When prey is unobtainable (for example, too high up in a tree or the cat is restricted to indoors but can view prey outside, through a window ) the cat can often vocalise in a manner known as chittering, also sometimes described as chattering. The mouth is opened and closed rapidly, creating a series of short sounds interspersed by short intervals of silence. The sound is often individual to the cat and may differ in pitch and frequency but it can include a series of clicking-like noises (made from chattering the teeth) and/or a series of high pitched staccato squeaking-like sounds. The cat’s body language suggests it is in an aroused state as the eyes are usually wide open and fixated on the particular unobtainable prey (or occasionally an item simulating prey-like movement), the whiskers are usually directed outwards away from the face and often a tail quiver is observed together with the chitter.
The cat is fixated on an unreachable item and has begun to chatter. The fixed stare and opening of the mouth can be seen in this picture.