In the Arctic, most bird species have to breed on the ground. Some manage to find a place on a cliff, but that limits the places where you can breed. Birds that breed on the tundra have a lot of space to choose from but are also vulnerable for predation. Often they solve this by breeding on small islands (which of course limits the space they have to breed), or by making sure the eggs and chicks are well camouflaged.
There is another way. That is to be very aggressive. Parasitic Jaegers (or Arctic Skuas) opt for that choice. When you come close to their nest, they will first try to lead you away by pretending to be injured. They will make a sad noise and walk away with flapping wings, pretending to be injured. When this doesn’t work and the intruder continues towards the nest (knowingly or unknowingly), they will start to attack. And they do not mind what is coming too close, whether it’s an Arctic Fox, a harmless Reindeer, a human or a Polar Bear. Even though the bear in this case just kept on walking, she clearly didn’t like being bashed on her head all the time, often closing her eyes the moment the skua made another attack.