Cats are also known to make chirping noises when observing prey, or as a means of expressing interest in an object to nearby humans. When directed at out-of-reach prey, it is unknown whether this is a threatening sound, an expression of frustration, or an attempt to replicate a birdcall (or replicate the call of a bird's prey, for example a cicada). Since this feline expression often involves a mouth movement similar to the one they would use to kill their prey (their "killing bite"), they may be trying to practice this mouth movement in anticipation.