I like this game! It wasn't too hard. Often endless games like this have a point they hit where they're just impossible, but sometimes it's nice when a game just lets you play it until you're done.
I bought the hat. I know you said you would have added more hats if you had more time, but I like the idea that this bird more than anything just wanted his little santa hat. He doesn't seem to care that all the other birds are attacking him.
Excellent balance for a first game, I think! I hadn't noticed the top-of-the-screen invincibility issue. Maybe a blue bird that flies up and acts lke a rock, but opposite?
Also welcome to the world of pixel art! My pointer here is that some of the backgrounds make some of the sprites kindof hard to see, and the solution to this is more contrast on the objects you're meant to see, like shine and shadows on the worms and rocks and even the sticks against the darker backgrounds.
Had a good time! Looking forward to what you do next.