There are two theories why cats bring home live mice or dead mice or birds or someone’s underwear from next door.
Desmond Morris asserts that when cats bring home freshly caught prey, often a live mouse, they view their human caretaker as part of their family and on this occasion they treat their human companions as kittens (they usually see is as ‘pseudomothers’ or surrogate mothers). I suppose what he is saying is that the mother cat is bringing food back to the den and if it is live the purpose is to train the kittens how to kill prey. We know that mothers do bring prey back to the nest.
However, Dr Bradshaw says that male cats don’t bring prey back to the nest and female cats only do this when activited to do so by hormones and the presence of kittens (i.e. not humans that she sees as kittens). He also says that female cats don’t treat humans in this way on any other occasion.
For these reasons he has a different theory as to why cats bring home live mice. He considers his theory much more likely to be the real reason. The reason for the ‘gift’ brought through the cat flap (door) is that the cat simply wants to bring his/her prey home so that he can eat it in comfortable and familiar surroundings in his own time and without the possibility of another cat intervening and stealing the prey.
He says that the place where the prey was caught may well have scent marks nearby indicating the presence of other cats. It makes practical sense to bring prey home and eat it there.
My cat brings home live mice. He does this at night. He brings them into the bedroom. He plays with the mouse and then when it has died or is killed he eats it in the bedroom in my presence. I am often aware at this time; around 4 am.
My cat’s behavior fits Dr Bradshaw’s theory nicely. Sometimes cats who bring home prey abandon it and decide to eat commercially prepared cat food instead as they may find it preferable. On these occasions the instinct to hunt is then overridden by domestication and familiarity with pet food with which he has become accustomed to eat.
Note: Dr Desmond Morris is the famous biologist who wrote Catwatching and many other very fine books while Dr John Bradshaw wrote Cat Sense: The Feline Enigma Revealed. They are both renowned experts on animal behavior.