This is a neat concept, and given a little polish this could turn into a fun little platformer.
My biggest gameplay issue right now is that only letting the player press up once in bird form feels a little annyoing, since sometimes you need to make little adjustments to your vertical position and it doesn't make much sense from a narrative point of view and you can cheat the system by quickly pressing R twice. I would probably just remove this limitation altogether.
UX-wise, a stamina meter or some other indicator would be nice. Think of Celeste's sweat drops / red flashing, for example.
Finally, I'm not sure, but it might actually turn into a more interesting game if you limit the player's ability to turn into a bird. Right now, there's just no reason to ever be in human form, except for resetting the up arrow limitation. Having transformations be a limited resource might make for more interesting decision-making: Can I solve this section as a human or should I turn into a bird? This might in turn lead to easier level design.
Graphics-wise, the things that are there look pretty good already, but the game would love a little more color I think. Particle effects may be a nice touch, too.
I like where you're headed with this. I think it might be worth to keep on working on it and turn it into a more ambitious game. Well done!