Why does my cat chatter her teeth at birds through the window?

This rapid jaw chattering, sometimes with a chirping sound, shows up almost exclusively when a cat is watching prey she can see but can’t reach, like a bird or squirrel through glass. The leading explanations are a mix of predatory excitement and a kind of frustration response, an involuntary jaw movement tied to the mismatch between the hunting instinct firing and the prey being unreachable. It is not a sign of pain or distress and does not need any intervention. Indoor cats do it more than outdoor cats simply because they encounter this exact unreachable-prey situation more often.