Why do some birds hide their bills during courtship displays?

In the context of reproductive behaviour, some birds hide their bills during courtship displays because the bill is their primary weapon. By concealing it, the bird reduces the potential for aggression and signals a non-threatening, friendly intention to its potential mate.1

This behavior is a form of display—a ritualized social signal—that helps mitigate the natural hostility or wariness that can occur between potential partners during the breeding period. Because courtship often involves close proximity, such displays serve as an essential communication system to facilitate the transition from individual interaction to the consummatory sexual act.1