Wind and gravity provide constant acceleration. If you want "turbulence" you can modulate them by keying them. You can use an animation applied on a higher track from your other animations to get various effects. If you use 2 animations of different lengths, the combined wind/gravity will not be repetitive, nearly random. Lastly, you can write code at runtime to control the wind to provide truly random behavior.
Physic constraint parameters are independent, unless the "global" checkbox is checked.
We don't have plans to allow building out entire scenes using physics. What we have is efficient and easy to use. A more complex physics engine that handles more forces and collisions cannot be as efficient or as easy to use. Take a look at Box2D, it provides extensive physics features like what you describe, but it also massive and very complicated. That said, we do have a list of cool features we want to add to physics!
Spine is primarily an animation tool and our physics is designed to be combined with keying. For your bird example, you can key the rotation of the branch moving down when the bird leaves it. Your keys can act like forces and then let physics take over. For example, set a stepped key to rotate the branch downward, reset the physics, then immediately move it back up with another stepped key. The branch will return to where it was using physics, simulating the bird jumping off. I don't have a branch image, but if you can pretend Spineboy's head is a branch: 😃
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Doing that with only physics may be challenging, but combining keys with your physics makes it possible to get all kinds of movements very easily. The keys look like this:
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(The flash of the head moving too far could be improved, I did this quickly!)
Note that physic constraint gravity is "real". It is applied as a constant acceleration according to Newtonian physics. Strength is a spring force that wants to return the bones to the unconstrained pose, so you may want to set strength to zero to have your bones fall realistically.
