A rookery is a colony of breeding animals; specifically crows, although the term is used more generally to refer to birds and sometimes to marine mammals as well. The term is used both to refer to the site of the colony, and to the collective group of animals which inhabits it. Rookeries can be both artificial and natural; archaeological evidence suggests that humans have been breeding animals in facilities that they have built specifically for this purpose for thousands of years.
One of the common names for the European crow is “rook,” a word which is probably onomatopoeic in origin, since rooks make a croaking, raucous sound. By 1725, people were referring to groups of breeding crows as rookeries, and the extension of the term to other species soon followed. Classically, people think of a rookery as a very noisy, chaotic place since a lot of animals are present and each creature is following its own agenda. Rookeries also tend to be very messy, due to excrement from animals and their young.
Most animals do not inhabit their rookeries year round. Rather, they travel to the rookery to meet up with other individuals, breed, and raise their young until the young are mature enough to manage on their own. In some cases, a rookery is removed from the animal's normal habitat and hunting grounds. While this might seem odd to nest far from home, this means that predators are less likely to attack the young animals, since a rookery is usually established in a remote, sheltered area.
Animals which breed in rookeries have adapted certain traits to help them deal with the intense environment. They are able to recognize the distinct cries of their young over the din of thousands of other individuals, for example. Some creatures even establish social routines, such as allowing other members of a rookery to care for their young. In a remote rookery, parents typically take shifts, where one parent looks after the young while the other seeks out food.
In the sense of an artificially established environment, a rookery is usually a structure which is designed to facilitate breeding of birds, like a dovecote. For humans, the difficulty in cleaning up and maintaining the rookery is outweighed by the convenience of having ready access to the creatures inside. A rookery which is established is typically smaller than a rookery in nature, with tens of individuals as opposed to hundreds or thousands.