How do birds generate thrust? I have been watching this video carefully and I want to know how the wings of birds generate thrust. This is because the wings are more or less flapping up and down --- generating the lift. But I do not see any appreciable change in the angle of attack of the wing, so as to generate a thrust component.
Please provide links to articles/papers which can make me understand bird flight better.
 A: Birds' wings don't just flap straight up and down. They angle their bodies in order to change the angle of attack of their wings, thus creating a component of thrust or drag, depending on whether or not they're slowing down or taking off. This can be illustrated by the corresponding Force-Body Diagrams:
Take Off
Slow Flight
Cruising Flight
Approach
A: There's another way to look at it as well.
Suppose the motion of the bird's body is roughly sinusoidal in the vertical plane - curving up, then down, then up, then down, so it is experiencing first upward acceleration, then downward, etc. all superimposed on the acceleration of gravity.
When it is at the bottom of a curve and experiencing upward acceleration, it can flap its wings downward, hoisting its body against that acceleration and doing work.
At the top of the next curve it can raise its wings, also possibly doing work.
The net effect is that in an up/down cycle its wings have done work, translating into kinetic energy.
You can simulate this by hanging a swinging weight from a string at the edge of a table.
When the weight is swinging through the center, shorten the string.
When it is at either end of the swing, lengthen the string.
The weight traces a figure-8 trajectory, and swings more and more strongly.
I think you also can see this in how children operate a schoolyard swing, or the kind of S-shaped motion used to propel oneself with a skateboard.
I think this is another way to look at the pitching wing angle producing forward thrust.
A: I believe the best way to explain this how I did it in 5th grade. Imagine a bird flying, when it flaps its wings downwards, the air that was under its wings is pushed into the air below the bird, this creates a high pressure below as the air molecules get really close and hence are ramming against one another. Now when it takes its wings back upwards, those cramped air molecules rush upwards to fill up that extra space. This sudden movement of these molecules is exactly what creates the thrust. Yup, its that simple. 
A: 
As shown in the figure, the red arrow is the flight direction of the bird, and the black arrow is the flapping direction of the wings.  The red lines are the cross sections of the wings.  When the bird flaps its wings down, the leading edge of the wings is lower than the trailing edge, which provides forward thrust.

As shown, the red arrow is the direction of the bird's movement, and the red line is the wing section.  The black arrow is the direction in which the wings are moving up.  As the bird moves its wings upwards, the leading edge of the wings is high and the trailing edge is low.  So there is no forward thrust, but backward resistance.  Because the wings use less force to move upwards than they do to move downwards, the forward thrust is greater, so combined, the wings provide forward thrust for the birds.
