Why do bird wings flap?

There are two rea­sons why birds flap their wings. Pri­mar­ily birds flap their wings to pull them­selves for­ward, sec­on­dar­ily to lift them­selves up.

Bird flight looks effort­lessly sim­ple but actu­ally is a com­plex­ity of inter­nal actions and reac­tions and out­side forces. The most impor­tant fac­tor about bird flight is that all birds are so light that in some sit­u­a­tions they are lifted up by the air they float in.

Bird                     Wing­spread      Weight
White-tailed Kite         39″             12 ounces
West­ern Sand­piper     14″               1 ounce
West­ern Gull                58″            34 ounces
Mourn­ing Dove            18″               4.2 ounces
Mal­lard                           35″             36 ounces
Barn Swal­low                15″                 .7 ounce
House Finch                     9.5″              .75 ounce
Sand­hill Crane             77″            10 lbs 7 ounces

Of course there are other fac­tors involved, the width of the wing and its shape, for exam­ple. But you can see from this chart why birds with short wings and heavy bod­ies have to beat their wings faster than those with longer wings and lighter bodies.

Ther­mals rise at up to four feet per sec­ond, the sink rate for glid­ing birds is between one and three feet per sec­ond. Rid­ing ther­mals helps birds fly for long peri­ods with­out any effort. With the air help­ing they need to pro­duce very lit­tle ‘lift’ from their wings . We need to keep that in mind as we begin to fathom how birds fly through the air.

One source of bird lift is by flap­ping their wings; when a bird presses its wings down on the air beneath it, the bird’s body is pushed up, that is – lifted up. (You can watch this in many of the PBS and other pro­grams show­ing films of for­ma­tions of Swans or Cranes in flight. Watch as the wings move down and you will see that the bod­ies rise.)

The rea­son the birds’ bod­ies aren’t pushed down on the upstroke that fol­lows is that the flight feath­ers ‘weath­er­vane’ and open slightly – like Venet­ian blinds –  on the upstroke and some air slips between the feath­ers. (More on this in another blog.) While the birds’ bod­ies are pushed down a bit, it is less then what they gained on the pre­vi­ous upstroke.

Another, lesser, source is the air­foil of the wing itself, specif­i­cally the upper air­foil sur­face of the wing. The pres­sure above the wing is reduced because the air pass­ing over the wing takes longer to reach the trail­ing edge than that flow­ing under­neath it and the wing (and bird) rises as a result.

Airfoil 1 Why do bird wings flap?

The mov­ing air­foil pro­duces lift

Unlike air­planes that get all their lift from the air­foils of their wings, a bird’s air­foil pro­vides only a por­tion of the lift needed to keep it aloft. For many birds it takes only a slight updraft – less than the hot air ris­ing from a chim­ney – to rise upwards with­out a flap. With their broad wingspan and light weight Vul­tures can be seen cir­cling for hours, coast­ing along with almost no effort.
Hawks and Kites as well can drift up to great heights on updrafts so faint that we humans would not feel them.

Updraft Why do bird wings flap?

Updraft form­ing a cloud

Some seabirds fly for miles with­out flap­ping, lit­er­ally ‘surf­ing’ long cor­ri­dors of updrafts. But that will be the topic for another blog.

How does the bird’s wing develop thrust?
The expla­na­tion found in most birds on books is that the wing is tilted down so the some of the lift, now angled for­ward, will pull the bird for­ward. This is sim­ply not true, for many reasons.

Airfoil 2 Why do bird wings flap?

Tilted air­foil wrongly thought to pro­vide thrust

First, since a bird’s air­foil pro­duces very lit­tle lift, tilt­ing it for­ward could not pos­si­bly pull a bird for­ward at 30 to 40 miles an hour which is typ­i­cal for birds.
Sec­ond, this is a log­i­cal impos­si­bil­ity. If for­ward thrust is depen­dent on air flow­ing across the wing, but air doesn’t flow across the wing until the bird is mov­ing, it can’t get started. (Cir­cu­lar rea­son­ing: the action pro­duces a reac­tion which pro­duces the orig­i­nal action.) This is why a short-tailed cat can’t catch its tail no mat­ter how fast it runs.)
Third, birds in flight do not tilt their wings down­ward. Just look at them in flight.

Inter­est­ingly enough, the air­foil above, the one that is shown in all the bird books you will find, is not the shape of any bird’s air­foil I know of. Actual bird wings have a pro­nounced under cam­ber or cur­va­ture; it is their under cam­ber that pro­pels them (thrusts them) forward.

Airfoil3 Why do bird wings flap?

Rep­re­sen­ta­tive air­foil of a bird’s wing show­ing the under cur­va­ture. (Undercamber)

Here is how that works. As the wing presses down, the air is slightly com­pressed and has to go some­where. Since the down­ward cur­va­ture at the lead­ing edge of the wing pre­vents the dis­placed the air from mov­ing out the front, it goes the only direc­tion it can – out the rear.
“Ah ha!” says Isaac New­ton, “For every action there is an equal and oppo­site reac­tion so the wing will be pushed for­ward.” Well yes, Mr. New­ton, that’s exactly how jet engines work. The rush­ing air out the tail of the engine pushes the engine — and the attached air­plane forward.

Airfoil4 Why do bird wings flap?

Newton’s Third Law of Physics at work

In a sense, the birds wing acts much like a pro­peller on an air­plane or motor­boat does. As it bites into the air (or water) the cur­va­ture pulls the pro­peller and engine for­ward.
The under cam­ber (cur­va­ture) of a bird’s wing is quite pro­nounced as seen here and can push out a lot of air:

Avocet Power Stroke  5178458 Why do bird wings flap?This beau­ti­ful Amer­i­can Avo­cet has just lifted off the ground. He is half way into the first power stroke. The under cam­ber is clearly shown as is the resem­blance to a curved pro­peller.
Under cam­ber is deter­mined by the mus­cu­la­ture and bones of the wing, which does not change dur­ing flight. With every power stroke the bird is pulled for­ward. Since birds are very light and their wings very strong they can fly really fast. Most can eas­ily out fly a human runner.

Dip fishing A233728 02T Why do bird wings flap?This first year Snowy Egret land­ing on the rocks clearly dis­plays the pow­er­ful under cam­ber which runs the full width of the wings. It is easy to deduce the power of these seem­ingly del­i­cate wings.

Bird flight is an incred­i­ble process, one that amazes me the more that I learn about it.

Note: The term ‘Power Stroke’ I have used here is the term to indi­cate the down stroke of a wing flap, the other three seg­ments of the wing flap are Upstroke, Upper  Tran­si­tion, and Lower Tran­si­tion. These are more fully described in my forth­com­ing book: Avia­nau­tics, the Art and Sci­ence and of Flap­ping Flight.

Your com­ments & ques­tions much appreciated

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3 Responses to Why do bird wings flap?

  1. Dawa mayalmu says:

    Thanks I got some idea from this page.

  2. Gerald Anderson says:

    As a retired aeronautical/meteorological sci­en­tist, I really enjoyed your blog. I once heard a lec­ture by a noted British sci­en­tist who said he was moti­vated in his career by try­ing to prove how bum­ble­bees could fly in spite of analy­ses prov­ing they couldn’t. He often had high hopes that were dashed by evi­dence. Late in his career he was most hope­ful of suc­cess, when he was totally destroyed upon find­ing that they not only flew, but made love while doing so! (do bees actu­ally “make love?”)

    Thanks for your texts and pictures.

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