Undercarriage failure?

I watched this black-headed gull fly in and make a very neat landing on just one leg.  My initial reaction was that the poor thing had lost a leg however a few moments later he switched legs and stood on just the other one.  Looking at the others in the group most of them were standing on a single leg with the other leg tucked up in their feathers.  I am guessing this is perhaps a heat saving measure.