Neil Gaiman

Rook: corvius frugilegus. Also a word meaning to cheat or steal. Also a piece in chess. Rooks are the most social of the corvidae. They build nests in rookeries (an obsolete name, incidentally, for a ghetto of thieves and whores), many hundreds of birds to a tree. They have enough of a language that even humans can tell the difference between their danger calls and their all-clear calls. They can imitate human speech. But there's something else: the mystery. It's a mystery from which we derive the collective noun we use for these birds. Like a murder of crows, a tiding of magpies, an unkindness of ravens...a parliament of rooks.

You'll get a field. Empty. Suddenly, the sky is black with birds, and they fall like a ragged black rain onto a field, covering it completely. Or almost completely... in the center of the field, there's an empty space. And in the middle of that space sits one lone rook. It caws, and calls, and caws some more. Then thousand little eyes stare at it, unflinching. Sometimes they call out, as if they're asking questions. It's like a parliament. It's like a trial. The lone rook continues to caw and the others wait. This can go on for hours. From dawn till near dusk.

Only one of two things could happen. Either the birds take wing as one, leaving the lone rook alone in the field...or, again as one, they fall on the bird, and peck it to death. Why? It's a mystery.

- Neil Gaiman, 'Parliament of Rooks', The Sandman Fables & Reflections