Several gulls showed the same strange behaviour just after they had flown up from a gathering place on the river. Apparently, after having gained some hight, they suddenly stopped and seemed to perform a strange 'sur place' in they air, while they were violently shaking their feathers dry. They often repeated this a few times, before flying to their sleeping roosts. Some individuals seemed to give some weird footkicks in the air, before scratching their plumage with their feet. I was lucky to be able to take a picture of this surrealistic kind of air ballet performed by an Audoin's Gull.