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.
The Bill of this Bird is white: The Breast and Belly white. All the rest of the Body black; but in some places dusky, 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. italicsThe type locality was restricted to South Carolina by the American Ornithologists' Union in 1931. 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 Juncaceae, from the Latin word juncus. Its modern scientific name means "winter junco", from the Latin word "of the winter".
These two or three subspecies have dark slate-gray heads, breasts and upperparts. Females are brownish-gray, sometimes with reddish-brown flanks. They breed in the North American Taiga from Alaska to Newfoundland and south to the Appalachian Mountains, wintering throughout most of the United States. They are relatively common across their range.
This subspecies has a medium-gray head, breast, and upperparts with white wing bars. Females are washed brownish. It has more white in the tail than the other 14 subspecies. It is a common endemic breeder in the Black Hills of South Dakota, Wyoming, Nebraska, and Montana, and winters south to northeastern New Mexico.
These eight subspecies have blackish-gray heads and breasts with brown backs and wings and reddish flanks, tending toward duller and paler plumage in the inland and southern parts of its range. Oregon dark-eyed juncos are also less commonly known as brown-backed dark-eyed juncos. This is the most common subspecies group in the West, breeding in the Pacific Coast Ranges from southeastern Alaska to extreme northern Baja California and wintering to the Great Plains and northern Sonora. An unresolved debate exists as to whether this large and distinct subspecies group is actually a separate species with eight (or nine, see below) subspecies of its own.
Sometimes considered a ninth subspecies in the Oregon/brown-backed group, this subspecies has a lighter gray head and breast than the eight Oregon/brown-backed dark-eyed juncos, with contrasting dark lores. The back and wings are brown. It has a pinkish-cinnamon color that is richer and covers more of the flanks and breast than in the eight Oregon/brown-backed dark-eyed juncos. It breeds in the northern Rocky Mountains from southern Alberta to eastern Idaho and western Wyoming and winters in central Idaho and nearby Montana and from southwestern South Dakota, southern Wyoming, and northern Utah to northern Sonora and Chihuahua.
This subspecies is essentially rather light gray on top with a rusty back. It breeds in the southern Rocky Mountains from Colorado to central Arizona and New Mexico, and winters into northern Mexico.
Sometimes included with the gray-headed dark-eyed junco proper as part of the gray-headed group, this subspecies differs from it in having a more silvery bill with a dark-colored upper mandible and a light-colored lower mandible, a variable amount of rust on the wings, and pale underparts. This makes it similar to the yellow-eyed junco ( Junco phaeonotus), except for the dark eyes. It is found in the southern mountains of Arizona and New Mexico. It does not overlap with the yellow-eyed junco in its breeding range.
Males tend to have darker, more conspicuous markings than females. The dark-eyed junco is long and has a wingspan of .Rising, J.D. (2010) A Guide to the Identification and Natural History of the Sparrows of the United States and Canada. Christopher Helm Publishers, London, . Body mass can vary from . Among standard measurements, the wing chord is , the tail is , the Beak is and the tarsus is . Sparrows and Buntings: A Guide to the Sparrows and Buntings of North America and the World by Clive Byers & Urban Olsson. Houghton Mifflin (1995). . Juveniles often have pale streaks on their underparts and may even be mistaken for ( 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. 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. It is known among bird song practitioners as an excellent bird to study for learning "bird language".
A flock of dark-eyed juncos has been known to be called a blizzard.
Oregon junco group diversification is likely a result of both genetic drift and selection. Geographically isolated subspecies, like J. h. townsendi and J. h. pontilis, had low genetic–environmental association. This meant that most of the differences between them did not grant a measurable benefit to fitness. Instead, most differences were likely due to isolation and small population size. Those would increase the impact of genetic drift, increasing differences between the subspecies. Isolation barriers created by unsuitable desert habitat likely led to this lack of gene flow. Other subspecies, like J. h. pinosis and J. h. thurberi, were much less geographically isolated. Their ranges often overlap, forming zones of intergradation where both subspecies exist and interbreed. Instead of geographic isolation, differences between these subspecies are probably driven by adaptation. This is supported by J. h. pinosis populations correlating with isothermality, or how much temperatures shift from day to night and season to season, and negatively with elevation. This indicates that they likely adapted to those specific conditions. Reproductive barriers could then come from those adaptations and differences in preferred habitat, rather than lack of interaction. There was also notable gene flow between J. h. thurberi, J. h. shufeldti, J. h. montanus and J. h. oreganus. These subspecies formed a latitudinal gradient, where subspecies bleed into each other and decrease or increase in a number in patterns correlating to latitude. Divergence was likely driven by local adaptation along the path north. Overall, it is likely that there is no single mechanism driving the historical diversification of dark-eyed juncos. Different subspecies feel different evolutionary forces with more or less frequency. However, multiple forces compound to drive divergence in all or most of the subspecies. It is likely that in the case of the dark-eyed juncos, these multiple factors have worked to accelerate change. Sexual selection and geographic isolation work to reinforce and amplify small changes established by genetic drift and natural selection. The exact mechanisms behind the speedy diversification of juncos remain a subject for more research.
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 conclusion is further supported by phenotypic similarities: the UCSD birds and J. h. pinosis share similar degrees of white in their tail and nonmigratory behavior. This realization undercuts the idea that urbanization drives the lack of migration, but not that it caused many of the other changes. More evidence comes from the UCSD birds occasionally interbreeding with overwintering J. h. pinosis individuals, but not J. h. thurberi. This is in spite of J. h. thurberi juncos being much more common in the area. If the UCSD population were interbreeding with both, it would most likely have been observed already. Since the founding population was quite small, and the nearest J. h. pinosis population far away, genetic drift likely had a large influence on how the population developed. However, a strong association between habitat variables and functional genes was found. This association is a sign of natural selection and adaptive traits, not drift alone. This indicates that genetic drift is likely not the only evolutionary force at play. Selection is likely magnifying changes initially driven by drift, allowing the UCSD population to diverge from its neighbors very quickly.
More proof of adaptive in the UCSD population can be found on the gene level. Two of the most differentiated genes between the ancestral J. h. pinosis population and the UCSD birds were linked to beneficial traits for urban environments. Specifically, an ABCB6 allele conferred better tolerance of heavy metals. As heavy metal poisoning is a documented issue for urban birds, this gene likely increases fitness in urban environments. Variation in KCNQ4 is linked to high-frequency echolocation in , and seems to correlate with making higher pitched calls. This is helpful in an urban environment because low-frequency urban noise often drowns out bird calls. With higher pitched vocalizations, calls would be less likely to get lost in that noise. Given the theorized survival advantages, the prevalence of these traits is likely to be driven by natural selection. Research remains to be done on which alleles in the UCSD population serve an adaptive function, and which are likely just consequences of drift. Especially with the discovery that they are more closely related to J. h. pinosis, much pre-existing research may be less relevant. However, they are still a very promising model system for how birds adapt to urban ecosystems.
One such factor may be differences in parasite communities. A study comparing urban and non-urban junco populations across California found that specialist blood Parasitism like Haemoproteus are nearly absent in urban birds, while generalist parasites such as Plasmodium remain common and increase with rainfall. This shift in parasite pressure could contribute to physiological or behavioral adaptations distinguishing urban juncos from their rural counterparts.
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