They’re not. And they’re not kosher, either, which is odd because we find them among other non-kosher birds: storks, cormorants, owls, herons, the hoopoe (Israel’s democratically elected national bird) and the ever-abominable falcon in Leviticus 11:13-20 (JPS Version, for you citers out there). Of course, I crosschecked other versions of the Bible and they all say the same thing. This is no mistranslation. The authors of the Bible really thought bats were birds. Of course, we know they’re mammals–like us.
Thanks to Richard Dawkins for pointing this out in his recent book The Greatest Show on Earth: The Evidence for Evolution.