My husband and I have two kittens about six months old. They are sisters. Recently we have noticed that they will sit and look out the window and when they see a bird they get excited,wag their tails, squeak, and they start clicking their teeth together making a clacking noise. Is this common to any particular breed? If so, which one and why do they do it? One of the babies is jet black and short-haired, the other is very fluffy with a Maine Coon appearance.