Birds (according to KennyW) sunbathe.
They fluff up there feathers and on an hot and sunny afternoon they will lie down on a path and enjoy the sun's rays penetrating through to their bodies.
This would be unbecoming behavior for a hummingbird, so he/she (she in this case) does the same but on a branch. Look at her head to one side, she's loving it.
I uploaded a video too