On a cloudy day last July, a group of conservationists near St. Louis, Missouri, hopped into a small boat armed with cardboard boxes and yellow leg bands just a few millimeters in diameter. They motored toward a concrete barge anchored offshore, where they rounded up 18 squeaking, fuzzy, golf-ball-size Interior Least Tern chicks into a box for banding. 

“Good luck, little guys,” said Jeff Meshach, deputy director of the St. Louis-based wildlife hospital World Bird Sanctuary, during a livestream of the banding. After placing a delicate yellow band on one leg of each chick, he gently spilled them back onto the barge—their nesting habitat parked in a pond along the Mississippi River. When they grow up, it's possible they'll return to nest here; the bands let the conservationists keep track of them. “Hope all of you come back next year and raise your own kids here," he said.

When Interior Least Terns were listed under the Endangered Species Act in 1985, the birds had vanished from much of their historic range, including this area near St. Louis where the Missouri River meets the Mississippi River. But they have since bounced back, and today roughly 18,000 birds raise their chicks on nest barges and sandbars across nearly 3,000 miles of river in 18 states. Earlier this year, the U.S. Fish and Wildlife Service recognized the Interior Least Tern’s recovery by removing it from the federal list of endangered species under the Endangered Species Act (ESA), effective February 12.

Environmental groups have cheered the delisting, though they emphasize the move shouldn’t mean an end to conservation efforts. “This is great news and definitely warranted,” says Mike Parr, president of the advocacy group American Bird Conservancy. “But it will need ongoing vigilance to ensure it remains off the Endangered Species Act list.”

The Least Tern is America’s smallest tern: Adults weigh less than two ounces and measure eight to nine inches long. The species is widely distributed, nesting on Pacific and Atlantic coast beaches as well as along rivers’ soft shorelines. But the birds that nest in the country’s interior is managed as a distinct population. During the summer nesting season, Interior Least Terns can be found along the Mississippi, Missouri, Ohio, Arkansas, and other rivers throughout the Central United States.

Those rivers have undergone significant change over the past century, and the birds have, too. Beginning in the mid-20th century, the U.S. Army Corps of Engineers dredged and streamlined complex river networks to construct a single wide, deep passage to make room for ships moving goods. These management efforts did away with many secondary tributaries and sandy islands where terns liked to nest. The few beaches that survived remained connected to the mainland, which made the terns’ shallow nests easy pickings for predators like foxes and coyotes. By the 1980s, Interior Least Tern populations had plummeted to fewer than 2,000 adult birds; they were listed as federally endangered in 1985.

Three splotchy Interior Least Tern eggs sit directly on the sand on the nesting barge near St. Louis. The eggs are well camouflaged amid the shells and sand; the species doesn't build a nest. Tara Hohman

The listing forced the Army Corps to protect the remaining Interior Least Tern habitat, and their numbers began climbing back up—but not significantly enough to ensure the population’s long-term survival. Then, about 20 years ago, Paul Hartfield with the U.S. Fish and Wildlife Service (FWS) approached the Mississippi Valley Division of the Corps with a proposition: Rather than working at odds, FWS and the Corps could work together.

In a recent interview with Audubon magazine, Hartfield recalled his argument. At the turn of the millennium, the Corps generally fought against the ESA and the FWS, viewing conservation policy as bureaucratic red tape that interfered with their construction projects. Instead, he argued, the Mississippi Valley Division of the Corps could save time and money by integrating conservation into their programs at a fundamental level. By making a few small changes, they could, for example, dredge rivers more intentionally to construct a healthy, complex ecosystem for a number of endangered species like pallid sturgeon, fat pocketbook mussels, and Least Terns.

“What had been lost in the river ever since they started managing it was habitat and depth diversity,” says Hartfield, who served as the Interior Least Tern lead recovery biologist for 10 years. “So, we agreed that what we would do is manage for habitat complexity and explore the possibility of opening up some of the [river] channels that had been closed.”

The Corps was skeptical at first, but they agreed to try. First, they carved experimental 200-foot notches in dikes that had been installed to streamline river flow and reduce erosion. The notches redirected some water into secondary streams that disconnected islands of sandy habitat from the mainland, protecting nesting birds from predators and human beachgoers. Dikes with notches proved to be just as structurally sound as those without—and ended up being cheaper to build because they used less material.

“It was a real wake-up call for the Corps,” says Hartfield. By 2013, nearly a third of all dikes in the Lower Mississippi (roughly 300) had been notched, and new dikes were constructed with this feature already in place.

The Corps also began to construct new sandbar islands for terns using sand routinely dredged from the bottom of the river, which they would otherwise dump in a heap offshore. During a trial of this technique on the Ohio River in 2002, terns began nesting on the new sandbars the very next day. One month later, there were 64 active nests.

A nesting barge floats in a pond along the Mississippi River near St. Louis. The barge is a floating flat surface covered in sand with a one-meter fence to keep predators out. Its sole purpose is to serve as nesting habitat for birds. Tara Hohman

They also began experimenting with placing barges in rivers to serve as nesting islands for terns. The nesting barge near St. Louis is an example of a project managed by the Corps in cooperation with conservation groups, including Audubon. “It’s a very unique and collaborative partnership we have,” says Tara Hohman, a conservation scientist with Missouri’s Audubon Center at Riverlands. “The Corps manages the Riverlands Migratory Bird Sanctuary where our Audubon Center is located, so we have a lot of our conservation work based around their managed areas.”

The collaborative efforts have paid off. About 80 percent of the Interior Least Tern’s habitat is now managed by the Corps in this way, and the population numbers roughly 18,000 individuals. The Corps is also applying this management approach to other endangered species conservation efforts, such as the Least Bell’s Vireo in southern California.

Hartfield, who drafted the recommendation to delist the Interior Least Tern before retiring this year, is confident that ecosystem-based management of the tern’s habitat will continue independently of its endangered species status. “Conservation is now part of that ecosystem,” Hartfield says.

The Corps-managed nesting barge near St. Louis, for instance, will persist as tern habitat for the foreseeable future. And conservation groups will keep tabs to make sure the birds also stick around even after their federal protections have disappeared. “Monitoring post-delisting is just as important as monitoring while listed,” Hohman says.

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