There are about 1 million territories of Carrion Crow in the UK. It’s considered a pest and farmers use a number of techniques to keep these birds away, e.g. scarecrows. A group of crows is known as ‘murder’ of crows, slightly more menacing than a ‘charm’ of finches or ‘parliament’ of owls.
The crow is slightly less gregarious than the rook, although they may form groups in roosts during the winter
Stephen's Wildlife 