I've always struggled to tell the difference between Rook, Crow and Raven, though I've had Jackdaws sorted for years (notably smaller – black cap above more slightly silvery head and neck). To remedy this situation I've really been trying to get to grips with the differences and positively identify large black birds over recent months. Here's what I'm fairly sure is a rook, on a feeder at the Lost Gardens of Heligan in Cornwall.
This is fairly classicly an adult rook by virtue of the grey area around the base of the bill, which itself is long and pointy compared to that of a carrion crow. That said, the head shape seems more typically crow-like to me (less of a crown) and in this example there's no evidence of the 'baggy trousers' that usually distinguishes a rook. As a result I'm left still fairly puzzled, but I'm going with my call that this is a rook. Furthermore, there were lots of them around all together and apparently this is a sign of rookedness.