Why does my cat chatter his teeth when looking at birds outside?

My dog chatters his teeth sometimes. Like hes nervous or something. But his actions dont match nervous🤔

It's an attempt to get their attention. Cats can "chirp" and "bark" in varying degrees to attract prey.

He warming up in case he gets out? Lol

Idk, that’s why I don’t like cats...they kill birds!

Mine does too

To my knowledge it is usually either a sign of excitement or frustration. In your case I'd assume it is expressing a slight frustration but interest in the prey that it is unable to reach it. I'm no licensed behavioral expert though.