9S' instinct on meeting someone new is to discreetly scan them to get an understanding of what they are physically. The stranger looks similar to Taako with the ears, so 9S is expecting his sensors to report back something similar: some kind of humanoid being that isn't a human, despite the appearance.

"They bite? But birds don't have teeth!" Sounds fake. 9S sounds distinctly skeptical. "Are you sure they're a kind of bird? Birds usually flee from danger."

Guess who doesn't have much experience with birds? This guy.