Why does a cat do that?

A while back I posted a very funny video of two cats chattering complete with translation. Imagine my surprise (and delight) when Joyce started doing the same thing while watching a blackbird snatch debris from the eavestrough. I wondered why she was doing this. I found the answer to this question and and a whole lot more at CatStuff. The answer is:

When a cat kills prey, it bites down quickly several times. The 'chatter' is simulating the cat's behavior at the time of attack.
Alternatively (according to a visitor): it's pure and simple frustration. When a cat is stalking or killing a bird, they're almost invariably completely silent. It's when they are shut off from the prey that they chatter.

I'd never seen a cat do this but evidently it's a common feline behaviour. Google "chattering cat video" for guaranteed cat chatter overload.