I am a sucker for paper themed anything honestly. The texture you used for the back along with the hand drawn assets radiate such a mood. As for the game play, the circular bar works and it definitely gives me a sense of executing a full "flap". If there was another indicator of performing a good flap, that would have had me wanting to do more perfect flaps. (sfx, particles, or maybe some transform animation)
The death sfx did give me a bit of scare and almost sounded strikingly dark / melancholic sounding compared to presented feel of the game. If that is the soundscape you guys imagined, then you did a great job!!