Flight in birds – Support and Locomotion (Senior 4 Biology) | Ganzaa

Support and Locomotion

Lessons

Senior 4 · Biology

Support and Locomotion

5 lessons

Lesson 4 of 5

Flight in birds

25 min

A bird that stands still in the sky

Over a lake, a black-and-white pied kingfisher beats its wings furiously while its head stays perfectly still a few metres above the water. Then it folds its wings and drops like a stone onto a fish. Hovering is one of the hardest things a flier can do: with almost no support from the thin air, every newton of its weight must be held up by the air its wings push down. Birds manage it because nearly every part of their bodies, from their bones to their lungs, is built for flight.

Definition

Lift: the upward force on a wing, at right angles to the airflow, produced as the wing deflects air downwards. Thrust: the forward force. Drag: the backward force of air resistance.

Aerofoil: a shape, rounded at the front, curved on top and thin at the back, that produces lift efficiently when air flows past it. Angle of attack: the angle between the wing and the oncoming air.

Keel: a deep ridge on the breastbone (sternum) of flying birds, where the flight muscles attach.

Wing loading: the bird's weight divided by its wing area, in $\mathrm{N\,m^{-2}}$ . Glide ratio: the distance travelled forward for each metre of height lost when gliding.

A body built for flight

Feature	How it helps flight
Feathers: flight feathers on wings and tail; contour feathers on the body	Light, strong and flexible surfaces. In each vane, side branches (barbs) carry tiny barbules with hooks that zip them together into a smooth, airtight sheet. Primary feathers at the wing tip give thrust; secondary feathers nearer the body give lift. Contour feathers streamline the body.
Hollow (pneumatic) bones with internal struts, some linked to air sacs	Strong but light, like a scaffolding tube
Fused bones: lower backbone fused with the pelvis, fused collarbones (wishbone), short tail bone	A rigid frame that does not flex under the pull of the flight muscles
Deep keel on the sternum	Large area for the attachment of the big flight muscles
Toothless, horny beak; no heavy jaw	Saves weight at the front of the body
Lungs with air sacs and a one-way flow of air; four-chambered heart	Rapid supply of oxygen for the very high rate of respiration flight needs
Streamlined body; legs tucked up in flight	Less drag

How a wing produces lift

A bird's wing has the cross-section of an aerofoil and meets the air at a small angle of attack (Figure 1). Air flowing round it is turned downwards as it leaves the trailing edge. To push air down, the wing must exert a downward force on it, so by Newton's third law the air pushes the wing up: lift. The same flow makes the pressure above the wing lower than the pressure below. Increasing the angle of attack increases lift, up to a point; beyond it, the airflow breaks away from the upper surface and lift suddenly falls (a stall).

Figure 1. Airflow round an aerofoil (simplified).

The flight muscles

The flight muscles can make up a far larger share of a bird's body mass than limb muscles do in most mammals. Both main muscles start on the keel (Figure 2). The large pectoralis major pulls the wing down in the power stroke, which produces lift and, through the twisting primary feathers at the tip, thrust. The smaller supracoracoideus (also called pectoralis minor) lies underneath it; its tendon runs up and over the shoulder joint like a rope over a pulley and pulls the top of the humerus, raising the wing. Keeping both heavy muscles low on the breastbone keeps the centre of gravity below the wings, which makes the bird stable.

Figure 2. The flight muscles on one side of a bird (front view, simplified).

Ways of flying

Flapping: the downstroke gives lift and thrust; on the upstroke the wing is partly folded and the feathers open to let air through, so little lift is lost.
Gliding: wings held still and outstretched; the bird slowly loses height but uses little energy.
Soaring: gliding in rising air, either warm air rising from the ground (thermals) or wind deflected upwards by hills, so the bird gains height without flapping. Vultures, eagles and storks soar with long, broad wings.
Hovering: rapid flapping while the body stays in one place, as in the kingfisher; it uses a great deal of energy, so it is done only for short periods.

Worked example 1: wing loading

Illustrative values (take $g = 9.81\,\mathrm{N\,kg^{-1}}$ ): a pigeon of mass 0.40 kg with a wing area of $0.065\,\mathrm{m^2}$ ; a duck of 1.2 kg with $0.10\,\mathrm{m^2}$ ; a soaring stork of 3.5 kg with $0.75\,\mathrm{m^2}$ . Calculate each wing loading and suggest what it means.

Pigeon: $\frac{0.40 \times 9.81}{0.065} = 60\,\mathrm{N\,m^{-2}}$ .
Duck: $\frac{1.2 \times 9.81}{0.10} = 118\,\mathrm{N\,m^{-2}}$ .
Stork: $\frac{3.5 \times 9.81}{0.75} = 46\,\mathrm{N\,m^{-2}}$ .
Each square metre of wing must hold up that many newtons. The duck's high wing loading means it must fly fast and flap hard, and needs a long run to take off; the stork's low wing loading lets it fly slowly and stay up on weak rising air, ideal for soaring.

Worked example 2: gliding and muscles

(a) A gliding bird travels 120 m forward while losing 10 m of height. Calculate its glide ratio, and how far it could glide after leaving the top of a thermal 300 m above the ground. (b) In a 400 g pigeon (illustrative), the pectoralis major has a mass of 64 g and the supracoracoideus 8 g. What percentage of body mass are the flight muscles, and what does the ratio of the two muscles suggest?

(a) Glide ratio $= \frac{120}{10} = 12$ , that is 12 : 1. From 300 m: $300 \times 12 = 3600\,\mathrm{m} = 3.6\,\mathrm{km}$ in still air.
(b) $\frac{64 + 8}{400} \times 100 = 18\,\%$ . The pectoralis is $\frac{64}{8} = 8$ times heavier: the downstroke does almost all the work of producing lift and thrust, while the upstroke only lifts a light wing back into position.

Canadian Museum of Nature: "How Do Birds Fly?". Watch how lift, thrust, gravity and drag act on a bird, and which features of its body make flight possible.

Warning

Common mistakes

Saying lift happens because "air over the top must meet the air underneath at the back at the same time". That is not true. Lift comes from the wing turning the airflow downwards, with lower pressure above and higher pressure below.
Saying the same muscle raises and lowers the wing. Muscles only pull: the pectoralis pulls the wing down, the supracoracoideus (through its pulley tendon) pulls it up.
Saying bird bones are "weak because they are hollow". Struts inside make them strong for their weight.
Forgetting that gliding always loses height in still air; only rising air allows soaring.

Tip

Memory hook. For flight adaptations, think "Light, Lift, Lungs, Lock": light bones and beak; lift from aerofoil wings and feathers; lungs with air sacs for oxygen; and a locked, rigid frame of fused bones with a keel for the muscles.

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Practice: flight in birds

List four features of a bird's skeleton that adapt it for flight, and explain one.
Explain how an aerofoil-shaped wing produces lift.
Describe how the pectoralis major and supracoracoideus move the wing, and explain why both can be attached to the keel.
A bird of mass 2.0 kg has a wing area of $0.50\,\mathrm{m^2}$ . Calculate its wing loading ( $g = 9.81\,\mathrm{N\,kg^{-1}}$ ).
A vulture glides 900 m while losing 50 m of height. Calculate its glide ratio.
Explain why hovering birds can hover only for short periods.
Suggest why large soaring birds take off more easily from a cliff or into the wind than from flat ground on a still day.

Show answersHide answers

Any four: hollow bones with struts; fused vertebrae and pelvis; fused collarbones (wishbone); deep keel on the sternum; short tail bone; toothless beak. For example, the keel gives a large area for the attachment of the big flight muscles.
The wing meets the air at a small angle of attack; air flowing over and under it is deflected downwards; pushing air down produces an equal upward push on the wing (Newton's third law), and the pressure below the wing becomes higher than above it: lift.
The pectoralis major runs from the keel to the underside of the humerus and pulls the wing down. The supracoracoideus runs from the keel, but its tendon passes over the shoulder like a rope over a pulley to the top of the humerus, so when it contracts it pulls the wing up. The pulley changes the direction of pull, so both muscles can sit low on the keel, keeping the centre of gravity low.
Weight $= 2.0 \times 9.81 = 19.6\,\mathrm{N}$ ; wing loading $= \frac{19.6}{0.50} = 39\,\mathrm{N\,m^{-2}}$ .
$\frac{900}{50} = 18$ , that is 18 : 1.
Hovering needs very rapid flapping to push enough air down to support the whole weight without any forward speed; this uses ATP very fast, more than the bird can keep up for long, so the muscles soon tire.
Lift depends on the speed of the air over the wings. Jumping from a height or facing into the wind gives airflow over the wings at once; on flat ground on a still day a heavy bird with low muscle power must run and flap hard to reach enough airspeed.

Quick check

Match each feature of a bird to how it helps flight.

Tap one on the left, then its partner on the right.

Key points

Birds are adapted for flight by feathers (vanes locked by barbules), hollow strutted bones, fused bones, a keel, a light beak and efficient lungs with air sacs.
Aerofoil wings at a small angle of attack deflect air downwards, producing lift; too large an angle causes a stall.
The pectoralis major (downstroke) and supracoracoideus (upstroke, via a pulley tendon) both attach to the keel.
Birds flap, glide, soar in rising air and hover; wing loading and wing shape suit each lifestyle.

Sources and credits

5.9.6: Birds, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for feathers, pneumatic bones, the keel and flight muscles, and air sacs. Adapted by Ganzaa.
"Bird flight", Wikipedia contributors, licensed under CC BY-SA 4.0. Used for lift, gliding, soaring, hovering and wing loading. Adapted by Ganzaa.
Figures 1 and 2, the table, worked examples (all masses and areas are illustrative), practice and quick check: written by Ganzaa.
Photos: Noukayami, CC BY-SA 4.0; Prasan Shrestha, CC BY-SA 4.0; Gribkov, CC BY 4.0; all via Wikimedia Commons.
Video: "How Do Birds Fly?", Canadian Museum of Nature, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A bird that stands still in the sky

2A body built for flight

3How a wing produces lift

4The flight muscles

5Ways of flying

A bird that stands still in the sky

A body built for flight

How a wing produces lift

The flight muscles

Ways of flying