I only hope that when it happens to me I don't look quite so daft as this young Blackbird. It spent loads of time just near the bird bath wallowing in the hot sunshine.
Once it came out of its trance it was off to the water too cool down. I remember those days on the beach when I did much the same thing as a youngster!
Lots of other birds also partook of this ritual. The Dunnock above manage to stay a little more dignified. I also say Magpies, Collared Doves and Wood Pigeons indulging in similar behaviour. There are two things that are probably happening here. The bird is panting, and has its beak open as a form of heat regulation. By spreading its feathers and often with open wings the heat probably encourages any parasites, picks etc to come to the surface, where the bird can remove them when it preens.
I didn't see the Robin sun-bathing in this way but it did seem extra keen to keep returning to the pool to bathe.