If you're confused by the sight of a mama cat seemingly biting her young kittens on the neck, don't be. In placing her teeth onto the back of a wee kitten's neck -- or nape -- she isn't biting him per se, but rather carrying him from point A to point B.
Mother cats, like mothers of many other species of animals, tend to seek out safe, private and calm nesting locales for birthing and rearing their youngsters. If she for any reason begins to have doubts about her spot of choice, she might, using her teeth, retrieve her little ones via their scruffs, one at a time. When you see a mother cat biting her kitten's nape, she's actually probably in the process of taking him -- and his littermates -- to a more suitable and peaceful setting. Mother cats make great efforts to ensure that all of the siblings stay tight as units.
When kittens initially notice their mamas' teeth clutching their nape regions, their responses to the situation are usually pretty immediate. Not only do they stay completely silent, they also dutifully stop moving entirely. They usually curl their tails up tightly against their undersides, too. All of this usually is advantageous for a couple of reasons. Since they don't whimper or wail, potential predators can't hear them. Since they keep all of their tiny body parts so close to their bodies, they also generally don't disrupt the determined queen's movements.
Once mother cats arrive at their desired destinations with their little ones, they promptly let them loose in the new nests. To reassure and relax them after their journeys, the mama cats generally immediately and lovingly lick the kittens, too.
"No" to Humans and the Scruff
Mother cats manage their kittens by biting the slack skin on the backs of their necks all of the time, but it isn't a good idea for humans to ever try to replicate the action using their hands. Mother cats know exactly how to do it in a safe and reliable way, but that doesn't mean that humans do, and because of that, the situation can be potentially hazardous. If you need to carry a cat, do so by putting a single hand below his back legs, and the other hand in the back of his front limbs. If you ever have any uncertainties regarding the right etiquette in holding felines of any age, consult your veterinarian.