On Xmas Day I was driving on the B3224 near Willett Hill in the Brendons.
Coming round a bend, I came upon perhaps a hundred corvids stood mostly in a dense flock on the road. At first I thought there must be some spilt grain but on investigation there was nothing in the way of feed. The birds flew up and joined a flock of hundreds, if not a thousand of their kind, wheeling around in the air.
I was more aware of the mass spectacle than the individual birds but I think they were mostly jackdaws and rooks. A few were mobbing a frustratingly distant and silhouetted large raptor with a wedged though not forked tail. They didn't seem to be focussed on a recently ploughed field or anything like that. Are such large flocks a well known phenomenon and why would they occur? Cold weather movement maybe?
One for Bill Gulliford perhaps!
Think there must be some very large movements of Corvids occurring ? Hard weather to come ? Just after reading your post I watched a continuous stream of Carrion Crows flying South South West over my garden in East Brent, they continued for about 10 minutes and must have numbered near a 1000 birds, half an hour later another contingent of 3 or 400 went by. Certainly not local birds, there are two pairs living in the village and I know them personally !
I agree with the suggestion that they were after salt. On occasion I have seen the odd one or two pecking at the salt lick blocks which are put out in the fields for cattle and sheep. As regards the location, I used to live at Willett when i was a child, and I remember several rookeries around there, especially a huge one at the back of Willett House.