Overall the prototype was really fun!
one thing i think worth looking into is the flapping. Since you start falling as soon as you finish the flap, you need to release and press it again very quickly, which feels very clunky and unnatural.
Another thing that worth mentioning is that the current version of the vertical sections feels unsatisfying and slow, while this game shine brightest on the fast pace screens.
Here are some ideas for what can be done to fix it:
Focus almost entirely on horizontal screens.
Add some sort of diagonal/curved glide that will let the player finish high rooms more fluidly and quickly.
Another option is momentum based glide that will carry you up relative to how long you dived before it (like in the platformer "the king bird")
Another hiccup that can be fixed easily is vertical screen transitions, i found myself going back and forward before finally settling on the top one. Something to catch you on the other side (like a floor) or getting a up boost on screen transitions (so you have more time to flap on the new screen) should do the job.
Another thing i had to mention is that this game has very large speed potential, so you might want to consider an in game level timer super meat boy/the king bird style.
I hope this feedback will help you and good luck with further development! I'l be waiting for the full release 😃 (also i love the character art)