I guess... Technically... Flappy bird is a bullet hell, if you just pretend the pipes are bullets!
Fun idea, I noticed that when I went down to no health, the game let me continue playing until I had picked up a health powerup, and then gone back down to 0 health. Also, it's a bit frustrating when the bullets from the vultures block the only route through the moving walls, but that would be VERY hard to fix.