That's from late February to June, the nesting season, but the rest of the year, Cocker gradually concludes, they are roosting, often split between a late summer/early autumn roost and a later one - the latter reflecting a large concentration of birds. Roosts are usually in the middle of woods, even though these are birds that feed in grasslands (they've been recorded flying up to 32km to feeding grounds for the day - "as the crow flies"!), and he concludes that protection from weather, uninterrupted nights (they're usually in very calm places) and perhaps to some degree predators (although there's few of these now) , are an important part of the roosting behaviour.
But the biggest advantage for rooks in these huge gatherings is, he concludes, like in rookeries, the spread of information. It is, however, ore complicated than that. In the Yare Valley roost he studied, numbers ranged from 10,000 to 40,000 at the year's peak, numbers depending to a large degree on continental European visitors, who leave snow-covered territories for warmer western wintering grounds.
"For non-resident naive individuals the primary value might lie in following resident birds out to otherwise unknown feeding sites. The resident population may thus enjoy a dominant status in the froost and occupy more central locations in the trees. They can monopolise the best perches for thermal protection or defence against predators."
It's well known that corvids are intelligent - the Caledonian crows having shocked researchers by inventing tools - but Cocker also finds real world examples. There's the M4 rooks who've learnt to get to waste at the bottom of bin bags by gradually tugging them up the side of their frame, holding the bulked plastic under one foot, and those birds who'll bury food for later consumption. (For jays this is standard, for rooks seemingly more learned behaviour.) So we learn in the Aberdeenshire vernacular a self-seeded tree is "craw(rook)-sown".
Cocker explains how rooks mainly eat insects and arthropods in the upper topsoil, particularly worms, which explains theiir distribution as a bird of pastures and cropland. (Although they'll also eat small mammals, eggs and grain.) They stab 5-6cm into the ground, then uses Zirkeln, open-billed probing, to find their prey. Forest is not for them.
"They occupy vast swathes of the Mongolian and Manchurian grasslands, right through to the outskirts of Beijing and the shores of the Yellow Sea. To the west they've conquered the immense oceanic expanses of Russia and Asian grassland from about 160E to a point half a world away on the Baltic coast. ...Rooks were dependent on the westward spead of stock grazing and cereal agriculture ... to make their own entry into Europe. So when you next pass a rookery remember to stop and listen. Amond the spring-summoning cacophony you'll hear the faintest echo of a Neolitic axe."