Why does a cat move its kittens to a new nest?

When the kittens are between twenty and thirty days old, their mother usually moves them to a new nest site. Each kitten is picked up firmly by the scruff of the neck and, with mother's head held as high as possible, is carried off to the fresh location. If it has to be transported over a long distance the mother may grow tired of the weight and let her head sag, switching from carrying to dragging. The kitten never objects, lying limp and still in its mother's jaws, with its tail curled up between its bent hind legs. This posture makes the kitten's body as short as possible and reduces the danger of bumps as it is unceremoniously shunted from old nest to new one.

As soon as the mother arrives at the new site she has chosen, she opens her jaws and the kitten drops to the ground. She then returns for the next kitten and the next, until the whole litter has been transported.

After the last one has been moved, she makes a final trip to inspect the old nest, making doubly sure that nobody has been left behind.

This suggests that counting kittens is not one of the cat's strong points.

It is usually stated that this removal operation is caused either by the old nest becoming fouled or because the kittens have outgrown it.

These explanations seem logical enough, but they are not the true reason. A cat with a large, clean nest is just as likely to set about moving its litter. The real answer lies with the wild ancestors of the domestic cat. In the natural environment, away from canned cat food and dishes of milk, the mother cat must start bringing prey back to the nest, to arouse the carnivorous responses of her offspring. When the kittens are between thirty and forty days old they will have to begin eating solids, and it is this change in their behaviour that is behind the removal operation. The first, old nest had to be chosen for maximum snugness and security. The kittens were so helpless then and needed protection above all else. But during the second month of their lives, after their teeth have broken through, they need to learn how to bite and chew the prey animals brought by the mother. So a second nest is needed to facilitate this. The primary consideration now is proximity to the best food supply, reducing the mother's task of repeatedly bringing food to her young.

This removal operation still occurs in domestic cats – if they are given half the chance – despite the fact that the feeding problem has been eliminated by the regular refilling of food dishes by their human owners. It is an ancient pattern of maternal feline behaviour which, like hunting itself, refuses to die away simply because of the soft lifestyle of domestication.

In addition to this 'food-source removal pattern', there are,of course, many examples of a cat quickly transporting her litter away from what she considers to be a dangerous nest site. If human curiosity becomes too strong and prying eyes and groping hands cannot keep away from the 'secret' nest, strange human smells may make it an unattractive abode.

The mother cat may then search for a new home, simply to get more privacy. Moves of this kind can take place at any stage of the maternal cycle. In wild species of cats, interference with the young at the nest may result in a more drastic measure, the mother refusing to recognize them as offspring any more, and abandoning them or even eating them.

What happens, in effect, is that the alien smells on the kitten's body make it into an alien 'species' – in other words, into a prey species and the obvious response to such an object is to eat it. Domestic cats rarely respond in this way, because they have become so used to the scents and odours of their human owners that they do not class them as alien. Kittens handled by humans therefore usually remain 'in the family', even if they have acquired new scents.