The prey looks rather large, but do you suppose it is the fledgling of the gull trying to stop the eagle? Do you know of some other reason the gull would go after the eagle?
Gulls and eagles battle it out all the time. Big flocks of gulls drive off eagles. However, weak/young/unfocused gulls are prey for eagles.
Gulls eat fish, eagles eat fish, eagle eats gull leaving more fish for eagles. Gull runs because it wants to live to eat more fish, fights eagle who is eating gulls’ food, doesn’t give a crap who eagle eats as long as not him or his fish.