Why does a cat chatter its teeth when it sees a bird through the window?

Not every owner has observed this curious action, but it is so strange that it is a case of 'once seen never forgotten'. The cat, sitting on a window-sill, spots a small bird conspicuously hopping about outside and stares at it intently. As it does so it begins juddering its teeth in a jaw movement which has variously been described as a 'tooth-rattling stutter', a 'tetanic reaction' and 'the frustrated chatter of the cat's jaws in the mechanical staccato fashion'. What does it mean?

This is what is known as a 'vacuum activity'. The cat is performing its highly specialized killing-bite, as if it already had the unfortunate bird clamped between its jaws. Careful observation of the way in which cats kill their prey has revealed that there is a peculiar jaw movement employed to bring about an almost instantaneous death.

This is important to a feline predator because even the most timid of prey may lash out when actually seized, and it is vital for the cat to reduce as much as possible any risk of injury to itself from the sharp beak of a bird or the powerful teeth of a rodent. So there is no time to lose. After the initial pounce, in which the prey is pinioned by the strong claws of the killer's front feet, the cat quickly crunches down with its long canine teeth, aiming at the nape of the neck. With a rapid juddering movement of the jaws it inserts these canines into the neck, slipping them down between vertebrae to sever the spinal cord. This killing-bite immediately incapacitates the prey and it is an enactment of this special movement that the frustrated, window-gazing cat is performing, unable to control itself at the tantalizing view of the juicy little bird outside.

Incidentally, this killing-bite is guided by the indentation of the body outline of the prey – the indentation which occurs where the body joins the head in both small birds and small rodents. Some prey have developed a defensive tactic in which they hunch up their bodies to conceal this indentation and in this way make the cat miss its aim. If the trick works, the cat may bite its victim in part of the body which does not cause death, and on rare occasions the wounded prey may then be able to scrabble to safety if the cat relaxes for a moment, imagining that it has already dealt its lethal blow.