Migrating Juncos

In our household, every few days we have to take out a bucket of scraps. It usually consists of eggshells from breakfast, potato peels from dinner and scraps of fruits and vegetables. This time of year, we find it convenient (and quicker) to simply dump these scraps a couple strides out of the house in the spot where the garden would be in the summer.

This year is no exception. One day, I was looking out the window and saw several birds in the snow, picking through the scraps.

They were Dark-eyed Junco’s (Junco hyemalis). These are smaller birds, often between 5.5 and 6.3 inches in length (14-16 cm)¹. These birds are best described as slate-colored on top and white beneath.

For me, I often think of these birds when I think about migration. This is because I rarely ever get to see these birds. The only time these birds make an appearance outside of my window is during the winter months, after everyone else has left. 

Dark-eyed Junco’s breed in the coniferous or mixed-coniferous forests of Canada, the western U.S. and in the Appalachian Mountains¹. Junco’s don’t stay in parts of this range year-round. Birds that breed in Canada and Alaska will migrate to the southern United States to escape the northern winters². During winter, they can be found in open woodlands, fields, parks, roadsides, and backyards¹.

As it turns out, these birds have some interesting aspects to their migration. For instance, a study in 1976 found that Dark-eyed Juncos will migrate different distances depending on what their sex is. The females have been found to migrate further south than the males do. The study found that the percentage of females was likely to increase with precipitation and temperature and decrease with the latitude and the amount of snowfall³. The same team reanalyzed the data and found that temperature best explained the presence of females in winter populations⁴.

Interestingly, the same distribution was seen in an altitudinal migration undertaken by a population of Junco’s that just migrated up a mountain. The males were more commonly closer to the peak, while the females wintered further down. Although the pattern is the same, the authors suggested that the driving force behind it is likely different⁷.

Why do these birds distribute themselves like this? One popular theory is that birds would migrate further depending on how dominant they were in the hierarchy. Males tend to be larger and more dominant than the females, who therefore are forced to fly further south since the males have taken all of the best spots closest to the breeding rounds. However, a study in 1989 suggested otherwise when birds that wintered north or south of each other were brought together. The birds were either paired with another bird of the same social status, or with a bird of a different status. If the dominance theory was to hold, the birds from the north would be dominant over the birds from the south. What was found instead was that it went both ways. Some birds from the north were more dominant, but some southern birds were more dominant in their pairings. This study suggested that dominance does not play an important role in determining the latitude of a bird’s winter residence⁸.

A few years later, another study provided evidence for another theory, the endogenous theory. In this study, juncos were kept in the same conditions to compare their behavior. The study found that females will begin fall migratory activity twelve days earlier than the males do. They are often still active after the males have stopped. This suggests that environmental influences don’t cause the differential migration seen in juncos. However, the physiological mechanism that controls this timing is not known¹³.

Another interesting aspect of their migration is their survival. A study in 1982 compared the migrations and winter mortality between northern and southern populations of juncos. As would be expected, populations wintering further north experienced higher winter mortality. Interestingly, the data suggested that the annual mortality of northern and southern winter populations were probably the same. Since the males are the ones who winter further north, you would expect less males and more females the following year. However, this study did not find this. This suggests that the southern populations suffer a heavier mortality during their longer migrations, offsetting the advantage of wintering in friendlier climates⁶.

A third interesting aspect is their timetable. Rather than everyone leaving at the same time, it was found that the birds left at different times. Adult juncos tended to leave before the immature juncos. The same study also found that females tended to precede the males⁹.

Birds in different regions during the winter have been found to be adapted to survive in different areas. Juncos in Indiana were found to be able to fast for an average of 43 hours⁵. In comparison, birds in Oregon, where the winter is milder, were able to fast only for a night and the following day¹². For both groups, winter survival is dependent on finding food.

In order to keep warm during winter, small birds need to increase their rate of aerobic metabolism to produce enough heat. Juncos do this by increasing the amount of oxygen they take in¹⁰. Juncos are also capable of increasing their cold tolerance during the winter, lowering the temperature in which they begin to experience hypothermia¹¹. Junco’s also increase their body mass by storing extra fat. They will also raise their metabolic rate during winter¹².

It was enjoyable watching these small birds as they hopped from one possible snack item to the next. Then, due to some unseen disturbance, they all left the scrap pile, flying into the cover offered by the trees.

If you would like to read about another bird that occasionally winters in Ohio, click here!

If you would like to read about a bird I saw earlier this winter, click here!

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“Dark-eyed Junco: Identification” All About Birds <https://www.allaboutbirds.org/guide/Dark-eyed_Junco/id&gt; Accessed 21 February 2021 ¹

“Dark-eyed Junco: Range Map” All About Birds <https://www.allaboutbirds.org/guide/Dark-eyed_Junco/maps-range&gt; Accessed 21 February 2021 ²

Ketterson, Ellen D., and Val Nolan, Jr. 1976. “Geographic Variation and Its Climatic Correlates in the Sex Ratio of Eastern-Wintering Dark-eyed Juncos (Junco hyemalis hyemalis).” Ecology 57:679-693 ³

Ketterson, Ellen D. and Van Nolan, Jr. 1979. “Seasonal, Annual, and Geographic Variation in Sex Ratio of Wintering Populations of Dark-eyed Juncos (Junco hyemalis).” The Auk 96: 532-536 ⁴

Stuebe, Miki M. and Ellen D. Ketterson. 1982. “A Study of Fasting in Tree Sparrows (Spizella arborea) and Dark-eyed Juncos (Junco hyemalis): Ecological Implications” The Auk 99: 299-308 ⁵

Ketterson, Ellen D. and Val 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.” The Auk 99: 243-259 ⁶

Rabenold, Kerry N. and Patricia Parker Rabenold. 1985. “Variation in Altitudinal Migration, Winter Segregation, and Site Tenacity in Two Subspecies of Dark-eyed Juncos in the Southern Appalachians.” The Auk 102: 805-819 ⁷

Rogers, Christopher, Tad L. Theimer, Val Nolan Jr. and Ellen D. Ketterson. 1989. “Does dominance determine how far dark-eyed juncos, Junco hyemalis, migrate into their winter range?” Animal Behavior 37: 498-506 ⁸

Chandler, C. Ray, and Robert S. Mulvihill. 1990. “Wing-shape Variation and Differential Timing of Migration in Dark-eyed Juncos.” The Condor 92: 54-61       ⁹

Swanson, David L. 1990. “Seasonal Variation of Vascular Oxygen Transport in the Dark-eyed Junco.” The Condor 92: 62-66 ¹⁰

Swanson, David L. 1990. “Seasonal Variation in Cold Hardiness and Peak Rates of Cold-induced Thermogenesis in the Dark-eyed Junco (Junco hyemalis).” The Auk 107: 561-566 ¹¹

Swanson, David L. 1991. “Seasonal Adjustments in Metabolism and Insulation in the Dark-eyed Junco.” The Condor 93: 538-545 ¹²

Holberton, Rebecca L. 1993. “An Endogenous Basis for Differential Migration in the Dark-eyed Junco.” The Condor 95: 580-587 ¹³