An indoor cat sees a bird outside.
Its jaw starts moving.
You hear chirps, clicks, or chatter.
The behavior is common enough to be recognizable.
The exact reason is not completely settled.
It often happens during intense prey focus
Cats commonly make these sounds while watching birds or other small animals.
The cat is alert.
Its attention is locked on potential prey.
And very often, the prey is just out of reach.
That pattern suggests the sound is connected with hunting motivation.
Excitement may be part of it
The cat can see something it strongly wants to chase.
That alone can create a high level of arousal.
The vocalization may be one part of that excited hunting state.
Frustration may matter too
The bird is visible.
The cat cannot reach it.
A window, screen, or distance blocks the chase.
The behavior may partly reflect frustration created by that mismatch.
This fits nicely with why cats like boxes, another behavior that makes more sense when you remember that domestic cats still carry strong hunting instincts.
What about the strange jaw movement?
One idea is that the jaw movement resembles part of the biting action cats use on prey.
The recorded explanation notes that possibility.
It does not present it as a settled explanation.
Are cats imitating bird sounds?
That idea has been proposed.
But the recorded explanation is careful: prey-sound imitation is not proven as the general explanation for ordinary chirping at birds.
What do we actually know?
The strongest pattern is behavioral.
Cats often chatter, chirp, or click when intensely watching prey they would like to chase.
The sound is associated with that focused hunting context.
So when your indoor cat starts chirping at a bird through the window, it is probably extremely interested.
The glass is the part ruining the plan.
And that’s the short explanation.
Related explanations: Why do cats like boxes?, Why do cats slow blink at people?, and Why are cats’ tongues so rough?