“Chattering: This odd sound indicates frustration. Cats that watch critters through the window may chatter when unable to reach the evil squirrels.”
Our cat does this while watching birds through the window. I’ve also seen her do it when a housefly got away from her.
Last year there was a huge crow that used to land on our balcony and tease the cats through the window. Crow’s aren’t dumb. He wouldn’t show up if the door was open.
I used to laugh myself sick at my five monsters chittering, chattering, growling, hissing, lashing tails...
And that bird would just strut back and forth in front of the door... just taunting them...
Did that for about a month and then stopped. Every now and then, I would catch one of the cats at the door... just looking at the sky wistfully...