The Arctic Tern is a very aggressive bird fiercely defensive of its nest and young. It will attack humans and large predators, usually striking the top or back of the head. It is capable of drawing blood on humans and repelling many raptorial birds, polar bears, and smaller mammalian predators such as foxes and cats. Recent studies have shown average annual roundtrip lengths of about 44,100 miles for Arctic Terns nesting in Iceland and Greenland and 56,000 miles for birds nesting in the Netherlands. These are by far the longest migrations known in the animal kingdom.