The Dark-eyed Junco is the best-known species of the juncos. This bird is common across much of temperate North America and in summer ranges far into the Arctic. It is a very variable species, much like the related Fox Sparrow and its systematics is still not completely untangled.
Adults generally have gray heads, necks, and breasts, gray or brown backs and wings, and a white belly, but show a confusing amount of variation in plumage details. The white outer tail feathers flash distinctively in flight and while hopping on the ground. The bill is usually pale pinkish.
Males tend to have darker, more conspicuous markings than the females. They are 20 cm in length. Juveniles often have pale streaks and may even be mistaken for Vesper Sparrows until they acquire adult plumage at 2 to 3 months. But junco fledglings' heads are generally quite uniform in color already, and initially their bills still have conspicuous yellowish edges to the gape, remains of the fleshy wattles that guide the parents when they feed the nestlings.
The song is a trill similar to the Chipping Sparrow's except that the Red-backed Junco's song is more complex, similar to that of the Yellow-eyed Junco. Calls include tick sounds and very high-pitched tinkling chips.