School-boy error: Putting the same ambient sound in both slots on a screen. This doesn't make it louder; the two slots aren't exactly in sinc so it gives the sound an ear-bleeding echo.
Despite my ears filling with blood, I did enjoy this. I think you overused the homing-ball-throwing-bird which is one of the toughest enemies in the game, but still, it was pretty and fun. It was good trying to work out what it is you were passing, and then at the end recognising all the things you saw when you were tiny. The backtracking didn't worry me too much, though things-popping-out-of-the-ground-without-warning were also a bit overused.
More variation in enemies would be good, but not a bad level.