The dark-eyed junco (Junco hyemalis) is a species of junco, a group of small, grayish New World sparrows. The species is common across much of temperate North America and in summer it ranges far into the Arctic. It is a variable species, much like the related fox sparrow (Passerella iliaca), and its systematics are still not completely resolved.

Taxonomy

The dark-eyed junco was formally described by the Swedish naturalist Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae as Fringilla hyemalis. The description consisted merely of the laconic remark "F[ringilla] nigra, ventre albo. ("A black 'finch' with white belly") and a statement that it came from America. Linnaeus based his description on the "Snow-Bird" that Mark Catesby had described and illustrated in his 1731 The Natural History of Carolina, Florida and the Bahama Islands.

<blockquote>The Bill of this Bird is white: The Breast and Belly white. All the rest of the Body dark grey; but in some places black, inclining to Lead-color. In Virginia and Carolina they appear only in Winter: and in Snow they appear most. In Summer none are seen. Whether they retire and breed in the North (which is most probable) or where they go, when they leave these Countries in Spring, is to me unknown. [italics in original] The dark-eyed junco is now placed in the genus Junco that was introduced in 1831 by the German naturalist Johann Georg Wagler. The genus name Junco is the Spanish word for rush, from the Latin word juncus. Its modern scientific name means "winter junco", from the Latin word ' "of the winter".

Subspecies

Either 14 or 15 subspecies are recognized. Body mass can vary from . Juveniles often have pale streaks on their underparts and may even be mistaken for vesper sparrows (Pooecetes gramineus) until they acquire adult plumage at two to three months, but dark-eyed 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.

thumb|230x230px|Slate-coloured dark-eyed junco <br />(J. h. hyemalis)

The song is a trill similar to the chipping sparrow's (Spizella passerina), except that the red-backed dark-eyed junco's (see above) song is more complex, similar to that of the yellow-eyed junco (Junco phaeonotus). The call also resembles that of the black-throated blue warbler (Setophaga caerulescens) which is a member of the New World warbler family. Calls include tick sounds and very high-pitched tinkling chips. The migrant female J.&nbsp;hyemails experience delayed growth in the gonad to allow time for their seasonal migration. They then migrate down to the northeastern United States, where the resident subspecies is the Carolina dark-eyed junco (J.&nbsp;h.&nbsp;carolinensis). Female Carolina dark-eyed juncos have large ovaries and, therefore, do not experience gonadal growth delays because they are residents in the area. In winter, dark-eyed juncos are familiar in and around towns, and in many places are the most common birds at feeders. However, during the breeding season, insects comprise nearly half of the diet of adult dark-eyed juncos. Young dark-eyed juncos also depend on a diet of mainly insects.

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File:Junco nest.JPG|A dark-eyed junco nest with eggs

File:Junco hyemalis2.jpg|A fledgling pink-sided dark-eyed junco (J. h. mearnsi) at about one month after hatching, Yellowstone National Park

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Diet

Dark-eyed juncos mostly feed on insects and seeds, along with berries.

Evolution

Postglacial theory and diversification

thumb|On a tree branch in the middle of the winter.|187x187pxDark-eyed juncos have been widely investigated as a model for rapid speciation. This is due to exceptionally high phenotypic diversity, as seen in the large number of color patterns, over what seems to be a very short amount of time. Current estimates of dark-eyed junco (J. hyemalis) evolution place diversification from yellow-eyed juncos (J. phaeonotus) at 18,000 years ago, based on mtDNA. One theory for the cause of this expansion is postglacial theory. This theory claims ancestral junco populations expanded further north across North America as glaciers melted. Receding glaciers would open up many novel habitats, with new selective pressures. Under those conditions, natural selection can impact populations very strongly, since there are many open niches. Thus, even short periods of isolation can cause populations to diverge. Postglacial theory is supported by yellow-eyed and dark-eyed juncos sharing a dominant haplotype in their mitochondrial DNA, which indicates a recent burst in population. Yellow-eyed juncos are relatively reproductively isolated. Thus, a trait is more likely to be found in both species because of ancestry than gene flow, since the likelihood of interbreeding occurring often enough for the trait to be so common is much less likely than it being inherited from a single ancestral event. The range of red-backed (J. h. dorsalis) and grey-headed juncos (J. h. caniceps) in the south of North America also provides evidence, as the two seem to represent successive steps in developing dark-eyed forms. The red-backed junco is very similar to the yellow-eyed junco in appearance. It also has the most southern range of the dark-eyed junco species. Under postglacial theory, this population would be older than the others, since populations expanded northwards. The gray-headed junco, which is found further north, has the same lighter beak as the rest of the junco complex. Together, they show the number of dark-eyed junco-like traits increasing as they move north. If the postglacial theory applies, northern junco subspecies would have diversified later. This makes their wide range of coloration more notable, since it would have to arise even faster.

Oregon junco group diversification is likely a result of both genetic drift and selection. One of the most notable differences of the UCSD population is that they do not migrate like other populations of Oregon juncos. Instead, they remain on campus year-round. This differentiates them from other junco populations that migrate to the UCSD campus only in the winter. This lack of migration was likely influenced by the San Diego area's mild, Mediterranean climate. This climate also results in longer breeding seasons than exist at higher latitudes. This allows UCSD juncos to have as many as four broods per year, rather than the one or two of nearby populations. Greater brood size, in turn, may have acted as a selective pressure for parental involvement. Since birds breed more within a season, initial mating is less important, and males who are involved in care are more likely to breed again in that same season. UCSD resident birds also flock in pairs more often than overwintering birds, which may have resulted from the same selective pressures.

Despite other junco populations existing nearby, the populations diverge much more than expected. The degree of difference between the UCSD juncos and other local juncos was closer to what would be expected with geographic isolation. Since the nearest populations (located in the mountains) are of the subspecies J. h. thurberi, it was assumed the UCSD birds came from an ancestral J. h. thurberi population. However, genome analysis reveals that the population was likely established from the coastal subspecies J. h. pinosis 20–30 generations ago, which are conditions that make the founder effect very likely to be relevant. This shift in parasite pressure could contribute to physiological or behavioral adaptations distinguishing urban juncos from their rural counterparts.

A paper published in December 2025 reported that a study of the dark-eyed junco population at the University of California, Los Angeles (UCLA) reports that the bill sizes and shapes of the birds changed during the anthropause. Prior to the COVID-19 restrictions the bill morphologies of the junco campus population were significantly different from local wild populations, thought at the time to be at least partly due to their diet being mostly the remains of processed foods consumed by campus humans. Birds born during the lockdown had bills that more closely resembled local wildland birds. After the lifting of pandemic restrictions the bills of hatchlings quickly returned to pre-Covid morphology.

References

  • Dark-eyed junco ID, including sound and video, at Cornell Lab of Ornithology
  • Dark-eyed junco—Junco hyemalis —USGS Patuxent Bird Identification InfoCenter
  • Juncos: What do we know?—An expert discussion of atypical individuals, the fine points of subspecific identification, and the proper understanding of the cismontanus population, from the ID-Frontiers mailing list (January 2004), supplemented with photographs and paintings.