Skuas are well known to fiercely defend their nests. Intruders will know they have come to close to their nest. First they will make threatening flights above the head of the intruder, but if you get even closer, they really start to hit you. I’ve heard stories about people being knocked unconscious by a Great Skua.
As I walked on Fair Isle, I apparently had walked too close to a nest and the parents really wanted to let me know. I quickly moved back a little, afraid for my own head, but also not wanting to disturb the birds too much. As I had moved back, one of the adults made one more threat flight to me, before returning to the nest.