wow, great still!!
here's my thoughts, i'm sure you're aware of most of them, since its a WIP...
the bird's motion is very linear. i'm sure you're changing that as i type this, though. wings could use some secondary motion.
the raindrops in the still look black. seems odd, but if this is your own world, then its fine.
is all that cool grass and foilage going to be in full motion, cuz that would be sweet! (but hard, no?) and here's something tricky - are the plants and grasses going to flick and bend when they get hit with raindrops?
amazing work so far!