My cats are indoor cats but every day I sit out in the back yard with them for a few hours so they don't have to be cooped up inside all the time. I have bird feeders in the back yard which do attract birds. Both of my cats will try to stalk the birds, naturally, but as they move towards them I've noticed that both of them will open and close their mouths very rapidly and I swear they make a chirping sound. The sound they make doesn't sound like the birds but it sure as heck doesn't sound like any cat I've ever heard either! What in the heck are they doing?