Crows are very territorial and attack anything they perceive as a threat. Its the poor old herons who get it round here. They will fly at them from quite some distance off....
A friend of mine has a fantastic photo of a buzzard and a heron having a go at each other, on the ground. The heron is standing over the buzzard with its beak wide open, while the buzzard has its wings spread, to make itself look bigger.....