of the feathers. Filoplumes lie under the contour feathers and are thought to provide the bird with feedback on contour feather activity.
Found only in certain taxonomic groups such as pigeons and herons, powder down feathers are never molted. Instead, they grow continuously but disintegrate at the tips into something like a fine talcum powder. The powder permeates the other feathers, presumably to provide waterproofing, although the exact function is not well understood.
Feathers and flight:
The combination of light weight, strength, and shape--combined with precision control--is largely responsible for giving birds their special ability of sustained flight. Readers interested in learning more about feathers and how birds fly are invited to participate in the Cornell Lab of Ornithology's acclaimed Home Study Course.