A Muster Rook is a rook of ordinary size, glossy black with a purple sheen, its face bare and grey-white around the base of a long, pale beak. In the zone a rook’s caw comes out as a short, harsh word, and leaves the same brief white afterimage in the air that a shouted order does. A flock wheels above the battle in thousands, dark against the strobing light, and their cries fall over the field like a second army’s. They smell of feathers and the turned earth of a field.

They are seen only over battlefields where the zone has woken, in flocks that arrive as the fighting begins and wheel overhead until it ends. Where they go when the zone fades, no one knows.

Rooks repeat what they hear, and in the zone they hear commands. A flock takes up the clearest, simplest orders, the ones the zone itself favours, and caws them back across the field, over and over, a little out of time with the officers who gave them. They settle on the dead after the fighting, as rooks do.

In the Grammar of War a command’s force grows with the number who speak and believe it, and a flock of rooks repeating an order adds weight to it, without any of the judgement of the one who gave it. An order to hold that the rooks keep cawing after the general has called the advance sets the two against each other, and the zone’s Law of Contradiction follows: soldiers freeze, the ground cracks, the air shimmers.

Nothing is known of their nesting or their lives outside the zone. Some say they are ordinary rooks the zone takes up as it takes up soldiers; others that they are part of the zone, and fade with it.

They give nothing. Commanders who know the zone give their orders quietly and briefly, as its own advice says, to give the rooks less to take up; some have tried to turn the rooks' repetition to their own use and found it serves no side. Old soldiers say "the rooks are drilling" of a battle going wrong.

On the Material Plane rooks over a field before a battle are an old omen of slaughter, and in some armies an officer whose orders the rooks seem to repeat is held to be favoured. The tale that the rooks are the voices of the Word-Generals is common among soldiers who have survived the zone, and nothing shows it is true.

One battle, they say, the rooks won: a flock that cawed "Flee!" over both armies until neither side was left on the field, and a valley that was spared because of it.