This behaviour is a part of the very brief courtship, before mating. The bird on the left is the male. Here the male seems to put his wing tenderly around the female. The following moments appear rather brutal to us, when the male hits the female's head quickly and quite violently with his bill. But these are all human interpretations of the courtship. In fact these moments are just part of the bonding activities of a pair, which finally must lead to an offspring.