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By late May, New York City is full of baby birds. Speckled young robins have fledged their messy nests and hop along after their parents, still hoping for an offered worm. Young Red-tailed Hawks huddle in nests built on both tall trees and high-rises. The parks’ ponds are dotted with Wood Duck and Mallard ducklings. But unbeknownst to most of the city’s 8.5 million human residents, another urban bird, both striking and abundant, is also raising young within city limits. Tucked into trees on little-known islands throughout the New York-New Jersey harbor are raucous colonies of Black-crowned Night Herons—the largest breeding population of the species in the Northeastern United States. But according to the scientists who study the birds, the herons are in trouble and could vanish in just over a decade. 

Nesting alongside several other species of long-legged wading birds, including Great and Snowy Egret and Glossy Ibis, the night herons occupy islands that range from natural islets just off the coast of densely populated Bronx neighborhoods to abandoned human-constructed landmasses far out into New York Bay. For a quarter of a century, New York City Bird Alliance, an ̽����ѡ chapter, has monitored them all, visiting 20 islands in total throughout the last two weeks of May each year and individually tallying every nest, egg, and chick. Despite the incredible liveliness of the occupied islands in the breeding season, NYC Bird Alliance’s two decades of robust data reveal a stark reality: As reported in a , the harbor colonies have declined by 27 percent since 2000—driven by a particularly steep drop-off in Black-crowned Night Heron numbers. 

Historically the most abundant species in the breeding colonies, the herons have lost more than half of their breeding population on the islands. “It’s kind of shocking,” says Dustin Partridge, NYC Bird Alliance director of conservation and science. When the researchers began digging into the data, he says, they expected to see a decline but not a real risk to the local Black-crowned Night Heron population. “But what we’re showing is that they are going to disappear at some point in coming years.”

Until the new analysis, the harbor colonies looked like a smashing success story. Sixty years ago, the islands hosted no such rookeries. Although New York’s waters once hosted massive wading bird colonies that astounded observers with their scale, the birds disappeared by the early 20th century due to hunting, habitat loss, and increasingly polluted waters. They stayed away for decades—until shortly after the passage of the Clean Water Act in 1972. Just two years later, local birders spotted 50 pairs of wading birds nesting on a small island off the north coast of Staten Island. Over the next two decades, the herons, egrets, and ibises returned to other islands throughout the New York-New Jersey harbor, including in Jamaica Bay and the East River. By the late 1990s, the new study shares, the harbor colonies supported as much as a quarter of the long-legged waders breeding in the Northeast United States, including Black-crowned Night Herons. NYC Bird Alliance’s analysis predicts that the herons could completely vanish from the area by as soon as 2037, with cascading effects on the species throughout the region.

It’s not yet clear what is driving the decline, but NYC Bird Alliance has some theories. The stocky herons, mostly white with a blue-black head and wings and striking red eyes, can be seen throughout the year near the water in New York City’s parks and wetlands, but are especially visible and abundant during the breeding season, when both adult sexes grow bright-white head plumes. Occasionally they even turn up on city streets, Partridge says. “They are pretty resilient,” he says, pointing to their ability to navigate a developed urban landscapes. But scientists who study the species have discovered that Black-crowned Night Herons are particularly sensitive to environmental pollution. “Their young just fail to thrive if there’s some sort of contaminant present.”

If the culprit is a contaminant, just what it could be remains a mystery, Partridge says, because on paper the Hudson River continues to get cleaner each year. In fact, the waters around the city are currently seeing a to the estuary, thanks to tighter regulations and concerted clean-up efforts. Black-crowned Night Herons’ sensitivity makes them a sentinel species for the ecosystem. “They signal when something’s amiss,” Partridge says.

Potential contaminants affecting the herons could include pesticides, the now-illegal but still abundant industrial chemicals called polychlorinated biphenyls (PCBs), or heavy metals, all of which could also impact human health. Other possible culprits include predators—namely nest-raiding raccoons, which can eradicate nesting waders from an island within two years—and human disruption; the nearer islands are popular destinations for jet skiers, whose revelries can cause adult birds to abandon their nests.

In addition to continuing the annual colony counts, NYC Bird Alliance is launching investigations into the potential sources of the Black-crowned Night Herons’ precipitous decline. This May, the team placed acoustic recorders on the islands in order to gauge whether and how often the colonies are disturbed by human visitors throughout the breeding season. They are also making plans to begin collecting feathers and eggshell fragments to test for environmental contaminants. “We’re kicking off a bunch of new work,” Partridge says.

One component of that work is a campaign to add the Black-crowned Night Heron to the New York state list of threatened and endangered species. It’s good timing: Last year the state added species to the list for and has committed to making more updates. Although the Black-crowned Night Heron is globally abundant and not listed by IUCN or the federal government, the species has already been declared endangered in Pennsylvania and Maine and threatened in New Jersey. State listing would help support efforts to protect the birds, including addressing the root cause or causes of the decline, once identified. In order to build support for the listing, NYC Bird Alliance is reaching out to the public, aiming to underscore what the city dwellers—both human and avian—share in common. “They stay up late at night, they can show up anywhere, and they’re nesting right here alongside of us,” Partridge says. “We’re going to really be talking about them as New Yorkers and as a New York City bird.”

On one of the final surveys of the 2026 season, a team of NYC Bird Alliance scientists and volunteers visited a small island in the Bronx that once hosted a healthy colony of nesting herons—until human disturbance drove the birds away. Black-crowned Night Herons haven’t built a nest on the island since, but last year the team spotted several roosting adults. If the birds resume breeding on the island, it would be the first time NYC Bird Alliance has seen the species return to an island after abandoning it.

Bushwhacking around the overgrown island revealed no nests, but the team heard at least one adult Black-crowned Night Heron; their call is a harsh cry reminiscent of a small dog’s bark. Their eventual return still seems possible, and that is key to NYC Bird Alliance’s long-term plan to save the harbor herons. Once they can ensure the islands’ safety, NYC Bird Alliance scientists hope to draw Black-crowned Night Herons back to currently unused islets, using decoys and audio cues.

Reaching that point will require a much better understanding of what’s causing the species’ decline, but that’s exactly what NYC Bird Alliance is setting out to do. Although their forecast for the harbor herons is sobering, the team remains hopeful. “Good news is we’ve discovered it soon enough,” Partridge says. “We have 10 years to act.”