Spearow & Fearow

I do love how many pokemon exist just to pad out the biodiversity of the setting. They weren't content to have just one normal-type bird near the start of the game, but threw Spearow into the mix among all those Pidgey. The designs are very, very close to one another, but Spearow is a bit rarer, and according to the pokedex, it's meaner and more territorial.
Spearow really diverges from Pidgey when it evolves into Fearow, a large, vicious avian that seems to combine aspects of a hawk or falcon with a stork. Its Japanese name is literally just Onidrill, which is pretty fearsome. It's basically just meant to be the "mean," killer bird of prey in comparison to Pidgey.

I like Spearow and Fearow more than Pidgey. Fearow is especially interesting, not really resembling any particular real-world bird.