# Flight Mechanism in Birds

The flight of birds apparently seems to be simple; a bird lifts its body and drives itself forward by beating its wings against the air- current. But the process is not so easy. It involves many complicated and delicate steps. Pigeon, like other flying birds, is actually a living aero plane.

It applies all the aerodynamically principles of a plane and use the same mechanical equipment’s. In order to fly efficiently, the body of pigeon is built up in a special design. The details of flight mechanism are still incompletely known. However, the following account will give an idea.

Major aerodynamic principle solved by the bird during flight

When our hand is stretched out from a running motor car or train, it is felt that the hand (which is slightly convex on the upper surface) is pushed upward and if the air-current is strong it can support the hand in air. This upward pushing is called lifting (Fig. 9.11 A).

If the air-current is not strong enough, then the drag (resistance to the motion of a body through air or water) of the centre of gravity will bring the object downwards. It means that in order to remain suspended in air, there must be enough force to negate the drag force and that must be proportional to the weight of the individual.

The same principle is applicable to the flight of a bird. In pigeon, the forelimbs are modified as wings. These wings not only work as the surface where air-current acts to lift, but also are specially built to produce necessary air- current required for the lifting of the body.

Flight does not mean only the support of the body in air; together with lifting the bird will have to move forward. Both these functions during flight are carried out by wings — either by flapping or gliding. Of these two aspects of flight, lifting and moving forward, the bird spends more energy in the former than the latter.

## Perching mechanism

The hind limbs of pigeon are typically built on the reptilian plan. Pigeon, like other flying birds, has the ability to perch on the branches of the tree. Some muscles in the legs are modified in such a fashion that the toes can close round the twig automatically when it sits on the tree.

There are four digits in the hind limbs which are flexed by two sets of tendons. The tendon of the hallux arises from the flexor perforans muscle (Fig. 9.12).

The tendons of the three forwardly directed digits are formed by the trifurcation of the tendon coming from peroneus muscle. The tendons are so oriented that a pull upon any tendon flexes the toes.

When the bird settles on the branch of a tree, the legs are bent and they put the flexor tendons on the stretch. With the exertion of the pull, the toes are bent spontaneously around the perch. A bird can go to sleep in this position without any fear of falling off.

To unlock the feet, it is obligatory for the bird to raise its body to straighten the leg and loosening the tendons which have been pulled tight over the ankle during perching.

It is interesting to note that the foot contains no muscle, but the working of the digits is controlled by tendons coming from the muscles situated in the upper sector of the legs.

It has been claimed that some other muscles also assist in the process of perching. The role of ambiens (a thigh muscle originating from the pubis and travels along the entire length of the leg to join the muscles of the toes) in perching is a disputed issue.

### Chief modes of flight in birds

There are four main types of flight in birds and all the types may be used by the same bird at different times.

1. Gliding or Skimming:

The simplest mode of flight in birds is the gliding. After making rapid strokes some birds hold their wings motionless (spread) and glide for a considerable distance without flapping their wings.

This type of flight may be compared to coasting downhill on a bicycle.

## Gliding

• The wings must be firmly braced to the body,
• The elbow and wrist joints must be rigid,
• The wings must present a relatively firm leading edge to the air.
• By the initial flapping of the wings, the birds acquire the required momentum, and if the momentum is not produced from time to time, the birds lose height.

The gliding flight can only be exhibited for a short time and best seen in the gulls circling a moving ship in the sea for fish or scraps thrown overhead.

## Flapping flight

It is the most common type of flight and may be compared to swimming breast stroke. In this type the wings move upward and forward, downward and backward and then more rapid upward than downward.

The upstroke is very rapid but due to partial folding of the wing curvature on the wing surface and the set of feathers, a minimum amount of resistance is created. The tips of wing do not work simply up and down but they, roughly describe a figure of 8.

In the down stroke (or power stroke), they move obliquely backwards and downwards and their distal portions tilted upwards, thus it helps both to lift the bird and propel it forward. Before the beginning of the upstroke the elbow is flexed and in the upstroke the wings are partly folded.

This type of flight is seen in duck (Fig. 9.13), crow and in other passerine birds. The wing beats vary according to the size of the bird and speed of flight. As a small bird sparrow has 13 strokes per second whereas in pelicans the stroke is 11.5 per second. Long pointed wings indicate strong sustained flight and are possessed by swifts, falcons and pigeons and also other bird that take long journeys.

## Soaring flight

It is the most highly specialized and spectacular type of bird flight. The soaring flight of vultures, hawks, gulls, pelicans, falcon, stork, albatross (Fig. 9.14) and other birds was described in detail by Hawkins (1913). It consists of two phases — circling and gliding. In circling phase, the bird’s path consists of a series of circles or loops with a continuous increase in height. The wings are fully extended and the tips of the primary feathers are widely separated.

Principles of Hawkins (1913) summarized by Cone in 1962.

• Soaring begins at a definite period of the day and is closely correlated with the intensity of sunshine.
• The time of onset of wing depends on the wing loading, of the birds. The higher the wing loading, the greater is the height at which the birds soar.
• In circling flight, the diameter of the circle increases with the wing loading.

## Hovering flight

The typical hovering flight of humming birds was described by Stolpe and Zimmer (1939). It probably represents the principles of active flight. In this flight the body becomes vertical and motionless. The wings beat downwards and forwards during down stroke, backwards and upwards during upstroke. The tips of the wings represent a figure of ‘8’.

The principle of the humming bird’s wing is essentially the same as that of a horizontal propeller blade of a helicopter. Hovering is best seen among humming birds (Fig. 9.15). Among Indian birds hovering is best seen among pied kingfisher and the kestrel, and among black winged kite and fishing eagles to a certain extent.

### Flight Modifications in Pigeon

Pigeon is a typical representative of flying birds. It is actually a ‘living airplane’. It flies in air by the same aerodynamically principles employed by a plane and utilizes similar ‘mechanical’ equipment’s, viz., wings, propellers, steering gear, etc. for helping in taking off and landing.

This animal exhibits peculiar anatomical modifications for leading a life in air. It shows actually double adaptations— aerial and cursorial and or scansorial. As a consequence, the forelimbs have converted into the wings and the hind limbs have virtually retained the reptilian plan and have undergone less modification.

To fly in air, a heavier-than-air organism must possess the following unavoidable prerequisites, without which Volant life becomes impossible.

## prerequisites

1. Organs for flight,
2. Lightness and rigidity,
3. Evolution of extra energy with provision for high power,
4. Speed and
5. Balancing and control.

Let us now consider the ways how pigeon has modified itself to lead a perfect life in air. The reasons for flight is also applicable in other birds.

### Organs for flight

• The forelimbs have transformed into the wings. The wings are the sole organs for flight. These organs have complicated structural constructions consisting of a framework of bones, muscles, nerves, blood vessels, feathers, etc.
• The humerus is a strong long bone with a prominent ridge for the insertion of the flight muscles.
• The radius is a nearly straight and slender bone.
• The ulna is stouter and slightly curved.
• Only three digits are present. There is no trace of fourth and fifth digits.
• The small bones of the wrist, hand and fingers have become highly modified — both by the loss of bony units and by the fusion of the remaining pieces into strong bony complexes. Such fusion reduces the chance of dislocation and mechanical friction during action.
• The wings spring from the anterior region of the trunk. During rest the wings remain folded against the sides of the body. During flight the wings are expanded.
• The surface area of the wings is increased by the development of feathers.
• The patagia are vestigial. A fold of skin stretches between the upper arm and the fore-arm which is called the alar membrane or pre-patagium.
• A similar but smaller fold extends between the trunk and the proximal portion of the upper arm.
• There are twenty-three remiges in pigeon, of which eleven are primaries and twelve are secondaries.
• The interstices between the quills of the remiges are closed by several rows of coverts to make the wing a continuous area to oppose the buoyancy of air.
• The lower surface of the wings is concave while the dorsal side is convex. This configuration makes the down stroke of the wings more powerful.
• The action of the wings is controlled by the flight muscles. The muscle fibres composing the muscles are long and striated to withstand fatigueness after prolonged activity.
• The important flight muscles which assist in flight are:
• Pectoralis major. The powerful down-stroke of the wing is caused by this immense muscle.
• Pectoralis minor. The elevation of the wing is caused by this muscle.
• Coracobrachialis muscles. These small muscles aid in depressing the wing.
• Tensores patagialis muscles help to keep the patagia tensely stretched when the wings are extended.

(xvi) The flight muscles are highly vascularized structures. The subclavian artery is very stout and divides into two branches: a pectoral artery to supply the flight muscles and an axillary artery to the wing.

### Lightness and rigidity

• The skeletal framework of a pigeon is very stout. It has attained mechanical perfection by using all possible architectural principles to get maximum strength and rigidity.
• Many bones are either rod-like or ‘T’-like.
• The sternum with keel is a typical ‘T’-like bone.
• Besides the typical long bones, the coracoid is a stout rod-like bone to withstand the pressure of the flight muscles.
• The long bones are pneumatic (Fig. 9.35) and are provided with a secondary plastering to make them rigid.
• The skull bones are thin and paper-like. These bones are firmly fused with each other. The posterior portion of the skull is spongy.
• Development of air-sacs makes the body light, because these sacs contain warm air to lighten the specific gravity of the body.
• Although the vertebral column is well- differentiated, the vertebrae particularly towards the posterior region) show the tendency of fusion. The synsacrum and pygostyle are typical examples of such fusion of posterior vertebrae.
• Absence of gall-bladder, urinary bladder, right ovary and oviduct (in female) minimise the body weight to some extent.
• Teeth are sacrificed in the modern birds. Jaw muscles and jaw bones are not too heavy as compared with the muscles and bones of reptiles and mammals.
• Oviparous rather than viviparous.

### Extra energy and power

• The secret of obtaining great and sustained power lies in the ability to convert chemical energy into mechanical motion. This is done through rapid and complete combustion of fuel. In living organism, this energy is liberated by the respiratory system.
• The lungs are proportionately smaller, but the efficiency is increased by the development of air-sacs that are developed from the lungs.
• The exchange of gases is very perfect in birds and the air-sacs help to convey oxygen directly to many tissues.
• The body temperature is higher which enhances the rate of combustion.
• The non-conducting coat of feathers prevents loss of surface heat.
• Crude power in the form of stored food-grains is kept ready inside the crop.
• The circulatory system as a whole is very efficient with a large and powerful heart.
• Rapid and high pressure circulation.
• High glucose content in the blood.
• A high basal metabolic rate (B. M. R.).

### Attainment of speed

• Speed is a must for aerial life. The body is fusiform and lacks any extra projection, which may offer resistance in the attainment of speed in air.
• Insertion of air-sacs in between the flight muscles like pads reduces mechanical friction and increases the mobility of muscular action.

### Maintenance of balance and steering

• An efficient equilibrating device is necessary for balanced flight. To equalize irregularities of air-pressure on two wings, pigeon can decrease the surface area of the wing by partial flexing.
• The air-sacs are so arranged on the two sides of the body that a proper centre of gravity is easily restored by shifting the contained air from one side of the body to the other.
• Most of the heavier visceral organs are lodged in the pelvis to maintain a proper centre of gravity.
• The tail is provided with rectrices and acts as a steering organ during flight.
• The brain is well-developed with a special development of the cerebellum.
• The optic lobes are well-formed.
• The organs for vision are highly specialised to give acute power of vision.

To have a wide visual field, the head can be rotated through an angle of 180° due to the possession of heterocoelous neck vertebrae. The jugular veins are long with an anterior jugular anastomosis.