In reply to Neil Henson:
> I don't think the beak looks right for a Rook. See rooks a lot at motorway services (for some reason) and their beaks are very large and distinctive.
Agree: a rook's bill is proportionally longer and a tad slimmer than a crow's - "dart like" is one description I've come across to differentiate it from the crow's. The crow's upper mandible also dips downwards towards the tip much more than the rook's, giving it a similar, but slightly less intimidating, profile to the raven's massive, hook-tipped bill.
The nasal feathering is also clearly visible on the bill of the highlighted bird. Juvenile rooks have this plumage feature as well, up until about the end of their second year, but it's rather scruffier than on the adult crow - and actually tends to smooth out the transition between the "dart-like" bill and the forehead. It's when they lose the nasal feathering towards adulthood that they develop the distinctive 'notch' between bill and forehead which gives them the domed head shape. It's also at that age that the root of the bill turns whiteish.
Bottom line: if it were a rook, the plumage would make it a juvenile and thus pretty unlikely to be behaving aggressively towards a mature carrion crow.
We spotted a juvenile rook on an East Lothian beach the other week; it didn't look the smartest cookie in the box. Unlike the beachcombing crows, four of whom were very quick to notice two humans sitting down in a sheltered spot out of the biting wind and getting out their sandwiches. They hung around hopefully at a discreet distance - and yes, more than a few morsels of our lunch did end up being tossed in their direction, which they appeared to appreciate.