Driving home through the lanes at the end of a day of teaching I passed two trees both full of rooks.
They were oak trees, nearly bare of leaves, and every exposed branch had at least one rook, all silouhetted against against the pale late afternoon sky.
"A parliament?", I mused, hurrying home. Not sure, thought that might be owls.
Turns out that the collective noun for rooks can be a parliament or a building.
Both words seem appropriate; the rooks were packed in like residents of a high-rise block of flats, on different levels in the trees (the penthouse apartments seemed the most popular) all in motion, as though some great debate (Brexit, maybe?) was in progress.