[Physics] How do birds generate thrust

aerodynamicsbiophysicsfluid dynamicsforcesnewtonian-mechanics

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.

Best Answer

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