But why does this particular gull chase the eagle over another gull? The gull does not want to eat the other bird,right? Do you mean that the gull wants to just chase off the eagle, and it just happened to be while the eagle had another gull in its grasp?
The chasing gull does not care about the prey gull, but only about driving off the hated eagle. Eagle gone, more fish for chasing gull who knows that as long as eagle has prey gull clutched in its claws, he is free to harass the eagle with impunity.