Head Tucked Ducks

In observing ducks over the years, I noticed that there are times when they turn their heads around and tuck them in. I've seen them do it while standing and while sitting. At first I thought that maybe this was purely a grooming behavior, but I saw many sitting and doing this without moving much, which seemed to indicate they were at rest as well. Looking into it further, I have learned that they do this when sleeping and sometimes while awake to keep warm, which in these shots makes sense as it is winter on the icy river that they are congregating around.