Awesome shot. That’s an Arctic Tern (the fox is Arctic as well). Terns nest on the ground and are very (”fiercely”, as WP puts it) protective of their nests and young. The fox may have been after the birds’ offspring right before this happened, but it may just as well have gotten itself too close to the nest without knowing about it, the terns won’t tolerate anyone nearby. (They attack people just the same.)