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.

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 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.

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.

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.

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:

Avocect taking off, first power strokeThis 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.

Snowy Egret, landingThis 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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8 Responses to Why do bird wings flap?

  1. John Whitmore says:

    As a pilot and avid bird watch­er, I now under­stand how birds fly and glide. Thank you for clear­ing up some mis­stat­ed facts.
    John W

  2. Donald V. Cook says:

    Hel­lo Richard-
    Very much enjoy­ing your bird pho­tos. Would like to obtain a copy of your Avia­nau­tics, the Art and Sci­ence and of Flap­ping Flight. I am inter­est­ed in the science/mechanics of bird flight.
    Thank you.

  3. Abishek Mukundan says:

    Very nice and use­ful post for high­ly qual­i­fied sci­en­tists.

  4. Dawa mayalmu says:

    Thanks I got some idea from this page.

  5. 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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