There are a handful of species of Juncos in North America. Most familiar is the Dark-Eyed Junco, formerly known as the Slate-Colored Junco. Why they changed that name is totally beyond me. Who can see their eye color?
Flocks of these sparrows - yes, they are in the sparrow family - are common around the US during migration and in winter, generally feeding on or near the ground, in fields, edges, and brush. The dual flash of white in their tail is an easy field mark in flight. They enjoy our bird feeders, and they do not mind snow at all.
They breed pretty much throughout Canada. Their arrival in the US in November, along with the White-Throated Sparrows, is a sign that winter is coming. They will begin to push north in March.
You can read more about these cheerful critters here.
Photo courtesy of R. Hays Cummins