(female narrator) Across their breeding, wintering, and year-round ranges, dark-eyed juncos inhabit nearly all of the U.S. and Canada, making them one of the continent's most common and abundant songbirds. But depending on where you've seen them, your concept of what they look like will vary from place to place. For example, the juncos studied at Mountain Lake, Virginia are part of a uniformly gray slate-colored group that breed across Alaska, Canada, and the Mountains of New England and Appalachia. But further west, other groups of dark-eyed juncos appear strikingly different.
The northwest corner of the state of Wyoming has some of the most iconic scenery and impressive wildlife in America, attracting millions of tourists every year.
The Grand Teton's rugged mountains are one popular destination, rivaled only by the geysers and geothermal attractions found at Yellowstone National Park.
A certain group of dark-eyed juncos also makes an annual visit to this region. But they're not here to see Old Faithful.
Instead, they come to breed in the lush mountain forests during the spring and summer months, taking advantage of the mild temperatures and abundance of insects.
Named for the colored feathers on their flanks, these pink-sided juncos also have brown on their backs and wings, features that clearly distinguish them from the slate-colored juncos.
The breeding range of pink-sided juncos is restricted to high-elevation forests in the Rocky Mountains of Wyoming and Montana, a smaller range when compared to their slate-colored counterparts. Moving farther west, another group known as the Oregon juncos can be found breeding in forest and woodland habitats ranging from British Columbia to Southern California.
On average, Oregon juncos are smaller in size. Their brown backs are similar to pink-sided juncos, yet their dark hoods could make them pass for an entirely different species.
In the mountains of eastern California, Nevada, and Utah, juncos take on another distinct look. Here their heads and bodies are a muted gray, giving them the name gray-headed juncos.
But perhaps a more distinguishing feature is the bright red patch of feathers on their upper back. Throughout North America, juncos typically breed in forest and woodland habitats, which are found at either high latitudes or high elevations. These environments can look as different from one another as the birds themselves. But all of them have common features that juncos require: trees, shade, grass, and ground cover for nesting and foraging.
Among the most extreme climates inhabited by dark-eyed juncos is the Black Hills of South Dakota, home to the white-winged junco. The rugged open landscape is dotted by ponderosa pines. Brutally cold winters turn into scorching summers.
The dry conditions make the area susceptible to wildfires. And juncos are among the first animals to move back into recently-burned habitats. This makes them a useful indicator species for wildlife managers.
White-winged juncos are the largest of all the juncos and have the most white in their tail feathers. They're named from the white bars on their wings, a feature thought to be a social ornament. Ornamental feather colors, such as white tails, dark hoods, bright backs, and colored sides, are used to attract mates and establish social dominance. These traits distinguish the different dark-eyed junco groups, raising the question, "Why are birds that look so different classified as a single species?"
At the Natural History Museum in San Diego, California, curator Phil Unitt oversees a research collection containing nearly 50,000 birds.
Highlights of the collection include exotic parrots and giant eagles. But still, the ordinary, extraordinary junco is among Phil's favorites.
(Unitt) The junco is especially interesting to me because, perhaps more than any other bird of North America, it has really conspicuous variation that still seems to be within one species. The complexity of the junco can be subtle or obvious. So you have a whole spectrum of variation from the gross to the subtle. You know, gross difference between an Oregon junco from the Pacific Northwest and a gray-headed junco. No one would ever think those are the same species.
(narrator) In examining the specimens by hand, researchers can collect detailed measurements of color, shape, and size
and make comparisons among different groups. Museum collections exist today due to the tireless efforts of field ornithologists-- men and women like Alden Miller, a professor at Berkeley, who, in the 1930s, set out on a decade-long quest to catalog and classify all the juncos across North America. Miller wanted to understand the process of diversification.
He spent months at a time in the field capturing juncos, mapping their locations, and taking detailed measurements of color, shape, and size, examining an astounding 11,774 junco specimens in the process. In the end, he identified twenty-one distinct groups of juncos across the continent. Fifteen of these groups inhabited the U.S. and Canada and had the common trait of dark brown eyes. These dark-eyed juncos could be lumped into six distinct groups that looked quite different from one another. And each group had its own breeding range. But Miller made a curious observation: in the zones where the different juncos' ranges met, it seemed that they could easily interbreed. For example, where the gray-headed junco and the Oregon juncos ranges meet in the mountains of Nevada, it's not uncommon to observe the two subspecies producing offspring together.
Here an Oregon junco mother tidies up and removes a nestling fecal sac. Moments later, the gray-headed father arrives with food. The hybrid offspring of mixed junco pairs are fertile and have traits blended from both parents. Their ability to interbreed in nature is the primary reason dark-eyed juncos are currently classified as a single species. But this left Miller puzzled. Why could groups that look so different interbreed so freely? He hypothesized that they were all closely related. But limited by the tools of his era, Miller had no way to further explore their evolutionary history. Today, Dr. Borja Milá from Madrid's National Museum of Natural Sciences has taken up Miller's quest.
(Milá) This has puzzled ornithologists for over a century. How can two juncos that look so different, in terms of plumage type-- how can they interbreed so easily when they come into contact? Of course, Miller, what he had was things like patterns of plumage color or eye color, bill color, and how those change across space. Now we have this very powerful tool, which is DNA.
Within DNA, we have like a footprint of the evolutionary history of a group. Genetic work has become an indispensable tool in the study of evolutionary relationships and the study of speciation in general. We can actually compare different populations or different species in terms of the divergence in the DNA sequence of a certain gene of interest.
(narrator) Using DNA from blood or feathers, researchers sequence specific gene fragments from multiple populations or species. The sequences are aligned and the mutations that differ can be compared. So two populations can be identical genetically or they can differ by several mutations. Quantifying that amount of divergence is very useful to then reconstruct that final genetic tree. Like a genealogical tree that allows us to understand the evolutionary history of the group.
(narrator) In this example, populations B, C, and D share a mutation in the third base pair that distinguishes them from population A, suggesting that these groups likely share a common ancestor that diverged from population A in the past. Also, populations C and D share a mutation in the eighth base pair that distinguishes them from population B, indicating that C and D are the most closely related and share a recent common ancestor. Population D also has a unique mutation that distinguishes it from population C. Because all four genetic sequences are quite similar, they likely share a common ancestor farther back in time. Once we have put together these evolutionary relationships between the different species or populations, we can actually understand better how these different physical characteristics, like plumage, color, or eye color, changes with time. Another very useful tool to us is what we call molecular clocks. Because we know parts of the genome mutate at constant rate, we can calibrate that rate and for a given level of divergence, we know it's equivalent to a certain amount of years.
(narrator) Milá and team used a mitochondrial DNA fragment called cytochrome oxidase 1 that typically distinguishes even closely related species of birds. Based on known divergence times from other species, the molecular clock of this gene has been estimated to tick at a rate of approximately 2% sequence divergence every 1 million years. But for the dark-eyed juncos, the genetic sequences were so similar that it was impossible to construct a typical evolutionary tree. Instead, the six major groups clustered together more like a bush, all differentiated from a common ancestor, the yellow-eyed junco, which inhabits the highlands of Mexico and Central America.
(Milá) Between dark-eyed junco forms we find very few genetic differences. That means different lineages, even though they look very different, in terms of plumage, they're extremely young. So they diverged very recently, just a few thousand years ago. In fact, we know this happened after the last glacial maximum, which took place about 20,000 years ago. So this, in evolutionary terms, is extremely recent. So this helps us to understand this capacity for interbreeding, which was very difficult to understand otherwise. Genetic data allows us to obtain a date for when that divergence happened.
(narrator) Combining Alden Miller's data on geography and physical characteristics with Borja Milá's genetic results finally allows for a comprehensive explanation of dark-eyed junco diversification. I think what's unique about the junco radiation is that--it's two things. Is that it's very rapid, but also that it's very recent. That about perhaps 10 to 20 thousand years ago, this yellow-eyed junco from the highlands of Mexico expanded northward and recolonized North America as the glaciers receded right after the last glacial maximum. And as these populations recolonized North America, they changed eye color from yellow to dark. Also, the bill changed from bicolor to a pink-colored bill. As populations became isolated in different parts of North America, they also developed--they evolved different plumage types, giving rise to the different dark-eyed juncos we see today, including the slate-color, the gray-headed, the pink-sided, Oregons, et cetera.
(narrator) It might be surprising to learn that the common dark-eyed junco represents one of the most stunning examples of rapid diversification in the entire animal kingdom. But there would be even more surprises to come, as researchers set out to examine other junco forms found south of the border.
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