Opengl application where a bird flies around a tree.

The bird's wings fly with an oscillating motion between -π/4 and +π/4.
To rotate the perspective you can use the arrows or you can drag the scene with the mouse.
By clicking the W key, the movement of the wings is blocked or reactivated.
By clicking the S key you can stop or reactivate the rotation of the bird.
Clicking the R key reverses the direction of rotation of the bird.
By clicking the TAB key:

• the first time the crown of the tree turns yellow;
• the second time the crown of the tree disappears.