Gulls playing helter skelter?

Do birds play? You have probably seen videos of crows sliding down snow and rolling down on car windscreens, or heard of the Australian black swans that enjoy surfing. These cases, and many others documented for a long time, are hard to explain in utilitarian terms (no feeding, or mating or predator avoidance is involved), it appears purposeless, and seem most fitting to actual play. Play behaviour, often solitary, some times involving objects, but also social, has been described in ten orders of birds including corvids, raptors and parrots amongst others. Some of this behaviour involve play flying, or flying in aerobatic ways. This afternoon, in the first mild sunny day of spring, thermals developed over the park, and a large mixed flock of gulls - Herring, Common, Lesser Black backed and at least a black-headed, later joined by a Carrion Crow - started to soar higher and higher, I could barely see them as they circled. My camera decided to run out of battery run out just at the same time (the photo above is the flock flying high this mid February).
 Then I noticed some gulls tumbling down. Not just gliding, but actually flipping over and going on their backs as they descended. Two of them seemed to be flying together, making a downward double spiral, opposite each other, occasionally tumbling. The Carrion Crow also dropped height by closing its wings as if enjoying the feeling of being kept up by the thermal. It was an exhilarating spectacle to watch, and the gulls gave the impression to be having a good time.