Flight is a (if not the) major adaptation of birds; and birds have mastered many different forms of flight.  It is a complex process, requiring constant adjustments of the body and feathers and a complex control system in the brain and nervous system.  Flight uses a lot of energy - but is the most efficient means of locomotion, using less energy per unit of distance than walking, running or swimming.  The energy cost of flight is the force behind many adaptations of bird anatomy.



There are four main forces that come into play in terms of flight:

  • Lift - the upward force of air pressure that counteracts gravity.
  • Gravity- the Earth's downward pull, felt as the bird's weight.
  • Thrust-  the forward force that counteracts drag.
  • Drag - the slowing (backward) force created by friction and turbulence.

When these four forces are balanced, a bird flies at a constant speed and direction.

A bird's wing is an airfoil (a structure with a shape that generates lift), much like an airplane's wing.  However, the plane's wing can only generate lift - thrust requires engines.  A bird's wing provides both lift and thrust.

The upper surface of the bird's wing has a stronger curve than the lower surface.  This means that air moving over the upper surface has farther to go than air moving under the lower surface.  This causes the air moving over the wing to travel faster than the air going under it.  According to Bernoulli's principle, faster air has lower pressure.  This means that higher air pressure under the wing will generate lift.

The amount of lift generated is partly determined by the angle of attack.  This is the angle between the direction of the air stream and a line connecting the leading and trailing edges of the wing.  In order to fly, a bird must be able to generate at least enough lift to balance its weight.

The amount of lift increases with both airspeed and the amount of air deflected (wing area).  An albatross (a large sea bird) can stretch out its wings and be lifted by a strong enough wind.  If the wind is not strong enough, the bird jumps off a height and the airspeed from its fall creates enough lift for flight.  Birds may launch themselves from trees or cliffs to attain the necessary airspeed, or may take a running start (loons).

Increasing the angle of attach increases the amount of lift generated- up to a certain point.  If the angle is too great, air will not flow smoothly over the wing and turbulence (drag) will increase.  This creates a stall (loss of lift).  Birds purposely stall when they want to land.

If you look at a soaring bird's wings, you will notice that the primaries spread out like fingertips.  The gaps between these feathers are called slots, and they allow the primaries to act like miniature wings: reducing drag at the wing tips and increasing lift.  Some slots, like that formed by the alula, can help control the flow of air to produce lift at slow speeds or high angles of attack (to prevent stalling).  This is especially useful during landing and take-off.

There are two main types of drag, in regards to a bird's wing.

  1. Profile Drag- Drag created by the friction between the wing and the air.  The thin leading edge of the wing works to minimize this drag.  Profile drag increases at higher airspeeds.
  2. Induced Drag- Drag created by turbulence as the air flows over the wing.  Induced drag decreases at higher airspeeds.

Drag is over come by thrust.  The two components of thrust are profilepower (the amount of power needed to overcome profile drag) and induced power(the amount of power needed to overcome induced drag).  Since induced power decreases as profile power increases, moderate flight speeds are the most energy efficient.  (Induced drag is great at low speeds, profile drag is great at high speeds.)

Minimum power speedis the flight speed at which the least amount of fuel is required to maintain flight.  This is typically between 30 and 60 kph.

The maximum flight rangeis the greatest distance a bird can fly on a given amount of fuel.  To achieve this, birds fly slightly faster than their minimum power speed - because the slightly higher speed gives them greater momentum, which carry them further.  This speed is called the maximum range velocity.


Flying in Formation

When geese fly in a "V" formation, they save energy - possibly up to 50%.  The wing tip of each goose in formation lines up directly behind the wingtip of the goose ahead of it.  This cancels some of the turbulence and saves energy.


Types of Flight

Soaring/Gliding Flight

Soaring is the simplest form of flight, and uses the least energy.  Since the bird doesn't flap its wings while soaring, it gradually loses altitude.  This sinking is a result of drag and gravity.  To counter the sinking, soaring birds (like vultures and hawks) take advantage of rising air.

Thermal soaring involves the use of warm, rising columns of air.  The bird continues to sink through this air, but the air is rising faster than the bird sinks - so there is an overall gain in altitude.

Slope soaring involves using air that is deflected upwards when it meets a ridge or an ocean wave.  Hawks soar along ridges, while seabirds can soar along the windward side of ocean waves.

Flapping Flight

Unlike soaring, flapping actively generates thrust on the downstroke of each wingbeat.  Since each primary feather can act as an airfoil, changing the angle of attack (on the downstroke) converts some of the lift into forward thrust.

Hummingbirds are able to fly forwards and backwards and to hover by changing two factors: the angle of attack and the direction of the wingbeat. 

Changes in wing position during flight control body orientation and airspeed.  The ability to move each wing independent of the other gives the bird the ability to steer.  Moving one wing slightly farther back than the other gives a curve in the flight path.  Moving one wing forward and one back allows the bird to make sharp turns.

Most small birds use the downstroke as the power stroke.  The upstroke is the recovery stroke, and produces little lift.  (Primaries are opened up on the recovery stroke to reduce air resistance.)  Powered downstrokes and simple recover strokes create vortex rings - doughnut-shaped rings of turbulent air.  These tend to occur at slower speeds.

Faster flight tends to convert the vortex rings into continuous vortex streams.  In either case, vortices have to be controlled so that they don't backlash (if the bird flaps to slow) ot interfere with the turbulence of the next wingbeat (if the bird flaps too fast).  As a result, birds increase flight speed by increasing the size and orientation of the wingbeats, not by beating their wings faster.

Aerial birds (swifts and hummingbirds) also use leading-edge vortices.  The sharp edges of the wing creates a vortex at the wing tip.  This vortex can pull the wing (and, thus, the bird) upwards and/or forwards, depending on how the bird channels it.

Landing on a perch or branch requires precise control.  Bats, flying squirrels, and gliding lizards touch down with their front limbs first, then bring the back legs to the landing surface.  Birds are the only vertebrates that stall directly over the landing site to land feet-first.

Tail feathers help control body position and stability during flight, aid steering and braking, and can add lift by improving air flow and reducing turbulence.

A cool video clip of an eagle owl flying towards the camera at 1,000 frames per second - click here.

Intermittent Flight

Many birds alternate between flapping and nonflapping as they fly.  There are two basic strategies:

  1. Flap glidingis when the bird (usually large birds like Cooper's Hawks) flap a few times, then glide.  This reduces energy costs at lower speeds.  Wings are extended in gliding.
  2. Flap boundingis when the bird (usually smaller birds like finches and woodpeckers) goes through patterns of flapping and nonflapping in sequence.  This reduces energy costs at higher speeds.  wings are folded or flexed during bounding.

Some birds, like starlings, use both flap gliding and flap bounding depending on the speeds they are flying at.


Wing Size and Shape

The size and shape of a bird's wing determine...

  • flight speeds
  • gliding ability
  • aerial agility (maneuverability)
  • energy consumption

Wing loading is the relationship between the birds body mass and their total wing area.  In other words, how much mass must be carried per unit of area of the bird's wing.  Birds with small wings compared to their body mass have a high wing loading.  Large wings compared to body mass means small wing loading - because the mass is spread out over the wing.

Loons have high wing loading, so they have to run over the water to generate enough lift for flight.  Hawks have low wing loading, so they can soar for long distances.

Aspect ratiois the ratio of lift to drag on a bird's wing.  Aspect ratio is strongly influenced by wing dimensions.

Long, narrow wings...

  • Have a high aspect ratio.
  • Produce more lift.  The leading edge generates the most lift, while the back half generates the least.
  • Reduce drag.  The most turbulence (induced drag) is generated at the wingtips.  Long wings separate wingtips, decreasing turbulence.
  • Are well suited for soaring and high speeds in open country, but sacrifice maneuverability.

Shorter, rounder wings...

  • Have a low aspect ratio.
  • Are well suited for maneuvering flight and explosive takeoffs (which require fast wingbeats).
  • Are an adaptation of species which live in areas of dense vegetation.

Bird Skeletons

Bird skeletons show the following adaptations for flight...

  • Bones are lightweight and filled with air.  Some have internal struts for support.
  • Instead of heavy jaws full of teeth, birds have lightweight, toothless bills.
  • Ribs are hinged and have ucinate processes - bony struts that overlap adjacent ribs for support.
  • The sternum (breastbone) has a large keel to anchor the major flight muscles.  (The size of the keel determines the flying ability.)
  • The coracoid (part of shoulder), furcula (wishbone), and scapula (shoulder blades) bones act together to keep the chest from getting crushed by wing strokes.
  • Large surfaces at wing joints allow wings to be folded up against the body, and to change positions/angles during different stages of flight.
  • Wing joints are made to withstand the wrenching forces of wing strokes during flight.
  • Fused hand and finger bones provide strength and rigidity.
  • The alula is attached to the thumb, and can be moved independent of the rest of the wing tip.

Muscle-Fiber Metabolism

Bird muscle is primarily made up of red muscle fiber and white muscle fiber, although they also have other, intermediate types of fibers as well.

Red muscle fiber provides sustained flight power.  These fibers are very efficient at aerobic respiration.  In fact, the flight muscles of small birds and bats have the highest capacity for aerobic respiration of all vertebrates.

White muscle fiber provides short bursts of power for short, explosive flights and fast turns/evasive maneuvers.  These fibers use anaerobic respiration.  As a result, lactic acid builds up and the muscle fatigues quickly.  The light breast meat on domestic fowl and grouse is mainly white muscle fiber, used for short, explosive flights. 


Flightless Birds

In addition to the ratites, there are many species of birds that do not fly: species of grebes, pigeons, parrots, penguins, waterfowl, cormorants, auks and rails.  These often inhabit isolated islands with few or no predators.  Without the need to fly, they gradually lose the ability.  The large, keeled sternum and large pectoralis muscles use a lot of resources to develop and maintain.

Rails delay the development of the keel and other flight apparatus until they are fully grown.  This gives more time to develop these structures, but leave the young rails flightless.

Foot-propelled diving birds, such as loons, grebes and cormorants, may see a reduction in wing size.  Some grebes are completely flightless as a result.

Wing-propelled diving birds, such as penguins and auks, modify their wings for efficient flight underwater - and lose the ability to fly through the air.  At some point, there is/was an intermediate form using wings for both flight and diving.


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