Quite fun little game. I especially like the cloud mechanic, which gives the gameplay another dimension aside from avoiding birds. The rising / falling of the balloon also felt quite good and responsive.
Just one or two small sidenotes: The levels don't really make sense, as the gameplay is oriented to flying as long as possible and if one reaches the land after a set amount of distance, there is no point in flying further anymore. I would do some kind of milestone progress where the goal is just flying as long as possible. Also, the clouds could be more indicative if they're dense or not as there were some clouds where it was not really clear.
But in total, I really enjoyed playing. Great work!