Played a few rounds, at first I didn't understand what made me fly or why was I always going straight to the ground, so maybe a tutorial would be nice.
Once you get it, it's pretty relaxing, the environment is simple but the parallax makes it look nice as you fly by. The birds are very cute and it is a cool feeling when you get a nice flock (the game over scene gets funnier with a lot of birds too!). I didn't feel like I was flying higher when I had a bigger flock though.
Perhaps the sound once you got another bird could be louder, I noticed a sound playing but it was very faint and only noticeable whe the background song had no notes playing.
Also, a score would be awesome for this one, maybe distance flown or birds in your flock.