Question Marks, their close ally the Comma, and Mourning Cloaks overwinter as adults. It isn't unexpected to see any of them flying about during a midwinter warm snap. In fact, I'm surprised that I didn't see more of them today. It was just the Question Mark in the photo, another Question Mark/Comma type (never got near enough to tell), and one Mourning Cloak. I would have loved to photograph the latter, which nearly hit me in the head as it coursed about, but it never alit and was soon gone.
These butterflies will retreat back to sheltered nooks and crannies with the next cold spell. But their appearance on a late winter day, with snow still persisting in shaded spots, was a nice harbinger of spring.