Special witch magic. During borrowing, a witch sends her mind into an animal and watches the world through its eyes. This has practical uses such as surveillance of the goings-on in the village (see Granny Weatherwax, in Lords and Ladies). A witch might also do borrowing when she's bored and wants to ride in a forest animal's mind for a little while. There is an inherent risk in borrowing: the witch's mind might forget to come back to her body and get stuck in the animal, and then, the mind will slowly lose its human characteristics, and become a few vague thoughts considered odd by the animal. After a mere few minutes of borrowing and then successfully returning to her body, even a very powerful witch may be influenced to act like the animal, for example, a witch who has just come back from borrowing a raven's mind might mistakenly decide to fly down the stairs instead of walking down the stairs (see Equal Rites). Many times witches talk about the diet the animal has. In A Hat Full of Sky Granny Weatherwax points out that you should never, ever eat voles.
Roundworld polymath Charles Foster, an Oxford University don and qualified veterinary surgeon, has possibly got as near as you can to Borrowing on a world without magic. Interviewed in Fortean Times no 339, he talks about trying to physically enter the world of various animal species to live and see the world as they do. His book Being A Beast is published in the UK by Profile Books and is reviewed here. An accessible article about his philosophy is here and makes interesting reading. (The FT article is not yet online).