Warblers in New Hampshire’s Forests Have Found an Ideal Way to Divvy Up Territories

From the Spring 2026 issue of Living Bird magazine. Subscribe now.

It’s early June in the White Mountains of New Hampshire, a clear, cool, calm morning—about an hour after dawn. The rising sun and a greening forest create an ever-shifting dapple of light and shade across the forest floor.

Carrying her phone in one hand, and a pair of binoculars around her neck, Abigail Archey—a rising junior majoring in biological sciences at Cornell University—strides in straight lines through the trailless wilderness. The uneven terrain is filled with rotting logs, ankle-catching swales, and clusters of shin-whacking hobblebush. As she traverses the forest, she rattles off bird species she hears: “Red-eyed Vireo. Wood Thrush. Black-throated Green. Black-throated Blue.”

A Few of Hubbard Brook’s Songbirds 

Like the hundreds of scientists before her who have walked this exact same ground at the Hubbard Brook Experimental Forest for the past 57 years, Archey is making history, one bird at a time. She records each bird on an ArcGIS mapping app on her phone. Some songs are barely audible, some clear and close. But there’s barely any movement to be seen. Nearly every bird is identified by sound alone. 

“If they weren’t singing, we wouldn’t know most of them were here,” she says. 

“Here” is Hubbard Brook. The experimental forest was started by the U.S. Forest Service in 1955 in partnership with a few Dartmouth College professors, establishing the 7,800-acre northern New England woodland as a place to study the long-term effects of forest management. In the decades that followed, Hubbard Brook became a multidisciplinary hub of scientists studying nearly every aspect of ecology and biology within the forest’s northern hardwood ecosystem, from snails to soil nutrients to snowpack. And, famously, birds. 

The rectangular Hubbard Brook “bird plot,” as scientists here call it, occupies just 10 hectares (around 25 acres, or about 0.3% of the forest’s total area), but it has played an outsized role in helping researchers understand the breeding season behavior of migratory songbirds. That’s because there’s a record of nearly every breeding bird that has spent time on this plot since 1969. The data comes from nearly six decades of researchers surveying this slice of forest at the height of songbird breeding season, recording every bird they see or hear and noting the locations and activities in order to get accurate counts of how many birds are present. 

There’s nothing like it in all of ornithology, according to Mike Webster, a professor of neurobiology and behavior at Cornell University. 

“I can’t think of any other examples where you find this much information over this much time for this many species,” says Webster, who also oversees the Cornell Lab of Ornithology’s Hubbard Brook Field Ornithology Program, which brings in undergraduates like Abigail Archey each summer to gain field experience and assist with research.

“Mapping individual territories was really just a way to count the birds,” says Richard Holmes, who was just two years into his career as a Dartmouth professor in 1969 when he started the Hubbard Brook bird plot. “The maps weren’t the main endeavor. They were a tool to help us get accurate numbers of breeding birds.” 

Now thanks to a new digitization project, these old maps are yielding surprising new insights about how communities of migratory songbirds shape their territories on their breeding grounds. In a study published in the journal Ecology Letters in 2024, scientists analyzed these maps to see how the territory sizes for various songbirds in the bird plot have changed from season to season. It turns out the birds maintain larger territories when there are fewer individuals in the plot, but can shrink their territories to accommodate more individuals in the plot when populations at Hubbard Brook go up—providing evidence that these warblers and other forest birds behave according to a theory known as ideal free distribution.

Scott Sillett, who leads the Smithsonian Migratory Bird Center, has studied the behavior of songbirds like warblers and thrushes since the mid-1990s when he got his PhD under Holmes at Dartmouth. He says this research sets a new bar for understanding the underlying dynamics of how songbirds establish their breeding territories.

“It’s one of those studies you kind of pinch yourself, like, ‘wow! we’re able to provide a really robust test of kind of a foundational theory in ecology,’” he says.

Sillett also says that it’s notable the study relied on an essential aspect of these songbirds—namely, their songs—to measure and track how they establish their territories. 

“Song is a way to be aggressive without physically attacking your neighbor,” he says. “It’s less costly. I mean, if you’re an organism with hollow bones, how much actual bashing into another animal do you want to endure?”

A year in the life of a newly fledged Black-throated Blue Warbler at Hubbard Brook plays out like an action movie. It starts late in the spring, when the young bird hero casts out from the nest on its own. Come autumn, the bird travels halfway across an unknown world, to the tropics and back, facing obstacles such as buildings, severe weather, and limited stopover habitat for resting and refueling. It perseveres, striving to return to where the journey began, its birthplace, to complete its quest—successful reproduction, the chance to bring a new generation into the world.

According to the long-term data from Hubbard Brook, there might be a dozen or more Black-throated Blue Warblers looking to breed on the bird plot each year, as well as a couple dozen Red-eyed Vireos, and anywhere from a few to a horde of birds of at least a dozen other species. How can all of these breeding birds—sometimes well over 100 individuals in all—find a breeding partner and set themselves up with the resources needed to achieve their own Hollywood endings?

Mike Webster—who is also director of the Cornell Lab’s Macaulay Library, the world’s largest scientific archive of natural history audio, video, and photographs—has studied bird communication in songbirds around the world, from the warblers at Hubbard Brook to Red-backed Fairywrens in Australia. Webster says vocal communication, such as songs during breeding season, play a huge role in giving each bird a fighting chance in a crowded playing field.

“Migratory birds need to establish a territory quickly, before other returning individuals lay claim to the best breeding habitat,” he says. “Songs allow these birds to immediately advertise their claim on unfilled habitat … they can just broadcast that ‘this area is mine.’”

According to Webster, birdsong is the perfect medium for staking a claim to breeding territory in forested habitat, because song travels well through the trees—unlike visual signals such as bright plumage, which would be obscured in dense forest foliage.

“Without songs, it’s hard to imagine that birds would be able to be efficient enough to breed successfully in the small window they have to be up here,” says Webster, noting that Black-throated Blue Warblers are only at Hubbard Brook from May to August. “Songs are like the glue that hold bird societies together.”

Richard Holmes, now a professor emeritus on the cusp of his 90th birthday, has been listening to the “zu-zu-zu-ZHWEE” of the black-throated blues at Hubbard Brook since he first flagged off the bird plot. As of 2025, Holmes was still making his way to the field station at Hubbard Brook to advise, observe, and participate in data collection. He says that even though the bird songs sound like peaceful melodies to humans, they’re actually pugnacious warnings about staking out turf.

“Most of the aggression we see occurs shortly after arrival, just before and during egg-laying,” he says. For half-ounce, 5-inch-long Black-throated Blue Warblers, that aggression takes the form of frequent chases, close encounters, and occasionally, physical attacks. Holmes says that within a couple of weeks, the breeding territory boundaries tend to settle out and singing becomes the birds’ main way of maintaining their declared spaces.

From the beginning of his studies at Hubbard Brook, Holmes established data-gathering  techniques, subsequently used by several generations of scientists and scientists-in-training, to determine how many breeding birds are occupying the plot. Key to the success is what Holmes and his collaborators dubbed “wander mapping.” That’s what Abigail Archey was doing on that early June morning, and it’s the key to making sure researchers aren’t missing any birds that might be present. It’s a technique that involves walking the plot each morning for a total of 16 hours per week, for five weeks straight, at the height of bird breeding season.

“‘Wander mapping’ was just what we started calling it. Really it should be called ‘territory mapping,’” says Holmes. Unlike straight-line, timed transects (which are also conducted through the plot), wander mapping gives the researcher agency to move freely so they can “cover all parts of the plot, and to follow individual birds if needed,” Holmes says. “They’ll end up finding nests, noting chases and fights between males, and documenting the presence of females, all of which add to the accuracy of our counts.”

Perhaps most importantly, the wander-mapping researcher must determine when two or more males of the same species are countersinging. Holmes says that countersinging is often detected when one male is singing and one or more of the nearby birds are reacting in kind. 

“So many times when one bird sings, another bird responds,” says Holmes. “It’s like these birds are having … a territorial rap battle. When you have multiple records of this countersinging behavior, you can see, okay, they’re moving closer together, they’re farther apart, and you can draw a boundary about halfway in between these birds.”

For more than 50 years, each bird detection and instance of countersinging was plotted on paper with a grid representing the bird plot. Each birdsong gets a dot (with a time) and an episode of countersinging is indicated by a line with arrows connecting two of the dots. At the end of each season, the researchers would use translucent  paper to transfer all the daily maps for one species onto a single map for the season. As the researchers analyzed the data for each species, the dense thickets of dots and arrows transformed into an interlocking series of circular blobs, each representing a territory. 

Beginning in 2023, the bird territory mapping project at Hubbard Brook went fully digital, with a smartphone app that allows each researcher to enter data immediately on to a shared map. Miranda Zammarelli helped with the digital transition as a PhD student at Dartmouth. Today she oversees the summer fieldwork and data collection at Hubbard Brook. She says that despite the technical upgrades, the process for determining the boundaries of bird territories remains much the same as it was when Holmes started it. 

“I look at the data for the season, and I make the boundaries myself trying to match as close as possible to the paper way,” Zammarelli says. “This is the way that they had been doing it for 50 years,” she says.

When Zammarelli arrived at Dartmouth in the summer of 2021, she immediately started brainstorming projects for her dissertation. Her advisor tipped her off about the 50-plus years of paper map files stored away in cabinets at Hubbard Brook—nearly 1,000 of them.

“I spent two weeks kind of zoning out at the scanner, thinking about the questions we could answer with these data,” says Zammarelli. “For me there was kind of the ‘a-ha’ moment of seeing all of these maps go by and actually being able to see some of the changes in circle sizes over time just by looking them over.”

Zammarelli noticed the territories would be bigger in some years and smaller in others. That led her to come up with the idea to test a long-standing pair of theories about how birds of the same species occupy habitats as an area gets more crowded. 

One of the theories—known as ideal free distribution, or IFD—holds that animals will spread across habitats to equalize their ability to survive and reproduce. That means individual territory sizes will decrease in proportion to each other when the population increases; as a result the animals divide up the available habitat more or less equally.

A competing theory—ideal despotic distribution, or IDD—holds that more dominant individuals monopolize high-quality areas, forcing others into poorer habitats, which would lead to the territories in the best habitats staying larger even as the surrounding population increases. While these theories had been tested in individual species, with varying results,  no one had tested them with multiple species over the timescale of decades. 

Zammarelli began by focusing on the territories of seven of the historically most abundant breeding songbird species on the bird plot: Black-throated Green Warbler, Black-throated Blue Warbler, American Redstart, Red-eyed Vireo, Ovenbird, Hermit Thrush, and Least Flycatcher.

In addition to the newly scanned territory maps, she also used long-term data about the overall abundance of each of these species—which years were boom years, which were bust—as well as additional information gleaned from the maps about which areas were the most popular places to set up a territory.

“We made these heat maps that showed us which areas have been the most preferred for each species to nest in, all-time. We use that as an indication of which areas in the plot are the higher quality spots,” says Zammarelli. For example, Black-throated Blue Warblers are likely to cluster in areas of the plot with stands of hobblebush, where nearly all of their nests are found.

Zammarelli says that understanding habitat quality is key to determining if the birds are free to select their territory, or if they’re subject to potential despots in their midst.

“If these birds were being despots, we would see that birds that are nesting in the most preferred areas would maintain the same size areas from year to year,” she says. “The idea is, if you’re in the best quality habitat, you’re not going to change your territory size just because more individuals are coming in. You’re the dominant bird, you’re going to hold on to that space and force everyone else into the low-quality habitat.”

However, Zammarelli’s analysis showed that songbird societies at Hubbard Brook forest tend toward the egalitarian. Across all the species she looked at, territory size decreased as local population increased—so more birds could fit into the space. She says the fact that birds avoided being habitat hogs on the more preferred sites makes a strong case that they operate in an IFD manner, a result that surprised Scott Sillett, the Smithsonian Migratory Bird Center leader.

“What surprised me was … we know that these warblers are territorial, they’re aggressive, they dive-bomb each other…. That’s why I think a lot of us expected it to be ideal despotic distribution, like, ‘well, of course, they’re little despots!’” says Sillett.

Zammarelli points to an example from the Ovenbird maps to show how it plays out.

In 2001, the researchers tallied around seven Ovenbirds on the 25-acre plot, one of the lowest counts of the whole study. That contrasts with 2009 when around 21 Ovenbirds, three times as many, crammed into the same area.According to the calculations from the paper, the average size of each individual territory in 2001 was around 5 acres, whereas the average territory size in 2009, when there were many more birds, was about 2.5 acres. 

“You can also see an increase in countersinging in the year when there are more individuals,” she says. “Theoretically, if you have more neighbors, you’re probably going to have to sing more to defend your space.”

Zammarelli stresses that this is just one example, and that countersinging rates were not part of their analysis in the research published in Ecology Letters.

Despite the dip and surge in the early 2000s, Ovenbirds have maintained a stable population at Hubbard Brook since around 2010, says Zammarelli, thanks to increasing areas with mature trees and sparse understory—the preferred breeding habitat for the species. According to Richard Holmes, Hubbard Brook is no longer the young forest it was when he arrived in the late 1960s.

“The forest was logged heavily through about the 1910s,” he says. “When I first came here it was still considered early successional, fewer old trees.”

Two of the most abundant birds from the first two decades of Hubbard Brook bird surveys—American Redstart and Least Flycatcher—prefer younger forests, and they have stopped nesting on the plot completely as the forest has matured. Populations of other species—such as Black-throated Green Warbler and Red-eyed Vireo, which prefer more mature forests—have shown a slight but steady positive trend, though there can still be huge variations from year to year. For example, Red-eyed Vireos on the plot have numbered as few as 13 in 1980, and as many as 32 in 2016.

Holmes is preparing to submit his most recent analysis of the Hubbard Brook bird abundance trends in a scientific journal, adding to the more than 140 journal articles published using Hubbard Brook bird data on which he has been an author. He says that while the overall population of songbirds on the plot decreased by around 60% between 1970 and 2000, the total songbird population on the plot has trended slightly upwards over the past two decades.

Zammarelli, who accepted a position as a postdoctoral researcher at the Cornell Lab starting in the fall of 2026, says that while there is no established relationship between either of the distribution theories and population trends, the fact that the overall population of birds is at least holding steady, and possibly increasing, is encouraging. She says that the birds’ tendency toward ideal free distribution could be a positive thing for the way these birds can weather changes to habitat and food availability.

“With ideal free distribution, birds should able to reproduce pretty equally on high- and low-quality habitats,” says Zammarelli. “So, if something happens to part of the habitat, there should still be individuals who can succeed in reproducing. With despotic distribution, [birds] in the best quality habitat might be the only ones that are reproducing successfully.”

Zammarelli also points out that when it comes to conserving forests like Hubbard Brook for birds, this study points to the value of adding less-than-ideal habitat to landscape conservation. The results of her research show that these songbird species have the ability to distribute themselves in a way that provides opportunities for equal fitness across patchworks of high-quality and low-quality habitat.

“We shouldn’t just focus all our efforts on the high-quality areas. We need to think about the lower-quality areas, because those areas still equal some reproductive output for birds,” she says.

“These birds have quite a bit of flexibility,” she says, “which has implications for how they can weather climate change or habitat loss.” 

Mike Webster, the Cornell Lab’s Hubbard Brook Field Ornithology program director, says that these kinds of insights and conclusions about bird population dynamics are only possible from long-term studies like the nearly 60 years of data gathered at Hubbard Brook.

“[This study] is really a testament to the power of long-term studies,” says Webster. “To understand how birds respond to climate change and other environmental disturbances over time, you need to go back to the same spot, year-after-year, and collect those data. There is just no other way of doing it.”