The simple answer to the title question is: Yes, hawks scavenge in winter. In fact, winter is the only time that I have gotten camera-trap images of hawks scavenging.
I'm not saying that hawks don't ever scavenge in other times of the year, but I find it really interesting how often they do in the winter months.
Here are a few other pictures of it. The pictures are a bit blurry due to condensation on the lense.