There are a lot of birds! There’s a recent global abundance study of birds which puts the estimate of birds at around 50 billion individual birds in the world. This is across roughly 9700 different species!
I think the Northern Mockingbird is interesting in particular, though. They’re known to be highly intelligent. If you are just a passerby in their nesting area, they’ll usually leave you alone. However, they can recognize individual humans! If you’re commonly near them they’ll remember you and have been known to try to scare you away. They’ll even remember if you’ve previously threatened or attacked them.