junco or snowbird, small seed-eating bird of North America closely related to the sparrows. Juncos have white underparts and gray (sometimes also brown) backs. They travel in flocks. The dark-eyed junco is the most common species, comprising in E North America the subgroup sometimes called the slate-colored junco, and several Western varieties. Juncos belong to the genus Junco and are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Passeriformes, family Emberizidae.