Cats are exceptional predators. If they have a chance to prove it, they may bring us some "surprises" when they come back. When they can't, they'll spend a good deal of their time looking out the window at what is ahead: flies, bees, people and, of course, birds.
If we approach it, we are likely to hear a somewhat curious sound. But, Why do cats chatter their teeth when they see a bird or other animal?
Cats chatter their teeth from being frustrated ...
Cats, when they see possible prey, chatter their teeth. Why? Well, there are two reasons: frustration or as training to improve the hunting death bite that would need to land on him in the wild. If they do not have the possibility of going outside, even if they always have their feeder full, they will do so because they feel frustrated, because they're looking at prey they can't reach. If we find our furry animals in this situation, we can make them feel a little better by offering them a toy they should / can hunt for followed by a can of wet cat food.
... or as hunting training
The chattering of the teeth is an instinctive behavior. They begin to develop it when they are very young, and they practice it throughout their lives. Failure to do so would put them at serious risk of being harmed while hunting, as even a small bird can injure them while trying to escape. So, Every time they have the opportunity, they practice this movement of the mouth, since tomorrow it can serve to satisfy their hunger.
Have you ever seen your dear four-legged friend chatter his teeth? If you have not done it and you would like to know how he does it, take a look at this video in which he sees a beautiful feline: