Crows & Seagulls 'fighting'

[peckpigeon]

Hello, I was wondering if anyone could tell me what the behaviour of the seagulls and crows I observed today means - if anything at all. I have never seen it before so found it interesting. I heard a strange caw from a crow coming overhead and looked up and saw that it was being pursued by two seagulls. They came round again sevaral times and try to attack the crow pecking at it with their beaks in mid flight - the crow would also give as good as it got. Perhaps this happens alot? or they were defending their territory or a cache of food somewhere? I know a pair of crows nest annually in a tree nearby. Thanks for any contributions.