Swallow (Hirundo rustica) on the Nest

A beautiful day on the West Midlands, once the cloud had burnt back.  I was on crossing duty for my wife, helping her to get over a busy road on her horse.  It gave me the opportunity to have some more practice with the swallows that now have three nests in the stables.  I was quite chuffed when I checked this image and found it was sharp.  Hand held at 1/10th of a second on a 600mm equivalent lens is pretty impressive, even with image stabilisation.

I wonder of anyone can tell me what the swallow is doing in the extra image?  It's feet are on the corrugated sheeting but it appears to have rolled through 90 degrees to expose its belly to the sun.  It lay there for some minutes, making me wonder if it was injured, but then flew off without a care in the world.