Why Warblers Flock to Wealthier Neighborhoods

At a meeting of urban wildlife researchers in Washington, D.C., in June, one diagram made it into so many PowerPoint presentations that its recurrence became a running joke. The subject, though, was serious: The diagram illustrated the links between structural racism, pernicious landscape features such as urban heat islands, and impacts to biodiversity, and it came from a study published in the fall of 2020 in the journal Science.

That study was “Ecological and evolutionary consequences of systemic racism in urban environments,” led by Christopher J. Schell, an ecologist at the University of California, Berkeley. It synthesized what a handful of urban ecologists around the country had begun demonstrating: that patterns of bigotry and inequality affect how other species experience life in cities.

Dr. Schell, who is Black and from Los Angeles, said he grew up with an understanding that “there is a ton of heterogeneity that exists in a city, and it’s not by accident that it’s that way.” Those variations could include the numbers of parks and street trees in different neighborhoods, whether a highway or rail line ripped through a community or whether an oil refinery spewed toxins into the air.

As a discipline, urban ecology is only about a quarter-century old, and until very recently its practitioners tended to treat cities mainly as a contrast to rural areas, without considering the wild disparities between and within cities. Dr. Schell wanted to show that urban heterogeneity in turn “is driven by systemic inequities,” he said, like “oppression, residential segregation, gentrification and displacement, unjust zoning laws, homelessness, so on, so on, so on.” Those issues don’t only impact people, he added: “How we operate influences the rest of the natural world as well as the social world.”

Over the past few years, a widening group of urban ecologists has been fanning out to study the overlap between environmental justice and biodiversity conservation, fields that had previously tended to keep to their own corners. Dr. Schell said that, in his lab, researchers “oftentimes do our own version of ‘six degrees of separation from Kevin Bacon’” to show how human actions ripple out to wildlife.

“Air pollution isn’t just restricted to people,” he said. “Other animals have lungs. Why would we not expect them to also be inhaling the same amount of pollutants that we generate?”

Madhusudan Katti, an ecologist at North Carolina State University who has worked at the intersection of biodiversity conservation and human well-being for most of his career, agreed. “Often the interests of other species and marginalized humans align,” he said. “It’s very much a colonial perspective to think about humans and wildlife as separate. We need to start thinking about humans and wildlife together in the landscape and mitigate things that will help both.”

Ecological fingerprints

This growth of urban ecology has been aided, in part, by Mapping Inequality, a sprawling, multiuniversity project from the University of Richmond’s Digital Scholarship Lab. It created a digital archive of “redlining,” the New Deal-era housing policy that enforced and perpetuated neighborhood segregation in the United States.

In 1933, the federal government created the Home Owners Loan Corporation, or HOLC, whose intent was to help Americans recover from the Depression. HOLC issued accessible home loans or refinanced mortgages to prevent default. To do this, it mapped more than 200 U.S. cities based on the perceived risk of lending money in various areas, grading neighborhoods from A to D and outlining them in corresponding colors, from green to red. Grades were based on the condition of the housing stock and on the race, ethnicity and income of residents. Neighborhoods with newer homes and more U.S.-born, white residents were usually graded A and outlined in green. Those with older homes and more immigrants and people of color were generally graded D and outlined in red. The redlined neighborhoods were deemed “hazardous” to invest in.

Ninety years later, nearly three-quarters of the redlined neighborhoods are still struggling financially, and nearly two-thirds are “majority minority,” according to a study from 2018. The human legacy of redlining is vast: poverty, unemployment, health problems, decades of lost wealth and opportunities.

The policy also left ecological fingerprints on many cities, effects that urban ecologists are now eagerly bringing to light. “There are just more people who have hardcore wildlife training who are starting to look at cities as a place to do their work,” said Eric M. Wood, an ornithologist and urban ecologist with California State University in Los Angeles and the Natural History Museum of Los Angeles County. “If you’d told me as a Ph.D. student, ‘Go study birds in L.A.,’ I’d have said, ‘No way. I’m going to Borneo.’”

When Dr. Wood first moved to Los Angeles in 2015, he was keen to apply his field skills in a big city. “I’m a birder and a natural history person, and for 25 years I’d gone out and identified all the birds and plants and insects,” he said. He planned to do the same thing in Los Angeles. To measure biological diversity in a given landscape, an ecologist needs to capture the range of environmental variability there — the heterogeneity. In a natural setting, that might mean looking at different elevations, hills that face north versus south or areas with wetter or drier soil. In Los Angeles, Dr. Wood soon found, the environmental variability was also based on neighborhoods’ socioeconomic status.

He and others recently surveyed birds across the sprawling metropolis and analyzed the findings against redlining maps. They found that predominantly white neighborhoods, which were often the ones “greenlined” on the HOLC maps, hosted a greater abundance of birds that generally live in forests, such as warblers, wrens and bluebirds.

In contrast, areas that today are predominantly Hispanic and were previously redlined have fewer of those forest birds and more “synanthropic” species, those often found in dense urban areas. (These include pigeons and sparrows but also crows and ravens, mourning doves, house finches and even a type of hummingbird.) In an article last month in the journal Ornithological Applications, the researchers wrote that the distribution of birds in Los Angeles today reflected “patterns of income inequality, both past and present, that carry over to influence urban biodiversity.”

As examples, Dr. Wood compared Beverly Hills, where the average home price is more than $3.6 million, according to Zillow, with Boyle Heights, a largely Hispanic neighborhood where the average home price is $628,000; it shows up as a large red blob on the HOLC map and has far fewer trees and green spaces. “You get loads of these birds that require insects for their life history, and they go to a place like Beverly Hills because there are trees and flowers,” he said.

These differing landscapes clearly matter to the birds. But is it important to people if they share their neighborhoods with a common raven rather than a yellow-rumped warbler? “The point is that there are just so many differences” between communities like Beverly Hills and Boyle Heights, Dr. Wood said. The birds were “an indicator of these broader conditions that are effectively bad for people.”

Another study, published in 2022, used publicly available genetic data from nearly 7,700 individual animals belonging to 39 species of vertebrates. It found that across 268 urban locations in the United States, the wildlife in neighborhoods with greater proportions of white residents had higher levels of genetic diversity and more evidence of connected populations of animals, which interbreed and exchange DNA. Genetic diversity is essential for wildlife populations to weather a catastrophe like a pandemic or a wildfire.

The finding revealed a blunt truth: Like a wall or a highway, systemic racism creates a barrier to wildlife movement. “The whole process changes your view of the world, honestly,” Chloé Schmidt, the paper’s lead author and a senior scientist at the German Center for Integrative Biodiversity Research, said of the study.

Dr. Schmidt, who is of mixed race, said that when she was growing up in New Jersey, her parents sometimes mentioned that the original deed to their house “said Black people couldn’t live there.” For her Ph.D. research, she had been assembling a database of genetic information about biodiversity, and when she read Dr. Schell’s paper, she realized that she had the data to test his ideas. “Redlining was so consistently practiced for so long in the U.S., we thought we could find a signal,” she said. Still, she was surprised by her own findings. “It was like, oh, god, how bad must this have been to still find a signal even when redlining was stopped in the ’60s,” she said.

From red line to redstart

Since the Industrial Revolution, wildlife across the planet has lost about 6 percent of its genetic diversity. The evolutionary effects of redlining are percolating through urban wildlife populations, but they are not yet set in stone. “There is still time to make positive change with environmental interventions that promote gene flow from more genetically diverse populations across the urban racial mosaic,” Dr. Schmidt wrote in a 2022 paper.

One way to spur that change, Dr. Katti and others argue, is to recognize and remedy a related problem: inequity in wildlife observation. Not only does the composition of wildlife differ between neighborhoods, but so does the incidence of people looking for wildlife. Diego Ellis Soto, a Ph.D. student at Yale, found that across the country, historically redlined neighborhoods were the least studied areas for bird diversity. Mr. Ellis Soto, who is from Uruguay, said he was shocked when he arrived in New Haven and saw how segregated the city was. In research published last month in the journal Nature Human Behavior, he found that neighborhoods that had been graded D had 74 percent fewer bird observations than those graded A, a fact that could affect conservation agendas. “How can we protect what we don’t have information for?” Mr. Ellis Soto said.

Dr. Katti, who has organized local bird counts in three urban areas during his career, has found ways around this challenge. Because birders tend to be white, with higher incomes, data from both National Audubon Society counts and the popular citizen-science birding app eBird are “skewed spatially in representation of higher-income neighborhoods,” Dr. Katti said. While eBird does receive a lot of data from urban sites, “it’s very patchy and unequal sampling,” he said.

The methodology for Audubon’s annual Christmas Bird Count varies by location, but in most places coordinators divide a count area into blocks and let volunteers pick a location anywhere within that block. “It’s all up to the volunteer where they want to visit,” said Jin Bai, a Ph.D. student in Dr. Katti’s lab. “And most likely, they want to visit somewhere more natural like a preserve, or somewhere away from people.” If you’re going out for a morning of birding, you’re unlikely to go to the park by the train tracks with trucks rumbling by. Unless you’re Mr. Bai: He goes birding in many formerly redlined neighborhoods and has documented several surprising species, including a yellow-billed cuckoo, an American redstart and a magnolia warbler.

Data recorded on eBird is widely used in scientific research and conservation, affecting projects like habitat restoration or captive breeding and decisions about whether to allow infrastructure. So gaps in bird observations based on socioeconomic factors have huge implications.

“The point we’re trying to make,” Dr. Katti said, “is that to use it for any kind of planning decisions, the data set is not reliable.” His bird-count methodology divides an urban area into grids of one square kilometer and then randomly chooses one point in each grid — and that’s where the volunteers go.

The influx of urban ecologists fanning out across these understudied landscapes is likely to shed new light on the twinned fates of humans and their nonhuman neighbors. Practitioners of urban ecology say their discipline is brimming with the potential to make discoveries with real-world impact. Mr. Ellis Soto, for example, is working with students in underserved New Haven schools, making hip-hop and bachata music from bird songs as a way to connect youngsters to the wildlife living around them.

“Now people are saying, ‘Heck yeah, I want to work in the toughest neighborhoods,’” Dr. Wood said.