Are birds native to Roshar?
Birds are not native to Roshar.
...Did they come over with the initial humans?
They did. The humans brought them over, along with a lot of other beasts. Like, horses aren't native to Roshar either. You can tell pretty easily the non-native things. Like, Roshar doesn't have any grapes. So the word wine got genericized in the same the way that the word chicken did. You'll see bunches of that. Like the way the weather works. Roshar does not have weather patterns like Ashyn did. They're used to seasons. Suddenly, the words for the seasons, all these weird things that you may have been wondering through the first couple of books, you're like, "Oh I see why they just call hard alcohol a wine," or, "I see why they call everything a chicken." It's kind of related to this.