Why do domestic cats "chatter" while watching birds?

The chatter is a fast movement of the jaw, with what appears to be an "ah ah ah ah" sound, while visually locked onto a bird. I have verified that our cat does this not only while inside the house looking out a window, but also does it if let out the door. She chatters while in "stalk mode", too. It's hilarious! We get waaaay too much entertainment from our kitty!

With a Google search, I have found several suggestions ranging from "frustration", "sonar", echo/vocal location, to supposedly "calling" the prey item or trying to replicate the sound of a prey item.

Has anyone come across a good answer for this?

The article I saw said it was the cat "practicing" in anticipation of the killing bite.

Sonar/echo location as a suggestions is pretty far off the wall. And I doubt any animal is going to come when a cat calls... short of another cat. Or a dog looking to whup up on the cat.

Both of my cats do that while they are bird watching. It is to funny. They get locked on and the chatter starts.

Maybe it's the cat version of "Get in my belly!"

My kitten does this all the time while he watches the birds, I think he's excited to see them and desperately wants to get out to catch them.

My cat does then when he sees just about anything from bugs, lizards, and birds.