I initially thought the bird was far too tall, but the more I look at it, the more I love it!
I like the wings, although perhaps a little more attention is needed to make them less blocky, and more organic? The shaping on the rest of the bird looks fantastic, well done.
There are only two big things for me - I feel like a little more distinction of where the bird ends and where the saddle begins (this would solve the tall bird problem), and the final thing, (a real nitpick) - there is a slight gap between the back of the rider's area and the back of the slope pieces build up behind it. This is due to gravity pulling the two constructions away from one another, and while not a piece or design issue, is only something which would come up from a practical perspective. Perhaps placing a 2x2 plate beneath the handle piece on the back of the saddle to connect the two areas?
Anyhow, nitpicking over, I love it, and will continue to watch with interest!
Edit: I would 100% buy this set, even with the above issues. I'm just offering the best criticism I can
Edit #2: I really like the way you've given the Tohunga the vehicle/creature/battle-assisting-mcguffin, and left the Toa as their own entity. It feels right, as the toa are more powerful, and are able to hands themselves. I also really like the Tree setpiece, it adds such a fantastic atmosphere, and from such a small build as well!