Part 1: Introduction to the Dark-Eyed Junco

DESCRIPTION AND IDENTIFICATION

The Dark-eyed Junco (Junco hyemalis) is one of the most abundant birds in North America (Bailey et al., 2017) with recent estimates suggesting the entire population is somewhere around 630 million individuals (The Cornell Lab of Ornithology, 2019). This passerine (order Passeriformes) is part of the family Passerellidae also known as the New World Sparrows (The Cornell Lab of Ornithology, 2019).  Often recognized by its burglar-like appearance here in BC due to its conspicuous black hood, the colouration of Dark-eyed Juncos actually varies greatly with geographical location; so much so that 5 out of the 6 recognized forms were considered different species until the 1980s (Cornell Lab of Ornithology, 2019). Although the plumage pattern of juncos differs depending on their location, juncos are generally dark-grey or brown in colour. Despite this variation in plumage, it is easy to identify a junco based on their flashy white outer tail feathers that are easily spotted while they’re in flight (McGlothlin et al., 2007), as well as their light pink bill.

If you were to take a walk through the woods at this time of year, you would likely hear the soft chirps of local Dark-eyed Juncos as they wander through the brush, possibly trying to encourage other juncos to follow.

Male juncos across the different subspecies typically have repertoires of 2-8 song types, and are often slightly improvised or modified by the individual (Reichard, 2014). During breeding season, male juncos don’t rely solely on their plumage to attract and stimulate potential mates, songs play an important role in mate selection. The advertising song of a junco is usually a single trill, consisting of a single pitch (Audubon, 2019).

Certain calls given by the dominant male junco are used as aggressive warnings for other male juncos to essentially “back off” (The Cornell Lab of Ornithology, 2019). Some studies have shown that despite similarities between the 6 recognized subspecies’ calls, male juncos actually respond more aggressively to the trills of other local males rather than an almost identical call from a junco of a different geographical location (Reichard, 2014).

DISTRIBUTION

With a rapid radiation through North America since the last glaciation (Reichard, 2014), dark-eyed juncos are found in nearly every corner of the continent, from Alaska to Mexico and BC to Nova Scotia.

Fondly called the “original snowbirds of middle latitudes,” many Dark-eyed Juncos in the US leave the mountains in which they inhabit throughout the breeding season for a slightly milder winter in the Eastern United States (Audubon, 2018). Some juncos do stay year-round, while others just over-winter.

HABITAT AND NESTING LOCATION

No one has ever accused the Dark-eyed Junco of being overly picky about its habitat. This is likely because juncos can be found inhabiting coniferous forests (pine, Douglas-fir, spruce, and fir), deciduous forests (aspen, cottonwood, oak, maple, and hickory), open woodlands, fields, parks, gardens and even roadsides all throughout North America (The Cornell Lab of Ornithology, 2019).

This lack of fuss over habitat carries over into their selection of nest sites as well. Typically, juncos are ground-nesting birds (among roots, in rock crevices, at the base of vegetation, etc.) but they can also nest in taller vegetation such as trees, and even man-made structures such as buildings, window ledges, planter boxes and even hanging flower baskets (Bailey et al., 2017).

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BEHAVIOUR

Dark-eyed Juncos are primarily socially monogamous birds, meaning that they tend to stick to one mate (Rice et al., 2013). During courtship, the male engages in a short-range song directed at a specific female (Rice et al., 2013) as well as proudly displays his tail whiteness. Tail-whiteness is an attractive trait to female juncos and could represent a male bird with a better diet (McGlothlin et al., 2007).

After a successful courtship, the female chooses the nesting location and builds the nest, while the male valiantly defends the area, chasing off intruders and angrily chirping at any disturbances (The Cornell Lab of Ornithology, 2019). The incubation period is typically 12-13 days and clutch sizes range between 3-6 eggs, each less than an inch in length (The Cornell Lab of Ornithology, 2019). Following current household gender-equality ideals, the male shares approximately half of the responsibilities during the nestling stage (Liebgold et al., 2013). Throughout this time, the recently hatched young are extremely dependent on their parents. After 9 days of being fed primarily insects, the young are capable of leaving the nest, but are often still reliant on their parents for up to 3 weeks.  (Canadian Wildlife Federation, 2020).

This almost sassy bird spends its time fluttering among the underbrush, confidently hopping and running along the ground in search of seeds and insects. During breeding season, juncos primarily feed on insects and can often be seen in pursuit of a fresh arthropod victim (Canadian Wildlife Federation, 2020). In the winter, juncos can be viewed pecking or scratching at the ground litter in search of seeds which make up the bulk of their diet (The Cornell Lab of Ornithology, 2019). Migratory birds such as the Dark-eyed Junco often display this shift in diet-preference and food intake in order to store up fat (Holberton et al., 2006).

Dark-eyed juncos often create flocks in the winter in which multiple subspecies may be found and a distinct hierarchy is formed (The Cornell Lab of Ornithology, 2019).

CONSERVATION STATUS

Dark-eyed Juncos play important roles in many ecosystems across North America as both agents of seed dispersal and regulators of insect populations (Canadian Wildlife Federation, 2020). Although populations are currently abundant and widespread, the North American Breeding Bird Survey suggests a yearly 1.4% decrease between 1966 and 2015 which resulted in an overall 50% decline in population (The Cornell Lab of Ornithology, 2019).

Not only are these juncos important in ecosystems, but they also provide good scientific models due to their abundance as well as their known behaviour patterns (Holberton et al., 2006; Reichard, 2014).

Dark-eyed Juncos provide functional aspects of both ecosystems and research, but maybe one of their most fundamental roles is the bringing of joy. As one of the most common birds found in North American bird feeders (The Cornell Lab of Ornithology, 2019), bird enthusiasts can gain endless amounts of fascination and intrigue just by simply viewing them.

Part 2: The Effect of Immune System Activation on Reproductive Success

All birds, including Dark-eyed Juncos, have three very energetically demanding types of behaviours throughout their lives: moulting, migration and reproduction. Often these events are timed so that they don’t overlap in order to increase the likelihood that the bird will survive another year, but not all energetically costly events can be so easily controlled (Graham et al., 2017). If these energy-depleting events occur simultaneously in an individual, the bird will likely resort to trade-offs in which priority is placed often on the behaviour that will more likely lead to survival (Graham et al., 2017). Some researchers decided to explore this effect of trade-offs in energetically expensive activities in Dark-eyed Juncos, specifically the trade off between an immune response and reproductive effort (Graham et al., 2017).

Over the course of two years, researchers scoured the area around Mountain Lake Biological Station (MLBS) in Virginia in search of Dark-eyed Junco nests (Graham et al., 2017). The nests that they discovered were checked daily in order to track the start of the incubation period (Graham et al., 2017). On day six of incubation, the mothers were captured using a mist net and injected with either a mild antigen or saline solution (as a control) (Graham et al., 2017).

The researchers found a significant correlation between the juncos injected with the mild antigen and the nest survival rate 6 days after hatching with 62% of hatched control nests surviving more than 6 days, while only 21% of the immune-responders (Graham et al., 2017). At the time of hatching the mothers injected with the antigen would have a spike in their immune response, which would likely lead her to try to conserve her energy (Graham et al., 2017). This attempt by the mother to ensure that she survives into another reproductive season may make her less available for the high-energy activities required for raising a young nestling, and could therefore decrease the chances of offspring survival (Graham et al., 2017).

A similar study was done by another research team but this time they tried to determine whether or not inducing an immune response before breeding would delay the onset of breeding activities (for example egg production) (Needham et al., 2017) Earlier-breeding individuals tend to have higher offspring survival rates, but in order to breed early the mother must start developing the yolk as well as undergo the final maturation of the follicle in the spring when resources are still sparse (Needham et al., 2017). If the mother is exposed to an antigen at this time, another trade-off is likely to take place.

Initially there was no difference in the mass, fat score or skeletal sizes between the control group and the experimental group, yet the experimental group laid their eggs 8 days after the control group on average (Needham et al., 2017). This delay in reproduction is likely due to a prioritization of the mother’s personal health over the creation of the next generation.

These studies show that even the smallest of disturbances (such as a mild antigen) before or during reproduction can lead to energetically costly behaviours that can lead to a decrease in reproductive success. Since Dark-eyed Juncos are so prevalent across North America, they provide excellent models for experiments such as these, whose results can hopefully be applied to other species who may be more ecologically threatened. Hopefully research like this can lead to solutions that can be applied to attempt to increase the nesting success of not only Dark-eyed Juncos, but many other more at-risk species.

CONCLUSION

Dark-eyed Juncos are one of the most prevalent birds in North America, and they can provide inspiration and intrigue to bird-watchers across the continent. Despite their burglar-like appearance the only thing they are likely to steal is your heart.

References

Audubon (2019) Guide to North America’s Birds. (Internet): https://www.audubon.org/field-guide/bird/dark-eyed-junco

Audubon (2018) Juncos are the Original Snowbirds. (Internet): https://www.audubon.org/news/juncos-are-original-snowbirds

Bailey, R., & Sherwood, M. (2017). Nest box use by the dark-eyed junco (junco hyemalis). Wilson Journal of Ornithology, 129(3), 620-625.

Canadian Wildlife Federation (2020) Dark-eyed Junco. (Internet): https://cwf-fcf.org/en/resources/encyclopedias/fauna/birds/dark-eyed-junco.html

Graham, J., Mady, R., & Greives, T. (2017). Experimental immune activation using a mild antigen decreases reproductive success in free-living female dark-eyed juncos (junco hyemalis). Canadian Journal of Zoology, 95(4), 263-269.

Holberton, R., Wilson, C., Hunter, M., Cash, W., & Sims, C. (2006). The role of corticosterone in supporting migratory lipogenesis in the Dark‐Eyed junco,junco hyemalis: A model for central and peripheral regulation. Physiological and Biochemical Zoology, 80(1), 125-137.

Liebgold, E., Gerlach, N., & Ketterson, E. (2013). Similarity in temporal variation in sex‐biased dispersal over short and long distances in the dark‐eyed junco, junco hyemalis. Molecular Ecology, 22(22), 5548-5560.

McGlothlin, J., Duffy, D., Henry-Freeman, J., & Ketterson, E. (2007). Diet quality affects an attractive white plumage pattern in dark-eyed juncos (junco hyemalis). Behavioral Ecology and Sociobiology, 61(9), 1391-1399.

Needham, K. B., Cook, N. J., Rutherford, A. R., & Greives, T. J. (2017). A pre-breeding immune challenge delays reproduction in the female dark-eyed junco junco hyemalis. Journal of Avian Biology, 48(10), 1348-1354. doi:10.1111/jav.01464

Rice, R., Reichard, D., Schrock, S., & Schultz, E. (2013). Low-amplitude songs produced by male dark-eyed juncos (junco hyemalis) differ when sung during intra- and inter-sexual interactions. Behaviour, 150(9-10), 1183-1202.

Reichard, D. (2014). Male dark-eyed juncos (junco hyemalis) respond differentially to playback of local and foreign song. The Wilson Journal of Ornithology, 126(3), 605-611.

The Cornell Lab of Ornithology (2019) All About Birds: Dark-eyed Junco. (Internet): https://www.allaboutbirds.org/guide/Dark-eyed_Junco/overview