Really good mood with both beatiful art and music. There are cases where seeing the birds is a bit hard like said below.
I feel like the gameplay is really too simple, reversing controls surprised me the first time but got used to it after that, a better fit to the theme would be found i think.
The low flying birds were quite rare, so i spent almost the whole game holding crouch, only switching key when the background changed which made the game even easier xD
For some reason the parallax was really not fluid, fullscreen or not doesn't seem to affect it nor restarting the browser.
Also sometimes you would get birds going at different speeds at the same time, which can make certain jumps impossible.
Cute game, some more explanation on why you run would be nice