Protecting the nest site.
The chase is constantly on for all raptors, gulls, crows, ravens, and vultures, even if these trespassers have no interest in the nest they can not fly over it without a serious confrontation. Once there is any type of chase the screaming begins and doesn't end until the trespasser is long gone. This is the male who just returned from a chase the nest is directly below, the female was on the nest at the time.
