Sure there are dumb birds, but in general they’re pretty smart, all because of the void left by the dinosaurs. It seems that when dinosaurs were wiped out, birds got smaller — but their brains stayed the same size, or in some cases grew larger.
So today, while you have, um, cognitively-challenged birds like emus, pigeons, and moas, most birds evolved to be smarter, with a few being notably smart:
The two groups of birds with truly exceptional brain sizes evolved relatively recently: parrots and corvids (crows, ravens, and kin). These birds show tremendous cognitive capacity, including the ability to use tools and language, and to remember human faces.