Why do cats chatter?

Every time sparrows land on the balcony Remy and Shadow crouch at the window and chatter at them. "Eh-eh-eh! Eh-eh-eh!" My mom's cat (who's indoor/outdoor) also does it, as she creeps towards birds. Why do cats do that? Wouldn't they catch more critters if they kept their thoughts to themselves?

I think it's more because they are unable to get to the prey, they are frustrated by the window between them and chatter to express the pent up excitement. I have only heard Neko do this once and Willie has not done it, but I was kinda worried When Neko did it. I thought he was like having a seizure until I saw he was looking at some birdies, then I thought it was adorable!

my cat makes a funny noise in his throat... not so much a chatter... then he'll do that whiny kitty cry because he can't get at the bird through the window. I have yet to hear him chatter at this point...

This is actually how adults cats talk to one another "in the wild". Only kittens mew, but our adult house cats mew to us because we keep them as permanent kittens. But amongst themselves, adult cats make those chattering sounds.

Ding ding ding! You win the prize, girl! I agree completely.
It's a combo of smacking lips and clacking teeth. The noise they sometimes make from the throat goes with it. It's serious excitement/frustration/desire to get to the prey. And of course, the Swishing Tail.

All my childhood cats were outdoor cats, and I never remember hearing that chatter. My previous indoor/outdoor cat, Kilgore, never did it, either. Now I have 2 indoor only cats. I've never heard Charles do it, but maybe he gets his frustrations out on the screen porch chasing Lizards. Venie sits on the cat tree looking out the window and chatters away when the ducks are on the pond. She is the first one I've heard to that. She actually sounds a little bit like a duck when she does it!

mjb, it's possible taht Venie does it because his ability to get ahold of prey is low. Your outdoor cats were able to catch things so they probably didn't get too excited over a fly like my cats do. My cats will chatter while they chase a fly and, as you know, they can hear a fly at fifty paces, so they chatter and chatter and follow it till it drops from exhaustion. The rest is called a snack. The sound they make IS cute.

Three of my cats are outdoor cats (they come in when it gets cold though) and one is an indoor/outdoor cat. They have all chased and/or caught animals, mostly mice.

I really think it would be frustrated that makes them make that noise because none of my have ever made a peep when they hunt.

The 'chatter' is sort of like a seizure, in a way. The cat is making the same jaw movements that it would make to sever the spine of its prey. Except, obviously, it doesn't have its prey. It seems to be something they do when they are frustrated by their inability to get to their target . . . its sort of like they are fantasizing. I do suspect, however, that it is not under conscious control . . . they're just thinking so hard about how that bird would taste in their mouths that their body reacts with that stereotyped movement. And it IS really cute.