The diet of the dark-eyed junco consists mostly of seeds, but during the breeding season it also feeds on insects and other invertebrates, including beetles, moths, caterpillars, grasshoppers, flies and spiders (3) (4) (6) (7). It also eats some berries (6) (7). The dark-eyed junco typically forages on the ground, hopping around the forest floor or venturing onto lawns, and sometimes scratching in leaf litter or scraping away snow to find food (3) (4) (6) (7). It may sometimes fly up from the ground to catch insects from low vegetation (3) and will also visit bird feeders, although it usually forages on the ground beneath (7).
During the winter, the dark-eyed junco forms relatively large foraging flocks, often with other small bird species (2) (3) (5) (6). In some areas, several dark-eyed junco subspecies may mix together in these winter flocks (3) (5). In contrast, male dark-eyed juncos are highly territorial during the breeding season, singing from prominent perches and chasing away intruders (3) (4) (6) (7).
The dark-eyed junco generally nests between May and July, although breeding is usually later in the north of its range than in the south (2). This species is monogamous (4), and during courtship both members of a pair may hop on the ground with the wings drooping and the tail spread out, showing the white outer tail feathers (6) (7).
The female dark-eyed junco selects the nesting site, which is typically in a small cavity on sloping ground, on a rock face, among tangled roots, under a fallen tree, or even beneath a building (2) (3) (4) (5) (6) (7). Nests may also occasionally be built above the ground, on a horizontal branch, on a building ledge or in a hanging flowerpot (3) (4) (7). The nest is built by the female and usually consists of an open cup of twigs, rootlets, dried leaves and moss, lined with grass, moss, feathers or animal hair (3) (4) (5) (6) (7). Nests inside small holes or cavities may consist of only a thin lining (3) (4).
The dark-eyed junco lays around three to six eggs, which are white, greyish or pale bluish-white, usually speckled with brown (2) (3) (4) (5) (6) (7). The eggs are incubated by the female for 12 to 13 days (3) (4) (6). The young dark-eyed juncos are fed by both adults and leave the nest at about 9 to 13 days old (2) (4) (5), becoming independent around 2 to 3 weeks later (4) (6). In some areas, the adults may go on to have a second or even a third brood in the same year (2) (3) (4) (5) (6).
The dark-eyed junco usually breeds from about a year old (4) and has been recorded living for up to 11 years (3) (4) (6).