How do birds generate forward thrust?

It is obviously related to the birds’ wing flaps – but what is the physics behind this? That is, how are birds able to move forward by beating their wings?

Sort of like how you move forward when you swim. The wings aren’t just moving up and down, it’s more of a complicated sort of rotary motion. Probably the best way to get an idea of it is to go over to Youtube and look for videos of birds flying in slow motion.

Their wings come forward and grab air, then throw it behind them. Their wings aren’t really just going up and down. Think about it as if they are swimming through the air."]In this video, it looks like the front edge of the wing is quite stiff and the rear is quite flexible, so that air is being forced backwards. But that may be a naive way of visualizing what’s going on.

Fascinating video (the correct link is this if anyone was having problems watching it).

I might be looking at it wrong, but to my eyes it almost appears as though the wings are pulling the bird forward during the downbeat (downflap???).

There are probably several equivalent ways of looking at it, but one way is that the bird is constantly gliding on a downward slope, while using the flapping motion to raise its altitude. The net sum of these motions is level flight (or something else, depending on the relative contribution of the two).

I might be looking at it wrong, but to my eyes it almost appears as though the wings are pulling the bird forward during the downbeat (downflap???).

The ridgid front part of the wing compresses the air under it as it comes down and the flexible rear edge of the wing remains a bit higher and that gives the air a pathway to escape backwards. More pushing than pulling.

It seems the wing behaves very differently for different birds and in different modes of flight.

The SmarterEveyDay guy had a good piece here on macaws that appears to focus on flight with rather low forward speeds. In this situation, the bird orients its wings for minimal air resistance on the upstroke - and in some cases, it even appears that the bird might be generating lift on that upstroke/backstroke. The same phenomenon appears to be happening with these Canada geese as they first take to the air, before they really get moving.

During high-speed cruise (example here), things are more like what you describe: on the downstroke, the wings are flying downhill, and the bird’s muscles are exerting effort to keep its body on a relatively level flight path. That descending flight path (during the wing downbeat) generates forward thrust in the same way that an airplane’s wings generate forward thrust when it’s in a descent.

You can actually achieve an analogous sort of motion using a skateboard. Imagine the ground is a series of hills with the peaks several feet apart, the peaks are all at one altitude, and the troughs are all at another (lower) altitude, perhaps a foot lower than the peaks. If you “pump” with your legs as you are on the downslopes and relax them on the upslopes, you can build up substantial forward speed after crossing several of these small hills, despite the fact that (at the finish) you are at the same altitude as when you started.