For birders, winter is a time for standing on frozen shorelines and struggling to identify distant waterbirds. That means focusing on bill shape, head shape, body proportions, and behavior, among other things. It’s understandable that you would be discouraged when you come across a bird that’s sleeping because all of those features will be partly or completely hidden. On the bright side, a sleeping waterbird reveals other useful field marks and in some cases is easier to identify than a bird that is awake.
Even though part of the head is hidden on a sleeping bird, what you can see is still useful. A bird that is awake is constantly moving — preening, stretching, alert, relaxed, courting, sparring, diving, etc. The head shape changes as feathers are raised and lowered with the bird’s mood and activity. A sleeping bird is just asleep. Its head shape stays the same.