Linda Poon is a staff writer at CityLab covering science and urban technology, including smart cities and climate change. She previously covered global health and development for NPR’s Goats and Soda blog.
How many squirrels live in New York City's Central Park? Finding the answer was surprisingly complicated.
If you’ve ever wondered how many squirrels live in New York City’s Central Park, there’s finally an answer: 2,373.
That number comes from the first squirrel census of Manhattan’s largest park, conducted by Jamie Allen and more than 300 volunteers who made it their mission to count and observe the rodents living in the 843 acres of green space.
And if you ask Allen why he did this, he’ll say, why not. A humorist and writer, Allen started wondering why no one kept count of squirrels while he was working on a short story eight years ago about his dog’s friendship with neighborhood squirrels in Atlanta.
“We kind of know other animal populations, like rats, in cities,” he says. (The conservative estimate is one for every New Yorker.) “It immediately became comical to me. Squirrels are an animal that we interact with on a daily basis, they’re disease-carrying, and they’re so common that we don’t even pay attention to them.” (It’s worth noting that most of the diseases squirrels carry don’t transmit to humans. Still, don’t go petting them.)
With that, Allen assembled a team of scientists, wildlife experts, and graphic designers and began counting the squirrels in Inman Park in Atlanta. After two counts, the team set their eyes on a more ambitious location: Central Park, which measures more than five times the size of his neighborhood park.
“It was the ultimate challenge,” he says. “And it’s the most famous park in the world.”
His team didn’t just count the squirrels. Just as the U.S. Census records demographics, housing data, and more, the Squirrel Census is filled with details about where each squirrel was spotted, what color its fur was, and whether there were clusters of them throughout the park.
Overall, the volunteers documented 3,023 squirrel sightings (this number includes squirrels that were likely counted more than once). Of that, 2,472 sightings (about 81 percent) were of gray squirrels, with various mixes of black, white, and cinnamon highlights. Another 393 were primarily cinnamon-colored, and 103 were black. All in all, they recorded 21 variations in fur color.
Volunteers also recorded the squirrels’ behaviors—whether they were running, climbing, eating, or foraging, for example. Some descriptions were colorful, others were clearly just for giggles.
One record logs a squirrel hanging in a tree “like an acrobat, hanging onto branch by its legs upside down.” Another “got bored.”
The project started out as something humorous, but there’s some real science involved. Early in the process, Allen enlisted the help of Donal Bisanzio, who at the time was studying urban epidemics at Emory University. Bisanzio helped him figure out how to tally squirrels—a crucial but complicated task for conducting a census. Squirrels are, well, squirrelly, meaning there’s a good chance that some would be counted more than once, and others might not be counted at all.
The trick is to divide and conquer. They drew a grid of 350 hectares—plots of land measuring 10,000 square meters—over Central Park. Think of them as something like Census tracts. Volunteers then fanned out and conducted two counts, one in the morning and another at night. The Squirrel Sighters, as they were called, spent 20 minutes per count searching for furry subjects, looking up in the trees and down in the bushes, and listening to the clawing and clucking sounds they make. Allen likens it to an Easter egg hunt; some volunteers found many squirrels, others saw none.
The team found the estimated “abundance number” after feeding their data into a formula popularized in the 1950s and ’60s by leading squirrel biologist Vagn Flyger, which takes into account the uncertainties of counting squirrels.
Then, to help readers visualize exactly where these bushy-tailed creatures live, Allen’s team member Nat Slaughter—a graphic designer and mapmaker—spent two years leading up to the October count creating two intricately detailed maps of Central Park. One maps the park’s terrain, including elevation detail and the various bridges, arches, and connecting paths, as well as points of interest like fountains and statues. That, along with smaller maps used to guide the volunteers, was created with the help of existing maps and of the city’s open data portal.
To include the level of detail he needed, Slaughter says he spent a lot of time in the park, sketching and taking notes, then feeding it into his computer. In doing so, he says, he had to correct for errors in the city’s data on things like elevation. “I was seeing a lot of hills that don’t actually exist in real life,” he says. The city used LIDAR technology, in which a drone or airplane shoots a light at the ground and the length of light is measured to determine elevation. The “phantom hills” are recorded when the drone or airplane mistakes, say, a building’s shadow as a hill.
Allen says being part of the project is about more than counting squirrels. In a way, he says, it allows you to experience the park differently than, say, if you were jogging through. As Slaughter told him, “It tunes the person to the environment,” and makes you notice things that you otherwise wouldn’t. Listen closely enough, and you can hear them rustling in the bushes, making the “kukking” noise, or crunching on a nut.
“Squirrels give themselves away by eating,”Allen says. “They’ll just be crunching on a nut and you’re like, ‘What in the world is that?’ And then you look up and there’s a squirrel.”
Asked how the census will further the academic literature on squirrels, Allen makes a clear distinction: This is not a study, and he’s not looking to prove or disprove any hypothesis. What researchers do with the observations is up to them. He says they will eventually release all the data into New York City’s open data portal.
For him, though, the census is simply his way of telling a story—about Central Park and its beady-eyed “citizens.”