Selected Birds of the Boreal Forest of North America



Dark-eyed Junco, Slate-colored subspecies
© Gay Bumgarner



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breeding and wintering grounds.
Dark-eyed Junco
Junco hyemalis

Breeding and/or wintering virtually throughout the continental U.S. and Canada, the Dark-eyed Junco, sometimes called the "Snowbird," is one of the most familiar birds at  North American bird feeders.    It is also one of the most variable, with 15 subspecies in five distinguishable groups that differ from one another sufficiently in size, proportions, plumage, vocalizations, migration patterns, and ecology that they once were considered distinct species (Slate-colored, Oregon, Gray-headed, White-winged, and Guadalupe Juncos).  It is abundant in the boreal forest during the breeding season.

Habitat
With such a broad range and variable ecology, it is not surprising that the Dark-eyed Junco occurs in a staggering array of habitats.  In the boreal region, it nests in both coniferous and hardwood forests, especially those with relatively sparse tree cover and dense understory.  Elsewhere, it occurs in virtually any habitat with sufficiently dense low vegetation for nesting.  Throughout its breeding range, it seems to respond favorably to  events such as fire and logging that restart forest succession,  resulting in increased density of shrub and ground cover.  During migration and winter it can be found in an even broader range of habitats, including areas with relatively little cover such as harvested crop fields, grassy lakeshores, lawns, and road margins.

Diet/Feeding Behavior
The Dark-eyed Junco is similar to many other sparrows in its diet and foraging ecology.  Winter diet is primarily weed and grass seeds but occasionally small fruits and waste grains; breeding-season diet is primarily insects and spiders,  more or less in proportion to availability.  On a year-round basis, the diet is usually about three-fourths seeds and one-fourth arthropods.  Juncos employ a variety of foraging methods, including gleaning, pecking, and scratching.  The species forms flocks, often numbering in the hundreds, outside the breeding season; these flocks are strongly hierarchical, with larger birds dominant over smaller ones and older birds over younger ones.  Thus, males, which are larger, typically dominate females, adults are dominant over young birds, and large subspecies dominate smaller ones where they coexist.  Foraging behavior shows typical risk-avoidance characteristics, with individual birds becoming less vigilant with increasing light intensity (when predators are less likely to attack) and flock size.  Well-fed birds are less risk-averse than hungry ones.

Breeding
Sixty-six percent of the global population of Dark-eyed Junco breeds in the boreal forest of North America.  The Slate-colored form nests throughout the boreal region, from the shores of the Bering Sea to the Atlantic, and south through the Appalachians.  Other forms nest throughout the western mountains to northern Baja California and west Texas.  The breeding system of the Dark-eyed Junco is categorized as "social monogamy."  Although birds of both sexes tend to have only one "social mate" with which they nest and defend a territory, they frequently copulate with neighboring birds.  As a result, males raise many young that are not their own.  Females that lose their mates quickly re-pair, often with neighboring males who may sire broods with their new mates while also rearing young with their original mates.  Such males rarely care for their new mates' original young, even though some likely are their own offspring.  The nest is usually on the ground, often on a sloping bank, and concealed under a protruding rock, among roots, under a log, or at the base of plant stems, but occasionally in a tree.  Usually four eggs are laid, with larger clutches more frequent early in the season and smaller clutches late.  The young are able to run before they can fly, if necessary, because their legs develop quickly.  Southern populations normally attempt to raise two broods per year.  Nest predation, particularly by rodents, is very high, and overall productivity is highly correlated with rodent density.

Migration/Winter Range
Most (including all northern) populations are migratory.  The peaks of migration are in October and March/April.  The winter range extends across southern Canada and virtually all of the lower 48 states into northern Mexico.  Juncos are found year-round throughout much of the West, Great Lakes region, Northeast, and Appalachians, though these areas experience complete or partial turnover of individuals.  Adults, especially females, tend to migrate farther (and begin migration earlier) in eastern populations.  For example, winter junco flocks are 20 percent female in Michigan and 72 percent female in Alabama.  Western flocks are less segregated due to greater mixing of populations.  Some montane populations exhibit altitudinal migration, with age and sex differential mirroring that of latitudinal migration.  Female mortality is probably higher than that of males during migration due to the greater distance but lower in winter because of milder weather; thus, net survivorship and sex ratios on the breeding grounds are about equal.   Migration is typically at night.  Flock composition probably changes from day to day during migration; individuals apparently seek one another out for group foraging and roosting during the day and may depart en masse at night but do not stay together during flight.

Conservation Issues and Status
Although still abundant in most areas, the Dark-eyed Junco declined at a rate of nearly 2 percent annually from 1980 to 2002 according to the Breeding Bird Survey.  Declines were particularly severe in Canada, especially western.  Similarly, the species declined nearly everywhere on Christmas Bird Counts from 1959 to 1988, with significant increases only in  Quebec and a few northern states, perhaps reflecting a shift in winter distribution resulting from global warming.  Since juncos prefer areas of open tree cover, including clear-cuts, logging per se probably is not the cause of population declines in this species.  Suspected causes include aerial spraying of forest insecticides and forest regeneration on the breeding grounds and as-yet unidentified factors during migration and winter.


References

Balph, M.H. 1979. Flock stability in relation to social dominance and agonistic behavior in wintering Dark-eyed Juncos. Auk 96:714-722.

Blancher, P. 2003. Importance of Canada's boreal forest to landbirds. The Canadian Boreal Inititative and the Boreal Songbird Initiative.

Ketterson, E.D., and V. Nolan, Jr. 1982. The role of migration and winter mortality in the life history of a temperate-zone migrant, the Dark-eyed Junco, as determined from demographic analyses of winter populations. Auk 99:243-259.

Nolan, V., Jr., E.D. Ketterson, D.A. Cristol, C.M. Rogers, E.D. Clotfelter, R.C. Titus, S.J. Schoech, and E. Snajdr. 2002. Dark-eyed Junco (Junco hyemalis). In The Birds of North America, No. 716 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The American Ornithologists' Union.

Sauer, J.R., J.E. Hines, and J. Fallon. 2003. The North American Breeding Bird Survey, Results and Analysis 1966-2002, Version 2003.1, USGS Patuxent Wildlife Research Center, Laurel, MD.

National Audubon Society. 2002. The Christmas Bird Count Historical Results
[Online]. http://www.audubon.org/bird/cbc.

Smith, K.G., and D.C. Andersen. 1982. Food, predation, and reproductive
ecology of the Dark-eyed Junco in northern Utah. Auk 99:650-661.

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