Why do cats chatter their teeth when they see a bird?
Because it is excited by prey it cannot reach: when a cat watches a bird through the window and makes that rapid clicking noise with its jaw, it is hunting excitement mixed with frustration, and perhaps a rehearsal of the killing bite. It is perfectly normal behaviour, but chattering while eating or for no obvious reason can be a sign of mouth pain.
What that noise is
It is one of the most curious things cats do: a cat is staring at a bird, a pigeon on the ledge or a fly on the ceiling, its body tense, tail twitching and pupils dilated, when suddenly it opens and closes its mouth very fast, making a stuttering or clicking sound. Sometimes it adds little trills or high-pitched meows. In English this is called chattering, and in Spanish castañeteo (the word for teeth chattering with cold), although it has nothing to do with the cold. It almost always happens when the prey is in sight but out of reach, especially behind a window.
The most widely accepted explanation: excitement and frustration
No definitive study explains chattering, but the explanation given by most ethologists and veterinary behaviourists is that it is a mix of hunting excitement and frustration. The cat has spotted prey, its whole body is ready to attack and its brain has set the hunting sequence in motion, but the glass or the distance stops it from completing it. That pent-up energy escapes as jaw movements. This fits with the fact that chattering almost always appears in that particular situation, and almost never when the cat can actually reach the prey.
A rehearsal of the killing bite
Another, complementary idea is that chattering is a kind of rehearsal of the bite cats use to kill small prey: a quick bite to the nape of the neck that separates the vertebrae. The jaw movements would mimic that action, performed in mid-air while the cat watches its prey. It is a reasonable and widely cited hypothesis, but hard to test. What is clear is that the behaviour is linked to hunting, because it appears even in indoor cats that have never caught anything: the instinct comes as standard, it is not learnt.
Do they imitate birds?
There has been speculation that the cat might be imitating bird calls to get closer to them. The idea rests on an observation made in 2005 in the Brazilian Amazon, documented by researchers from the Wildlife Conservation Society, in which a margay, a small wild cat, made sounds similar to those of the young of a small monkey, the pied tamarin, apparently to lure the adults; the observation was published in 2010. It is an isolated case in a different species, and there is no evidence that domestic cats chatter for that reason. It is best treated as a curiosity, not as the explanation.
The other sounds of the hunt
Chattering is not the only sound that goes with the hunting instinct. Some cats make little chirps or trills, a sort of meow with the mouth almost closed, while watching prey; others make a sound like babbling or a click of the tongue. Some cats never chatter and others do it every time they see a pigeon, and it has nothing to do with whether they are better or worse hunters. Breed plays a part too: many owners of Siamese, Orientals and other chatty breeds describe particularly varied repertoires of sounds. It is all part of the same state of excitement: the cat has spotted something moving, its attention is at its peak and its body is gearing up for a hunt that, behind the glass, never comes.
Is it bad? Should you stop it?
No. Chattering at a bird is normal behaviour, it does the cat no harm and it does not point to any problem. What can happen is that a cat that spends many hours watching prey it can never reach builds up frustration. If you notice that it stays very wound up afterwards, that the tension ends in an attack redirected at another cat in the house or at your ankles, or that it spends too long glued to the glass, it helps to follow up with a play session in which it can actually catch something, such as a feather wand that ends in a real capture or a toy it can bite. That way it completes the hunting sequence. It also helps to serve its food in puzzle feeders or hide small portions around the house so that it has to search for them and earn them, and to offer several short play sessions a day instead of one long one, which better matches its natural way of hunting: lots of brief dashes throughout the day and night.
Windows and balconies: watch the enthusiasm
The same instinct that triggers chattering is what makes many cats leap from balconies and windows after a bird or a fly. Focused on the prey, the cat does not judge the drop. If your cat starts chattering at an open window or on a balcony, make sure there is a strong, firmly anchored safety net. Ordinary fly screens will not hold the weight of a cat in full flight. Ideally it should be able to watch the outside world from a ledge or shelf next to a closed or protected window, which for many flat-dwelling cats is their favourite television.
When chattering means pain
There is another kind of chattering that has nothing to do with birds: the kind that appears when the cat eats, drinks, yawns or grooms, or with nothing in front of it at all. In that case it can be a sign of mouth pain. A common cause in adult cats is tooth resorption, in which the body itself gradually destroys the tooth and leaves the nerves exposed; others are periodontal disease and gingivostomatitis. It usually comes with drooling, bad breath, chewing on one side only, dropping food or preferring wet food. Some cats with nausea also chatter. If you notice any of this, have its mouth checked.
Chattering at a bird behind the window is hunting excitement mixed with frustration, and perhaps a rehearsal of the bite cats use to kill their prey. It is normal and there is no need to stop it, though it is worth playing afterwards and protecting windows and balconies. If your cat chatters while eating or for no apparent reason, think dental pain and see the vet.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.