It's important to understand gliding flight before we begin to talk about flapping wings. Go back and read the gliding flight page if you haven't already done so. Recall that the wings are angled slightly, which allows them to deflect the air downward and produce lift. The slight angle of the wings is called the angle of attack. If the angle of attack is too great, the wing will produce a lot of drag. If the angle is too small, the wing won't produce enough lift. The best angle depends on the shape of the wing, but it's usually just a few degrees! Notice that what matters is the angle relative to the direction of travel, not relative to the horizon.

The angle of attack determines the amount of lift and drag made by the wing.		The bird wing has a cambered, or curved, cross-section.

The airfoil has a rounded front edge to help reduce air resistance. Some wings also have a curved or asymmetric shape that helps deflect the air downward. While not strictly necessary, this is typical for birds. The inner part, near the bird's body, is more curved than the outer part. As you read on, see if you can figure out why!

The wings flap with an up-and-down motion. This may change in special situations, but we aren't going to talk about those until later. When the wings move up and down, they are also moving forward through the air along with the rest of the bird. Close to the body, there is very little up and down movement. Farther out toward the wingtips, there is much more vertical motion.

As the bird is flapping along, it needs to make sure it has the correct angle of attack all along its wingspan. Since the outer part of the wing moves up and down more steeply than the inner part, the wing has to twist, so that each part of the wing can maintain just the right angle of attack. The wings are flexible, so they twist automatically. This picture shows how the wing must twist in the downstroke, to keep each part of the wing aligned with the local direction of travel.

As the wing twists, and as the outer part of the wing moves downward, the lift force in the outer part of the wing is angled forward. This is what would happen if the whole bird went into a steep dive. However, only the wing is moving downward, not the whole bird. Therefore the bird can generate a large amount of forward propulsive force or thrust, without any loss of altitude.

The wing twists as shown to maintain the correct angle of attack for the downstroke.

The bird's wing produces lift and thrust during the downstroke.

The air is not only deflected downward, but also to the rear. The air is forced backward just as it would be by the propeller of an airplane. You can feel this blast of air when a bird takes off from your hand.

If thrust is produced in the downstroke, you might be wondering what happens in the upstroke. Since the wing is travelling upward, shouldn't there be a lot of drag, tending to slow the bird down? To avoid this, the bird does two things:

• The outer part of the wing points straight along its line of travel so it can pass through the air with the least possible resistance. In other words, the angle of attack is reduced to zero.
• The bird partially folds its wings, which reduces the wingspan and eliminates the draggy outer part of the wing. This is not strictly necessary though, and most insects lack the capability.

The inner part of the wing is different. There is little up-and-down movement there, so that part of the wing continues to provide lift and function more or less as it would when gliding. Because only the inner part of the wing produces lift in the upstroke, the upstroke as a whole offers less lift than the downstroke. As a result, the bird's body will bob up and down slightly as the bird flies.

The inner part of the wing produces lift, even during the upstroke.

The outer part of the wing is angled to pass through the air with little resistance.

What you've read so far is a basic description of how birds fly, when they are already up to speed and just cruising along. Birds also have other flying techniques, which they use when taking off or landing, or for other special maneuvers. Books on bird flight will tell you more about these techniques as well as the special adaptations birds have for flight.

Bird Flight Myths

Q: Doesn't a bird push its wings backward against the air?
A: No. Some insects fly this way, but it doesn't work well at larger scales.

Q: Doesn't a bird separate its feathers during the upstroke?
A: Yes, but only in special maneuvers like takeoff or landing.

Q: Do birds move their wings in a figure-8 motion?
A: Hummingbirds do this when they hover.

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