Very creative! Very much feels like real birds flying off! I'm also a fan of the incremental type of game play, although it does get a little slow near the end when waiting for all the birds to spawn. I like it when incremental games give you options to speed up the game.
Sadly no audio settings so I had to turn down my system volume. Funny voice lines though!
And I noticed you sometimes don't get your bonus pellet until 1 round after you buy it, or does it shoot twice maybe? Oh I just noticed if you hover over 2 birds, it shoots them both consuming 2 bullets.
The game also starts lagging a lot near the end, you should be able to fix this though, I'm guessing you're constantly checking which birds are nearby which is causing the lag. (I'm not sure how Godot handles collisions but you might want to look into Spatial Partitioning, it reduces collision-check lag a lot)
And personally I would make the flamingo upgrade have no limit, so we can dump our excess money into that :D