A collage of a male and a female house finch sitting on posts sticking out of a Manhattan high-rise
Library of Congress / British Library / Getty / Will Mullery / The Atlantic

Urban Bird Feeders Are Changing the Course of Evolution

More than 50 million Americans are conducting an unwitting experiment on a vast scale. I joined them from my Manhattan high-rise.

To my knowledge, no one has ever been killed by a plummeting bird feeder. Still, when you live on the 25th floor of a Manhattan high-rise, you can’t hang one outside the window and risk knocking off a pedestrian below.

Several winters ago, I sat at my desk lamenting this fact while gazing across the gray expanse of chimneys and water towers stretching to the East River and beyond. I was holed up in my apartment working on a book about a fish: the storied Asian arowana, a swamp-dwelling endangered species rumored to fetch up to $300,000 as a pet. After chasing the creature around the globe for more than three years, I was now confined indoors, buried in drifts of notes. Each time I attempted to force my thoughts underwater, I found myself staring out the window, longing to see birds in the city sky.

Having sworn off swamps, I missed a connection to the wild. Seeking a remedy, I discovered a small Maine company called Coveside Conservation Products, which makes a unique “Panoramic in-House Window Bird Feeder.” A semicircular mahogany platform enclosed with plexiglass, the feeder fits into an open window and juts inward, providing a front-row view of birds bold enough to enter. No part of the contraption dangles outside, presumably rendering it safe for urban use.

In reply to my enthusiastic query, however, Coveside’s owner, Jim Turpin, was less than a salesman. “Frankly, I’m not overly optimistic about attracting birds to feed in a high-rise setting,” he wrote, explaining that most species search for food at specific heights. He pointed me to the website of the Cornell Lab of Ornithology, where someone had inquired about luring birds to a 17th-floor balcony. The answer—that attractive foliage can help, but don’t hold your breath—wasn’t promising, considering I don’t have a balcony and scarcely overlook a tree.

Nevertheless, I ordered the feeder, filled it with birdseed, and installed it in my window, where it interrupted the soundproofing, so I found myself working amid a cacophony of sirens and jackhammering. Two hundred and fifty feet above ground—the height of the tallest giant sequoias in the Sierra Nevada—the wind howls more often than not. Despite my best efforts to insulate the edges of the wobbly wooden feeder, freezing January air whistled through the apartment, slamming shut any door left ajar.

My husband, Jeff, was not thrilled. But then, as a native New Yorker, he is not exactly Mr. Nature. Once, when we were dating, we went on a short hike in New Jersey, where he strode purposefully to the edge of the woods, leaned into the trees, and began uttering a convincing trilling call followed by a kind of whirring and honking. I was impressed until I realized he was mimicking a car alarm.

Now, as our thermostat plummeted, I told Jeff not to worry—that I just wanted to see if I could attract a pigeon or two, inevitably followed by a zillion pigeons, at which point I’d have to take the feeder down. Yet no pigeon appeared. Weeks passed. Chilly, noisy weeks. I began working in the industrial-grade construction earmuffs I’d bought when masons were repointing our building’s facade, grinding out the mortar between the bricks.

Then, one morning in March, as I brewed coffee in the kitchen, Jeff strode into the office and sprang back out again, announcing he’d seen a flash of red. I joined him, and we peered around the door until the startled visitor worked up the courage to return. A cherry gum ball of a head poked up from the ledge and cocked to one side, uttering an inquisitive chirp and inspecting the room from behind plexiglass. Once satisfied that all was clear, a sparrow-size creature with a blushing breast and triangular beak hopped into the feeder. I recognized it immediately as a house finch.

It was one of the few birds I could’ve identified, having reported, some years back, on a novel strain of bacterial conjunctivitis that jumped to the species in the mid-’90s from domesticated poultry around Washington, D.C. The epidemic moved like pink eye through a preschool, rendering many infected finches virtually blind. Millions perished—a heavy blow to a species ranked among the most successful alive.

Native to western North America, house finches weren’t introduced to the East Coast until 1939, when a Brooklyn pet shop released a small number that had been illegally trapped in California. Over the next 50 years, these plucky pioneers established a firm footing, spreading across the continent until they reunited with their western cousins on the Great Plains. Today the finches inhabit perhaps the widest ecological range of any living bird, having emigrated from their ancestral deserts all the way to the edges of the subarctic taiga, adapting to suburbs and cities alike.

A house finch (Miriam Molnár)

Though such a common species might not excite a seasoned backyard birder, I hardly qualified as such. I’d always liked birds, but I didn’t know a brown booby from a tufted titmouse. Only in writing about the aquarium hobby—an invention of the Victorian era, whose amateur naturalists spawned modern biology—had I come to recognize my own faunal illiteracy. I’d grown up in a time when courses such as ornithology and ichthyology had fallen out of favor, supplanted by the study of cells and DNA. While I’d never experienced the urge to keep a fish in a tank myself, I could understand the desire to draw nature close—even into one’s room.

According to experts, feeding birds is probably the most common way in which people interact with wild animals today. More than 50 million Americans engage in the practice, collectively undertaking an unwitting experiment on a vast scale. Is what we’re doing good or bad for birds? Recently, researchers at the Cornell Lab of Ornithology sought to answer this question, analyzing nearly three decades’ worth of data from a winter-long survey called Project FeederWatch. Preliminary results suggest the species visiting our feeders the most are faring exceptionally well in an age when one-third of the continent’s birds need urgent conservation. Still, what are the consequences of skewing the odds in favor of the small subset of species inclined to eat at feeders? What about when the bird we’re aiding is invasive, like our house finch?

Three or four times a day, my heart thrilled at his arrival, announced with a polite, question-like cheep. He’d alight on the ledge outside my window, stand on his tiptoes, and strain his neck to peek inside like a finger puppet on the hand of some unhinged window washer. If I held still enough, he’d materialize in the feeder swiftly and daintily, only to begin eating with all the grace of a toddler, seed flying everywhere, sunflower husks spilling from his chomping beak.

Traditional wisdom dictates that birds don’t become dependent on a free lunch. The idea traces to the mid-1980s when the wildlife ecologist Stanley Temple and his then-student Margaret Brittingham color-banded several hundred black-capped chickadees in the Wisconsin woods, mounted two specially designed feeders, then laboriously counted the sunflower seeds each bird ate. To the scientists’ astonishment, the chickadees obtained only 21 percent of their daily energy requirements from the feeders. The findings suggest that birds haven’t evolved to rely on a single ephemeral food source; rather, they’re constantly sampling the environment, forever scanning for backup options in a changeable world.

When Temple and Brittingham removed a feeder that had been stocked for more than 25 years from one of their study sites—a nature center at Devil’s Lake State Park—the chickadees there fared as well through the winter as those in the remote reaches of the reserve, where birds had never even seen a feeder. In other words, birds don’t forget how to forage just because they’re receiving handouts. Only in especially severe weather did feeders appear to undoubtedly help: chickadees with access to one had nearly double the chance of surviving the harsh Wisconsin winter—the difference coming down to a few frigid days.

Before long, our resident male brought home a dull brown female to whom, at dawn, he sang an ebullient warbling tune that pierced the pillows over our heads. One morning, after a particularly passionate serenade, Jeff noticed the female tweeting up a storm in the feeder as she tilted her head and shimmied her wings. He called me over in time to see the male respond by leaning down to touch her beak and regurgitate down her throat.

I had never seen something more beautiful. “I think I’m going to pass out,” I said, overcome with elation.

Jeff seemed to think this was a little much. But he was growing attached to the finches too, and now the prospect of babies loomed. I confirmed that what we’d seen was indeed a courtship ritual, so when the female disappeared for a few weeks, I hoped she was off nesting in a nice drainage pipe somewhere. (I’d briefly entertained the fantasy that she’d lay her eggs in the feeder until I considered the hazards of fledging from the 25th floor. To say nothing of the red-tailed hawks that occasionally cruised by, known to live on New York University’s nearby Bobst Library.)

Studies suggest that birds receiving supplemental food may have a better shot at reproductive success, laying more eggs, for example, earlier in the year. Sure enough, one sunny April afternoon, incessant squawking interrupted my work. When the source of this ruckus clambered into the feeder, I found myself staring back at a bewildered fledgling with downy white pinfeathers sticking up from its head. Eventually, five of these clueless youngsters squeezed inside, making an awful racket and harassing their father for food he couldn’t consume and disgorge fast enough. It was an appalling vision of parenthood—and one that bore repeating in a matter of weeks.

Since house finches lay up to seven eggs in a clutch, and raise several broods a season, flocks may congregate by the hundreds. Soon, birds lined up on our ledge like patrons outside a hot new restaurant. Word spread through the finch population of Manhattan. By midsummer, the line zigzagged across the eastern face of our building.

The raucous crowd attracted a few curious hangers-on: a dark-eyed junco, the Darth Vader of sparrows, his eyes obscured in an all-black helmet of a head; a mourning dove I called Lola, who monopolized the feeder for hours on end, vacuuming seed into an esophageal pouch called a crop for later digestion; and one warm summer evening, as twilight fell over the city, something large and fluffy that perched in silhouette against the darkening sky. Jeff swore it was a chicken. I feared it was a hawk.

Such clustering around feeders can accelerate the spread of disease. So I shouldn’t have been shocked when I strode by the window one August morning, causing the birds to scatter as usual, except one—a mussy-looking male who seemed oblivious to my presence. On closer inspection, I saw that his eyes were pink and swollen, encrusted with dried ooze. He hadn’t seen me because he was nearly blind.

I’d been dreading this moment, hoping it wouldn’t arrive, and now I felt nauseous. More than two decades since conjunctivitis first began showing up in house finches, the disease continues to afflict the species, rendering the eastern population less than half its former size. To protect the flock, I knew I was supposed to shoo away the sick bird, though he could starve on his own.

I stood watching the finch peck lethargically at the seed. Then, tears in my eyes, I began to bang on the plexiglass until he stumbled dizzily out onto the ledge. Following recommended protocol, I removed the feeder, cleaned it with bleach, and closed up shop for two weeks to disperse the colony, lowering the shades against the throng at the window whose plaintive chirping I did my best to ignore.

To my surprise, however, when the quarantine expired, the birds were there, awaiting the grand reopening. I didn’t see any more eye disease, but as summer passed and the weather grew colder, I began to worry about desert natives wintering in New York. I ordered a tiny space heater designed for pet parrots and got Jeff to wire it into the feeder.

A few weeks later, on a snowy Saturday, I sat watching with satisfaction as a berry-red finch, puffed up against the cold, stood basking in the electric warmth, inhaling his breakfast. No sooner had I turned my head to bite into my own bagel then a sudden thud and flurry of flapping propelled me from my chair. I raced over to the feeder, where all that remained was a sad swirl of downy brown feathers, red only at the tips—and a single hawk quill.

“Maybe the little guy survived,” Jeff said, stricken, when he returned from the gym and examined the forensic evidence. I wasn’t so sure and felt culpable for luring the hawk to a veritable vending machine of finches.

Predators in the apartment seemed like a turning point. I had to acknowledge that the feeder had become a major distraction. House finches are known to exhibit mild “feeder aggression,” and fights were constantly breaking out, dueling males tumbling by the window in cartoonish pinwheels. In lieu of actual food, our cupboards were stuffed with 16-pound bags of birdseed that we lugged up from the mail room all too frequently. At least every other week, I had to remove the feeder to scrub it with bleach and scrape off our ledges caked solid with bird poop. From the street below, it was nearly impossible to figure out what was attracting the finches to the building. Nevertheless, when my downstairs neighbor—who’d complained in the past about my clunky footsteps and propensity for dropping loose change—asked if I was seeing “exotic” red birds, I sheepishly came clean and waited for a cease-and-desist letter that surprisingly never arrived.

In truth, I would’ve welcomed an easy out. I wanted to take down the feeder myself but felt responsible for the finches. If the classic studies on rural chickadees found they didn’t become dependent on feeders, how much did that tell us about birds eking out an existence on the mean streets of Greenwich Village? The question is more relevant today than ever before, in an age when half the human population now resides in cities for the first time in history, a proportion that’s rapidly on the rise. Over the coming decades, the future of many wild animals will depend on their ability to adapt to an increasingly urbanized world. At the library, I searched out a book called Avian Urban Ecology, which said that little is known about the impact of feeding most birds in cities. There is, however, one stunning exception.

When I tried to contact the study’s author, Alexander Badyaev, I learned he was off the grid, trekking through the mountains of north-central Montana. He was doing what he’s done for 25 years: chasing house finches—in this case, populations isolated in valleys along the Continental Divide. In this region, house finches are relative newcomers, invaders from two separate fronts: those from the East, introduced via New York in the middle of the last century, and Western natives, which have undertaken a dramatic and unexplained dispersal within roughly the same time frame.

“For some reason, the finches just decided to take over the world,” Badyaev told me when we finally spoke several weeks later, noting that the species has also spread south, pushing deep into the tropics of Mexico. Though the reasons for this continent-wide colonization remain mysterious, its well-documented and recent nature—knowing who derives from whom and when—presents a rare opportunity to watch the stages of evolution in the wild and observe how past adaptations affect future ones. If Darwin saw his theory of natural selection illuminated in the finches of the Galapagos, Badyaev is looking deep into the house finch to resolve a question he feels Darwin never answered: How does life maintain evolvability, the facility to change and adapt to the future?

A pair of Baltimore orioles and a pair of house finches at an urban bird feeder (Miriam Molnár)

In the Southwest, for example, where Badyaev is a professor at the University of Arizona, house finches live on their native turf and nest on the cholla cactus, females shielding eggs against temperatures hot enough to hard-boil them. Meanwhile, some thousand miles to the north, where Badyaev earned his doctorate at the University of Montana, the same species hatches its young under snow cover, having undergone a transformation involving everything from hormonal signaling to eggshell architecture. To understand such radical metamorphoses, executed over a few short decades, his team has been measuring nearly every conceivable metric in hundreds of thousands of banded finches across dozens of populations, yielding one of the largest studies of wild birds in the world.

So when one of Badyaev’s undergraduate students, Clayton Addison, noticed that the male finches on campus in central Tucson were not singing a rapid trill that’s essential for attracting females in the nearby desert, the lab was able to dig into the data for answers. Comparing the beak sizes, bite forces, and diets of the two populations, the researchers showed that the urban finches rely so heavily on feeders that their beaks have adapted: they’ve become longer and deeper to accommodate the sunflower seeds typically on offer, which are much larger and harder than the small cactus and grass seeds that rural finches eat. This adaptation has altered not only how urban males sing, but also what urban females prefer in a mate. It’s a pattern that Badyaev has since found in other places where finches live in the shadow of humans, the same large beaks arising from a surprisingly diverse array of developmental pathways. Such varied routes to an identical end—a beak strong enough to crack sunflower seeds—may be one way that nature hides variability from the swinging axe of natural selection.

Evolutionary theory aside, however, I was stuck on one point: There’s such a thing as a finch Brooklyn accent—thanks to feeders like mine.

So should we feed birds? By doing so, we’re almost certainly changing them. A recent study in Science found that the beaks of great tits have evolved to be longer in the United Kingdom than in continental Europe, possibly due to the popularity of feeders in England. What’s more, species from hummingbirds to northern cardinals are wintering farther north than they have in the past, though whether as a consequence of feeders, the warming climate, or both remains a largely open question. (To say nothing of the garbage and waste grain from agriculture that we feed many birds without intending to.) Precisely because we’re altering nature so radically and in so many ways, bird feeders are bound to seem comparatively innocuous. Meanwhile, proponents argue that they connect us to wildlife, opening our eyes to the world beyond our backyards, including those species most in need of our help, such as seabirds and tropical birds.

Just after Christmas, Jeff planned a short trip to Panama and, in a move entirely out of character, surprised me with a birding tour—something he stipulated he was willing to do exactly once in his life. Early in the new year, we rose at 4 a.m. and stood yawning in the steamy darkness outside our hotel, waiting for a guide named Justo, who turned out to be the khaki-clad young man staring straight past us.

“I expected you to be more elderly,” Justo explained, as we drove to Pipeline Road, a roughly 10-mile, car-free gravel swath cut through Soberanía National Park, one of the best birding sites in the Americas.

The sun had barely risen before we’d logged keel-billed toucans, white-faced whistling ducks, magnificent frigatebirds, golden-hooded tanagers, and squirrel cuckoos. A few hours later, I was wilting fast. Swatting away whatever mosquitos hadn’t drowned in my sweat, I stared blindly into the wall of foliage, my thoughts wandering to lunch and the strange fact that I’d apparently married a birding savant.

“What’s that?” Jeff asked casually, pointing to yet another magnificent creature I’d failed to notice perched before my nose, this one a masterwork of color blocking with a blue head, golden belly, and black-and-white tail, its eyes ringed in yellow.

A gartered trogon!” Justo exclaimed. “Plus 10 points for Jeff!” (“Minus 10 points for Emily,” he added, frowning as I tripped over the spotting scope again.)

I’d like to report that Jeff went pro. But after returning to New York and filing away the checklist with the 67 species we’d spotted, he hung up his binoculars, just as he’d promised. We both, however, started noticing birds everywhere we turned in the city, which over the course of the year amazingly plays host to more than a quarter of the species found in North America. Ultimately, the pleasure I found in this urban aviary, hidden in plain sight, helped me finally understand the passion some people feel for fish, concealed as they are beneath the water’s surface. If birds rank among the best-studied animals today, fish are among the least, disappearing faster than we can discover them.

As for our flock of finches, we remained loyal to them for another six months, even after I spotted one with a beak full of wire “twigs” dismantling our screen to build her nest. Then, when I got pregnant and mentioned to my doctor how much guano I was scrubbing, the feeder came down. Though most of the birds dispersed, a few stayed put. Many months later, I still hear them singing or catch one peering wistfully through the window. Otherwise, the apartment is quiet again, sealed off from street noise and whistling wind.

Well, not entirely quiet. “It’s so nice hearing a baby crying in the morning,” our downstairs neighbor said recently, her improbable enthusiasm suggesting we had finally driven her insane. “It’s like the chirping of the birds.”