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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6 Responses to Why do bird wings flap?

  1. 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.

  2. Abishek Mukundan says:

    Very nice and useful post for highly qualified scientists.

  3. Dawa mayalmu says:

    Thanks I got some idea from this page.

  4. 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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