Wow, these look great. But I think you are not using cubes a lot, for birds and torsos it’s fine using spheres or cylinders. But you shouldn’t use them all the time. Now for more dynamic poses I will suggest staring with a gesture line, then exaggerated the pose (because structure stiffens the gesture). After that divide the body into 3 parts, shoulders, torso and hips. example: Hope it helped you. Good luck.

Gesture is really tricky! You essentially want to capture the 'energy' or 'flow' of the pose. I think the easiest way (I've found anyway) to learn gesture is to start with the Force Drawing videos about gesture by Mike Mattesi and apply those ideas to the Proko lessons. There's no real rules so to speak (your drawings definitely have energy/flow and are not stiff), but a way to think about it is per animation: how would the pose move? What is the character going to do next? Where is the movement or strongest forces through the body? Try to bring those things out... as said though, it's not easy, particularly early on while still grasping things like proportion, angles and line control! Oh and some poses are bad for gesture, a figure standing still, directly front on with hands by their sides doesn't give you anything to work with for gesture :)