Atmosphere, music, story. This is definitely one of the most immersive games I've played. Being a long-time fan of the submachine series, that means something :-)
So looking forward to the full game!
As for feedback, I would say it's a little too easy, as i could buy almost all the upgrades after a couple of runs. Further, the Sea gull blessing is also really powerful, so flying over sea isn't too much of a challenge anymore.
It also seems fairer to encounter less, but faster planes than 10 slow planes that crush your flock without remorse.
The flock should also keep responding if your mouse pointer goes out of the game screen. Now the flock suddenly stops. Don't know if this is possible though.
Reducing the graphics size, so you have more playing room would also be nice.
Keep up the good work!