A) THE FUNCTION AND FORMATION OF FEATHERS
The feather is the primary part of the bird which makes it unique from any other animal on earth. Feathers grow straight out of follicles in the skin, much like the way human hair and animal fur grow (1). Feathers grow from the follicles which are grouped in a pattern of tracts along the bird’s body. Birds look as though they are covered everywhere with feathers because they overlap, covering the bald areas between the tracts.
The first stage of a feather’s growth is a small pimple called a papilla—a kind of mound surrounded by a tiny trough. Papillae begin to appear on the skin of the embryo chick after about a week of incubation in the egg. By the time the bird hatches from the egg an epidermal cell called Malpighian is formed, which eventually becomes the actual feather (2).
The short basal part of the feather is called the calamus. There is an opening at the bottom of the calamus called the lower umbilicus. During the feather’s short period of growth, blood enters the young feather. Once growth is completed, the lower umbilicus is sealed off and blood no longer enters the feather. Once the feather is sealed off each feather can be moved by a separate muscle situated in the skin (3).
At this point the new feather will become one of six types: contour, down, semi-plume, bristle, or powder down. The longer, stiffer feathers that covers the adult bird’s body are known as contour feathers because they define the shape of the bird. Down and semi-plumes feathers are pieces of fluff that lie underneath the contour feathers for insulation and padding. Filoplume feathers are made up of a thin shaft with a few short barbs of barbules at the tip. They have a lot of free nervous endings connected to pressure and vibration receptors around the follicles. Bristle feathers have a very stiff rachis and few, if any barbs. They function as eyelashes and aid birds which catch flying insects.
Even though the quantity of each type of feather a bird contains is different, the basic construction of feathers is the same for all birds. The shaft which projects from the skin contain many branches on both of its sides. Together they form a flat surface called the vanes. The rachis carries the vanes of the feather. Each vane is made up of a row of barbs arranged side by side. The barbs are connected by smaller branches called barbules. The barb and barbules are linked together by means of tiny hooklets called barbiceli (4).
The system of barbules is one of nature’s most amazing bits of engineering. This structural device enables the feather to be waterproof and cut air precisely during flight. A large part of a bird’s day is spent maintaining the system of barbules (5). (See Figure I at the end of this section).
(B) THE WINGS
Amazingly, the bird’s wing physically compares to the human hand and arm which is displayed at the end of this section in Figure 2. The wings of a bird are composed of the humerus or the upper arm, the radius and ulna which makes up the forearm, and the wrist and finger bones. The wrist and handbones are fused together in order to provide a firm support base for the primary feathers. The chief function of the primary feathers is to propel and maneuver the bird during flight (6). Whereas the secondary feathers are primarily responsible for lift, which will be discussed further in a later section of this paper.
In addition to the primary and secondary feathers, the hand portion of the wing also supports a few small feathers on the thumb, or first digit. The thumb portion of the wing is known as the alula, and the feather makes up the bastard wing. The bastard wing’s function is to produce wing slots in order to increase the amount of air flow over the wing during slow labored flight (7).
The wing is also made up of muscles, tendons, nerves and connective tissue. It is through these items that the wing is able to move, even though the large pectorial muscle is accredited with doing most of the work (8). The pectorial muscle is the largest muscle in the bird’s body. It covers most of the area between the breast bone and the base of the wing. When the large pectorial muscle contracts, the wing is pulled downward. Contrasting the large pectorial muscle is the small pectorial and deltoid which are responsible for raising the wing. The deltoid muscle is also responsible for rotating the wing back out from the leading-edge-down position into which the large pectorial muscle turns it. The three muscles—the deltoid, large and small pectorial—provide the power needed for moving the wing (9). Even though the tendons, which run through a hole in the bones aid the large pectorial muscle in pulling the wing upward.
The skin made up of connective tissue covers all the above mentioned components of the wing. Furthermore, it is the skin in which the feather of the wings are inserted. Total control and proper functioning of the complicated appendage—the wing—is left up to the central nervous system which includes the brain and the spinal cord.
(C) THE FOUR DIFFERENT TYPES OF BIRD WINGS
The wings of birds vary in both shape and size. Both characteristics depend largely upon the function and the habitat of the bird (10). Wings have been classified under one of the following four groups: elliptical, high speed wings, high-aspect ratio, or slotted high-lift wings.
Elliptical wings are found on birds which live in a forested environment where they are forced to dodge quickly in and out of obstructions. Birds characterized with this type of wings have a low aspect ratio which means that the length of the wing divided by the width yields a low number. They are also highly cambered (flatter wing section or shaped) and the outer primary feathers are slotted. Elliptical wings are found in most gallinaceous species, as well as doves, passerines and woodpeckers.
The high-aspect ratio wings can be identified on birds by its length. The length of the wing greatly exceeds the width. Characteristically, this wing type is long, narrow and rarely has wing tip slotting. High-aspect ratio wings are most commonly found in soaring seabirds such as shearwaters, albatrosses and frigate birds. Birds who possess this wing type are also noted for their long distance gliding abilities.
High speed wings are found on birds that make long migrations. These wings have a low camber or a flattish profile and a fairly high aspect ratio. Such wing types taper to a relatively slender elliptical tip and tend to be swept backwards like the wings of a high speed jet fighter plane. The best examples of high speed wings are found on hummingbirds, swallows, falcons, sandpipers and plovers.
The fourth type of wings are the slotted high-lift wings. They have a moderate aspect ratio, deep camber and noticeable slotting in the wing tip. This type of wing produces an efficient soaring wing for birds that generally carry heavy objects for consumption such as owls, vultures, hawks and eagles.
(D) THE BIRD’S TAIL
Even though the wing of the bird plays an important role in bird flight, it is not totally responsible for flying. Since the wings are generally located just in front of the bird’s center of gravity, the body tends to move down towards the earth. During flight, the bird’s lowered tail is acted upon by the horizontal air stream, and is lifted in a compensating fashion. This provides the tail with great maneuvering power. The tail of the bird also aid the wings in supporting, steering, balancing and braking the body during flight (11).
(E) AERODYNAMICS OF BIRD FLIGHT
In order for a bird to fly it must obtain an upward force which is known as lift. The construction of a bird’s wing enables the bird to achieve lift. A wing which is shaped to achieve lift is called an aerofoil. The leading edge and the upper surface is more convexed than the lower surface.
When air strikes the leading edge of the wing it divides, some air passes underneath the wing while the remainder passes across the upper surface. Since the upper surface of the wing is curved, it has a longer surface than the underside of the wing. In order for air to travel to the rear of the wing at more or less the same time as the air on the underside, the air on the upper surface must travel faster than the air underneath the wing. The faster air travels across a surface the lower the pressure it exerts on that surface, as a result the wing’s upper surface experiences a lower pressure than under the surface and produces lift (12).
Lift, therefore, can only be produced by the wing when the flow of air is smooth over the surface. Lift is lost when the flow of air over the wing is separated—this occurrence is known as stalling. Stalling occurs when the wing is held at too high of an angle and the flow of air breaks away from the upper surface.
Air passing over the wing also exerts another force known as drag on the wing. Drag is the backwards pressure or force of the air opposing the wing in its movement through the air. Drag is equivalent to the amount of the bird’s wing which is exposed to the wind. The flatter the bird’s wing the less drag there will be, while the steeper the angle at which the wing is tilted (known as the angle of attack), the greater the drag. Drag on the wing can be caused by the air that has passed along the lower surface of the wing flowing up over the trailing edge of the wing and into the lower pressure area on the upper surface. This upward flow is strongest at the end of the wing and is referred to as wing-tip vortex. The flow of air caused by the wing-tip vortex spreads inward along the trailing edge of the wing and interrupts the desired smooth movement of air for a considerable distance causing drag (13). Both drag and lift are effected by the area and shape of the wing, the area of attack, and by speed.
Forward propulsion, known as thrust, is obtained by the large pectoral muscles when they drive the wing downward. In order to fly forward the downbeat must provide both upward and forward movement of the bird so that it both stays airborne and moves forward against the resistance provided by drag. Thrust is achieved by the flapping of the bird’s wing. As the wing is flapped the feathers tend to bend so that the backward edge of the wing is above the bones. (The wing bones are in the leading edge of the wing, thus the trailing edge is the feather.) As a result, even though the wing beats downward because of its shape, it pushes the air both downward and backwards, thus pushing the bird in the opposite direction-forward.
(F) THE FIVE DIFFERENT TYPES OF BIRD FLIGHT
There are five major types of bird flight, even though four of the five methods are closely related. The five major types of flight are known as flapping, gliding, soaring, fluttering and hoovering flight. Of the five, the mechanics of flapping flight is the most difficult for scientist to understand (14).
Flapping flight is the rapid and vigorous beating of the bird’s wing up and down against its body. It is this type of flying that enables the young bird to leave its nest and begin to learn the more advance flying techniques. Flapping flight is also the one used by birds when they take off in order to propel themselves into the air. The difficulties of understanding flapping flight becomes apparent when one considers all the aspects involved in this type of flight. A beating wing yields under pressure causing it to be very flexible. The shape, camber, expanse, sweepback, and even the position of the individual feathers may change pronouncedly. The different parts of the wing change in velocity and angle of attack during a single beat (15). The aforementioned variables are only a few of the mechanics of flapping flight that scientist are attempting to fully understand. However, some of the principles of flapping flight have been discovered largely due to high speed photography.
During gliding flight, the wings are stretched out stiffly into the air. The bird loses height due to the gravitational pull of the earth, however, gravity also gives the bird the advantage of acceleration during gliding flight. A good glider can glide a long ways horizontally with minimal loss of height by holding its wings at a slight angle to an air current, which allows the air to flow faster over the upper surface than over the lower surface of the wing thus creating lift. At the same time the resistance to moving air tends to drag the wing backwards. True gliding flight is made possible when lift and drag forces are adjusted to be equal to the weight of the bird, which is achieved by the angle at which the bird holds its wing causing the wind to hit it in a certain way (16).
There are two basic ways in which a bird can glide when the air is still. First it can launch itself from a perch and open its wings. This way the bird uses the energy provided by gravity so that by losing height it can travel forward. Secondly, it can depend upon thermals or obstruction currents like the glider plane does. Thermals are rising columns of warm air coming from the earth’s surface. Whereas, obstruction currents are caused when wind blows against large solid objects like mountains, cliffs and buildings and causes the wind to be pushed upward.
Soaring flight covers any technique of flight where energy is extracted from movement of atmospheric wind and converted into kinetic energy of the bird (17). Birds use soaring for two reasons. First it can be used to stay airborne using little energy while looking for food. Secondly, it is used for cross-country flight which offers an alternative method of travel instead of depending solely on power flight such as flapping.
The simplest method of soaring is slope soaring, where the birds fly in regions of rising air caused by the upward deflection of wind over a slope. Smaller birds can also use ocean currents for slope soaring. Two different types of wings seem to be best suited for soaring: the wide expansive wings of vultures and hawks, or the long narrow wings of gulls and albatrosses.
Fluttering flight generally refers to a bird that can remain stationary in one spot in the air without the help of a strong head wind (18). The hummingbird hanging suspended over a flower is an excellent example of fluttering flight. Some birds flutter at the tips of branches as they pick off insects that may gather there, such birds include the kinglets and warblers. Some species such as skylarks, purple finches and white-winged crossbills often give the most ecstatic part of their flight songs during fluttering flight.
Hoovering flight closely resembles fluttering flight. However, hoovering flight requires assistance from headwinds (19). During hoovering flight, the tail of the bird is usually fanned and pointed downward. The rear of the body is tilted downward, and the speed and angle of the wingbeats are regulated to support the bird yet prevent the wind from pushing the bird backwards. The osprey, hummingbird, common tern and belted kingfisher usually display excellent examples of hoovering flight.
FIGUREI The System of Barbules in a Feather
(figure available in print form)
FIGUREII Comparison of Bird’s Bone Structure to the Human’s.
(figure available in print form)