This must be the most pretty game I played this jam. Also the most infuriating x)
I hate that I have to redo the previous level every time I fall, and I hate it even more given that the bird doesn't always move the way I would expect it to. At one point I fell because hitting a wall sent me backward. Another time, I fell because the bird refuses to glide left or right when you flap your wings... The art style and the music tell me it's a relaxing game, but the gameplay feels like rage bait. You're supposed to make precision jumps and take risks, but taking risk is disincentivized because of how much it costs.
That being said, with a checkpoint at the beginning of the level rather than sending you back one level, I would have loved it. In my short time playing I felt that I was getting better at controlling the bird, and that just the first real upgrade (flapping) was already changing my approach. I understand that you wanted the player to intentionally fall down to get stuff they missed with the new upgrades they unlocked (or it's a way to soften the blow when they lose progression?), but if they give up before unlocking those upgrades it's all for nothing.
OK, I'm sorry for that rant, but I think it hit that bad because I really wanted to continue playing. The art is exquisite (special shout out to the pecking animation: it's amazing), and the sound design and music are pretty good too. It doesn't feel like a jam game, it feels like a finished game.