“You are not necessarily depriving him of his liberty, for territory is a form of natural cage and the word "liberty" does not have the same connotation for an animal as it does for a chest-beating liberal homo sapiens, who can afford the luxury of abstract ideas. What you are, in fact, doing is much more important, you are taking away his territory, so you must take great care to provide him with an adequate substitute, or you will get a bored, sick or dead animal on your hands.
The thing that turns a cage into a territory may be something quite slight, but it need not be size. It might be the shape of the cage, the number of branches or the lack of them, the absence or presence of a pond, a patch of sand, a chunk of log, which could make all the difference. Such a detail, trivial to the uninformed visitor, can help the animal consider this area his territory, rather than simply a place where he ekes out his existence. As I say it is not necessarily size which is of prime importance. This is where people who criticise zoos go wrong, for they generally have little idea what circumscribed lives most animals lead.”

The Stationary Ark (1976)