Pretty neat, and I get to be a birb!
Plenty of bullets, good visual contrast (I never had trouble seeing where something was), and the bullets fired in patterns, which is nice. I didn't get to the boss, so I'm afraid I can't comment on the design there.
By far the area most in need of improvement is pacing. I think it's mostly that the waves drag on longer than they should. I had a similar problem in a recent game, and what helped me was to remove *all* spawns that occur mid-encounter, then only sprinkle back in the ones that were absolutely necessary to make the encounter feel right. If after reevaluating you still feel that a nontrivial number of spawns are needed, it would help a lot to have some sort of feedback so the player knows that they're making progress--I had a few moments where I wondered if I was doing something wrong because the enemies kept coming. Also: if you haven't already, try making a simple UI timer to show total time elapsed when playtesting. This will give you quantitative backing for your intuitions about pacing, and it can be removed when you're ready to build.