A rookery is a noisy, crowded place where certain birds or animals gather to nest and raise their young. The word originally described colonies of rooks (large black birds related to crows) that build hundreds of nests close together in treetops, creating a constant racket of cawing and flapping.

Over time, the word expanded to describe breeding colonies of many different species. Penguins waddle into massive rookeries on Antarctic shores, sometimes with tens of thousands of birds packed together. Seals haul out onto rocky beaches to form breeding rookeries where they bark and jostle for space. Even herons, gulls, and other seabirds establish rookeries in trees or on cliffs.

What makes a place a rookery? Three things: lots of the same species gathering together, the purpose of nesting or breeding, and usually quite a bit of noise and commotion. A single bird's nest isn't a rookery, but thousands of birds nesting in one area certainly is.

People sometimes describe crowded, noisy human places as rookeries too. A cramped, bustling apartment building in old London might have been called a rookery. It's not the most flattering comparison, but it captures the sense of many individuals crammed together in barely organized chaos.