Birds: forever fascinating

What makes them so special? What makes them different from the rest of the living creatures? What makes us say, “as free as a bird”?

One evening, just as the sun set amid radiant clouds, there came a large flock of beautiful birds out of the bushes. They were swans, and they curved their graceful necks, while their soft plumage shown with dazzling whiteness. They uttered a singular cry, as they spread their glorious wings and flew away from those cold regions to warmer countries across the sea. As they mounted higher and higher in the air...”

 

Birds……….

What makes them so special? What makes them different from the rest of the living creatures? What makes us say, “as free as a bird”? It is their ability to fly, among other things.

 

In the course of evolution, these flying creatures acquired the feathers and bones adopted for flight. ‘Flying’ is defined as moving through the air with wings. But it is not simply flapping the wings up and down. To be able to fly, the wings should be able to generate lift and thrust.

 

When the birds ‘fly’, the air strikes the wing at an angle. The wing pushes the air downwards, creating ‘lift’. When a bird moves through the air, the air splits its path in front of the wing and meets up again at the back. Due to the curved shape of the wing, the bottom of the wing pushes the air, and creates an area of high pressure below and ahead of the wing. The fast-moving air creates an area of low pressure above and behind the wind. The higher pressure under the wing lifts the bird upwards, while the flapping of wings act as a propeller to thrust the bird forward. The wing is also deflected upward as a reaction to the deflected air underneath it.

Position of wings related to air flow                                    Position of wing and angle of attack


Pressure exerted on the wings


To create a lift, enough airflow should be generated under the wing. That is why the takeoff can be one of the challenging parts of a flight. For small birds, a small jump is enough to generate this airflow. Large birds need to take a run to generate this airflow. Often, large birds takeoff by facing the wind, and if possible, perching on a high branch or cliff, so that all they have to do is to drop off. Birds like herons, egrets and hawks jump upwards and push their wings downwards to create the lift. Birds like ducks, pheasants, and grouse don’t have strong legs. Therefore, they face the wind and run, flapping their wings, over a stretch of land or water, until they gain enough speed to liftoff. A hummingbird takes off from its perch by lifting its two feet and flapping their wings.

Hummingbird takes off from its perch


Landing on water

Birds flap their wings to generate both lift and thrust. But if they stop flapping and keep their wings stretched out, the wings actively produce only lift. Thrust is produced by gravity as the bird descends. When this happens, the birds glide. Gliders, like vultures, albatrosses, pelicans and storks, have large wings that generate a lot of lift without producing much drag.

 

To balance the gravity in the vertical direction, and the drag in the horizontal direction, a flying bird should produce both lift and thrust, respectively. In gliding, there is no active thrust production. So the birds resort to gravity to overcome the drag forces. Gliding always results in the bird loosing height. In order to maintain or gain height, the birds soar. Soaring is a special kind of gliding where the birds fly in to a rising air current. As the air is rising, the birds can maintain their height relative to the ground without flapping their wings. (see picture below)


soaring flight

Flapping flight is more complicated due to the structural movement and the resulting unsteady fluid dynamics. In flapping, the wings not only move forward relative to the air, but also flap up and down, bend, twist and sweep. In addition to generating lift, flapping also creates thrust to counteract its weight and drag. Flapping involves two stages: the power stroke, or the downstroke and the recovery stroke, or the upstroke. The angle of attack of the wings is increased during downstroke and decreased during upstroke. During upstroke, the angle of attack is reduced to zero to meet least resistance. The bird also folds it wings partially to minimize wingspan, thereby minimizing drag.

As the wings flap, there is very little vertical movement near the body. But the wing tips move up and down very steeply. So in order to maintain the correct angle of attack in each part of the wing, the wings must twist. As the wing twists, the outer part of the wing moves down, and the lift force on the outer part of the wing is angled forward. But as only the wings are moving downwards, and not the whole bird, the bird is able to maintain its height of flight. This is how birds can generate a large amount of propelling force without losing altitude. When birds flap, the air is not only deflected downwards, but also backwards, like it would be by a propeller.

Only the inner part of the wing produces lift during upstroke. So, the lift produced during upstroke is less than the lift produced in downstroke. As a result, there is a little up and down bobbing movement as the bird flies.


Force produced by the wings

Some birds hover, they remain in one place in the air, even when their wings are flapping. Hovering is generating only lift through flapping alone rather than as a product of thrust. This demands a lot of energy. The ability of a bird to hover depends on its size, moment of inertia of the wing, wing shape and the freedom of movement of the wings. Due to these limiting factors, only small birds and insects hover. Larger birds also hover artificially by flying into a headwind and using the thrust to fly slowly but remain stationary to the ground or water.  This is called wind hovering.

 

There are two kinds of hovering: symmetric hovering and asymmetric hovering.  Symmetric hovering is also called “insect stroke”, and is performed by humming birds and insects that hover with fully extended wings during the entire wing-beat cycle. Lift is produced during the entire wing stroke, except at reversal points. Larger birds, which cannot rotate their wings between the forward and backward stroke perform asymmetric hovering. The wings are extended during downstroke to provide lift, and are flexed back during upstroke to reduce drag. This asymmetric hovering is known as “avian stroke”.

The humming birds are among the most accomplished hovering birds. The wings of a humming bird remain straight throughout the entire stroke, and the stroke is in the shape of a lying figure eight.


Position of wings of hummingbirds in a stroke

Landing is another challenge for larger birds, with higher airspeed. Different species of birds have different ways for dealing with it. Birds like mallards, geese, and waterfowl species prefer to land on water. The swans can land only in water. These birds circle the wind to maintain their lift as they slow down to land. A cluster of feathers called the alula forces air over the top of the wing and helps maintain the lift as the birds slow down to land. Some birds aim for a point below the intended landing spot and pulling up beforehand. Other birds like egrets and songbirds perch on the trees.

 

Landing of large bird on water

Egrets perching on trees


Another fascinating thing about birds is the formation in which they fly. These flight formations are known as echelons and can be observed in migratory birds. The most common formation is the V shaped formation, known as the Skein, which is used by birds like cranes, geese and ducks.

There are two main advantages in this formation. The first, most obvious advantage comes from each bird flying slightly behind the other. This enables every bird in the formation to see the leading bird, the bird in front and the bird slightly to the side. This helps them to coordinate their flight paths, and not crash into each other, or get lost.  The other advantage, which is not easily seen, is that this formation allows the bird to expand less energy in order to fly, thereby increasing their flight efficiency. Each bird in the formation flies in a slightly lower altitude than the bird in front. This allows the bird to take advantage of the upward vortex force generated by the wings of the bird in front. This reduces the air resistance that the birds have to endure. In a skein formation, the bird in the front has to encounter the largest air resistance. When this leading bird tires, it falls back to the end of one of the arms of the V formation, and one of the nearest two birds take over. This rotation allows the lock to fly for a much longer time than would otherwise be possible.


V formation of birds when flying as groups

Looking up at the birds, man felt the desire to fly. Even before the time of the Wright brothers, many efforts have been made to find a way for man to fly up in the air. Leonardo Da Vinci even made a sketch of a wing-flapping contraption called the ornithopter. However, these efforts weren’t successful because they didn’t take into consideration the many complicated dynamics involved in the flight of birds. Today, even though many airplanes are designed to fly a great distance at a great speed, even though the flight mechanism of an airplane is similar to that of a bird, no man-made aircraft would ever be a match for the natural power, grace, and efficiency of birds.


I looked in my heart while the wild swans went over.
And what did I see I had not seen before?
Only a question less or a question more;
Nothing to match the flight of wild birds flying.
Tiresome heart, forever living and dying,
House without air, I leave you and lock your door.
Wild swans, come over the town, come over
The town again, trailing your legs and crying!

Edna St. Vincent Millay


Click here to read about the Locomotion of birds

Click here to read about vocalisations of birds

Click here to read about feeding behaviour of birds. 

Click here to read about the breeding of birds

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Be a Genius: Your source for Science and Technology Facts: Birds: forever fascinating
Birds: forever fascinating
What makes them so special? What makes them different from the rest of the living creatures? What makes us say, “as free as a bird”?
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Be a Genius: Your source for Science and Technology Facts
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