The simplest form of flight, gliding, allows some birds to float, seemingly effortlessly, for hours in response to the gentlest of sea breezes, while soaring birds can use warm currents in the air to rise to great heights and conserve energy while covering vast distances.
Even observing smaller birds, such as sparrows or pigeons in flight, makes us wonder how they achieve lift-off and how they remain in the air. Read on as we study the science behind bird flight and attempt to answer the question “How do birds fly?”
An aerodynamic and streamlined shape and the covering of feathers on its wings and body – combined with the physical forces of lift, drag, and thrust – are the crucial adaptations that allow a bird to take off, sustain flight, glide, and ultimately land safely when they need to return to land.
Four physical forces are necessary for effective bird flight: lift, drag, weight and thrust. The relevance of each force can be understood as follows:
We’ll be taking a look at the roles played by feathers, tails and wing shape in helping birds become, and remain, airborne, and discussing when birds develop the ability to fly, and why some birds will never make it off the ground. So keep reading to learn more about how birds fly.
Common Swift in flight
Wing feathers and tail feathers support a bird in every aspect of flight, allowing a bird to become and remain airborne, to balance, control direction and speed, and allow for braking and landing when needed.
A covering of feathers streamlines a bird for efficient flight. Wing feathers are spread and arranged for flapping (to gain lift), while the most subtle changes to positioning of a bird’s tail feathers allow it to change direction and, when needed, slow down and ultimately brake.
Birds’ wings are thicker at the front than at the back. The wing surface is more curved across the top than underneath, which causes air to move more quickly over the longer surface of the upper wing than the shorter surface on the underside of the wing.
This difference in airspeed between the wing’s surfaces causes lower air pressure on top and a more intense pressure below. This generates the lift that raises the bird’s wings, propelling it higher into the air.
When a bird flaps its wings, the feathers collectively twist and create more thrust. This propels the bird forward and higher up into the air, pushing the bird through the atmosphere This pushes the bird through the air, similar to how a swimmer will push through the water with every stroke as they change the position of their shoulders, hands, and arms.
To climb higher into the air, a bird tilts up the edge of its wings, increasing the angle between the wings and the airstream. The greater the angle, the higher the risk of encountering turbulence and stalling mid-flight. In turbulent air, a bird’s alula tuft of wing feathers spreads forward to form a slot in the front of the wing, keeping the airstream over the wing smooth.
A male Baltimore Oriole in flight
Wing shape and wing movement both play a vital role in helping a bird to fly. Three of the more common wing shapes, and how they affect the way a bird flies, are as follows:
Sparrows have elliptical wings
Common Buzzards have broad-soaring wings
Birds steer by twisting and turning their bodies during flight and adjusting their wings as needed to manoeuvre. Their tail is used as a kind of rudder, and can control steering, from subtle changes of direction from left to right, to sharper and more drastic swerves and turns.
To become airborne, a bird needs to move against an airstream. This means it must either take off into the wind, or create an airstream of its own, taking a short run and then jumping into the air, flapping its wings backwards and forwards (rather than up and down).
Some of the birds that are the strongest fliers are in fact notoriously weak at taking off.
One such example is the swift, which has underdeveloped feet and is rendered almost helpless if it becomes grounded. Instead, swifts become airborne after a rare rest by dropping into the air from a ledge.
Eurasian Roller taking off for flight
Taking off from water follows the same principles as take-offs that are initiated on land. Ducks and geese aim to get air moving fast enough to produce lift, by flapping their wings as they run at high speed across the surface of a lake or the sea. Once they have gained enough momentum, they are carried up into the airstream.
Yellow-billed duck taking off from water
Before landing – whether on the ground, on a bird feeder or on a branch – birds need to reduce their speed to avoid injury or damage to their feathers. One way they achieve this is by spreading their wings and tail as wide as possible and bringing their body into a near-vertical position. They then flap against the direction of flight and use their feet to absorb the shock of landing.
A bald eagle coming in to land on a perch
Landing on water is an important skill for ducks and geese to master, and for such species is far preferable than coming into land on solid ground. Water-birds use their large webbed feet to make contact with the surface of the water, braking by back-pedaling with their wings to slow themselves down.
A Dalmatian pelican landing in the water
There are around 60 species of flightless birds in the world, including penguins, ratites (emus, ostriches, kiwis, cassowaries and rheas), and some rails. These have typically evolved on islands or other remote locations where, over time, the species have adapted to a lack of native predators, which meant that the ability to fly was not required.
No bird species is born with the ability to fly, not even birds that are already fairly well developed on hatching. It takes a certain amount of time for flight feathers to develop, and it takes on average between 10 and 21 days after hatching for birds to accomplish their first attempt at flight.
Typically smaller birds master the art of flight earlier than larger ones. Bald eagles are not ready to take to the skies confidently until they are around 72 days old. One of the youngest birds to learn to fly is the tiny Chipping sparrow, between 8 and 12 days after hatching.
Juvenile Herring Gull in flight
A bird’s tail acts in a similar way to a rudder, controlling steering during flight and moving to enable braking.
As the surface of the Earth heats up unevenly, currents of warm air known as thermals rise into the atmosphere. Many large birds take advantage of these thermals, gliding on the up draught to gain altitude without having to expend a significant amount of their own energy to reach great heights.
Birds that use thermals to soar in this way, maintaining flight without flapping their wings, include buzzards, eagles, falcons, gulls, kites, herons, and albatrosses.
Turkey Vulture riding an autumn thermal
All birds, even flightless birds, have two wings. A broken wing can be a devastating injury for a bird to sustain, and survival rates for birds in such scenarios are not high.
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