Ospreys are a familiar sight on the edges of the Chesapeake Bay and in the tidal reaches of its tributaries, as abundant as sailboats on a sunny weekend afternoon in spring. An adult and juvenile osprey rest on a nest at the mouth of Raccoon Creek at its confluence with the Choptank River in Maryland. Osprey numbers have been swelling in the wake of artificial platform construction efforts and the banning of certain herbicides. (Dave Harp)

Drawn by warming weather, the promise of plentiful food and shallow water in which to hunt, they return to the Bay watershed every March. As remarkably adaptable birds of prey, ospreys can be found on every continent except Antarctica, but they have a special affinity for the Chesapeake. Nowhere else on the planet is there a larger breeding population.

Ospreys (Pandion haliaetus) may be flying high now, but that wasn’t always the case. Four decades ago, the population bottomed out at 1,450 nesting pairs around the Bay. Since then, their numbers have climbed more than eightfold, scientists estimate.

The osprey’s comeback story is a ray of hope for other recovery efforts, researchers say, if not an exact model that can be replicated for other species.

“You can go just about anywhere in the Bay and see osprey now,” said Bryan Watts, director of the Center for Conservation Biology of the College of William and Mary and the Virginia Commonwealth University. “I think they’ve become a really good symbol for how well we’re doing with the Bay.”

The turning point came in the 1970s after the federal government banned DDT, the chemical believed responsible for the osprey’s near destruction, said Barnett Rattner, an ecotoxicologist with the U.S. Geological Survey.

“Some of the most toxic chemicals affecting the osprey — the concentration of the chemicals in the food web — has decreased over time,” said Rattner, who studies the connection between toxic chemicals and osprey health. “There are still some problem areas from a pollution standpoint, but things seem to be improving.”

Standing watch over a channel marker, soaring in lazy parabolas, effortlessly snatching a fish — no matter the activity, ospreys are among the most recognizable bird species in the Bay region. The white breast and throat contrast sharply with the bird’s dark-brown back. Its crown is white, like a bald eagle, but with a brown band around the forehead.

Males and females usually mate for years, sometimes for life, but they don’t spend all of their time together, research has shown. They winter together in South America, but in the spring males fly northward first to scout nest sites. Once a nest is established — preferred spots are near water with good visibility — pairs return to it year after year.

The raptors also are distinguishable by their white feet and black talons, which are highly adapted for snagging fish near the surface of the water in midflight, Watts said. Ospreys tend to catch fish measuring 10–12 inches long. That partiality for larger fish nearly doomed the species, it turns out.

During the 1950s and ’60s, the osprey population crashed worldwide, along with other raptor species such as eagles and peregrine falcons. The main culprit, scientists would discover, was DDT — the pesticide that inspired Rachel Carson to author her seminal book, Silent Spring, in 1962.

The chemical had a greater effect on species at the top of the food chain, such as ospreys, Watts explained, because of a process known as biomagnification. After farmers sprayed DDT on their crops, some ran off into streams and into the Bay, where microscopic zooplankton ingested the substance. Small fish fed on the zooplankton; progressively larger fish fed on one another. Each step up the ecological ladder, the DDT became more concentrated in the tissues and internal organs.

DDT delivered a one-two punch to ospreys and other raptors, Watts said. Some birds were simply poisoned by large doses of the pesticide. But even nonlethal doses affected the hormone responsible for the calcium content of eggshells. The females began laying eggs that were often too soft to produce young.

Congress banned DDT in 1972. Osprey numbers have been on the rise ever since, Watts said. Today, he estimated, the Chesapeake region is home to around 12,000 nesting pairs in the spring and summer.

“[The Bay] is one of the most productive aquatic ecosystems in the world, driven by the all the shallow water. It’s the perfect area for ospreys to thrive,” he said. “And they have.”

Ospreys are again taking up seasonal homes in places where they haven’t been spotted for decades. When their population was waning, the raptors retreated to the mainstem of the Bay, Watts said. But as their numbers have multiplied, they have radiated up into rivers and streams.

Their upstream absence was so protracted that two or three generations of osprey researchers were unaware that such “tidal fresh” areas could support the birds, Watts added.

Their rebound might not have been possible, though, without a large-scale intervention inspired by the raptor’s own behavior, researchers say. At first, their habit of nesting on navigational markers was considered a nuisance because the giant piles of sticks would often obscure lights and signs. Wildlife officials soon realized, though, that building artificial platforms could accelerate the osprey’s comeback.

The structures are sturdier than trees and can be placed where predators, such as raccoons, can’t prey on their eggs. Between comprehensive surveys undertaken by Watts’ center in 1973 and 1996, the proportion of nests built by ospreys in trees plummeted from 31 percent to 7 percent as the birds switched their allegiance to channel markers and artificial platforms.

For a dramatic case study of the osprey’s renaissance, look no further than Poplar Island on the eastern side of the Bay in Maryland, just offshore Knapps Narrows and Tilghman Island. Poplar Island was well on its way toward disappearing until the Army Corps of Engineers in 1998 began rebuilding it with sediment dredged from the bottom of shipping channels serving the Port of Baltimore.

Today, amid the ongoing reclamation work, Poplar has become a sanctuary for osprey and other birds. In 2004, U.S. Fish and Wildlife Service biologists began stocking the island with osprey chicks recovered and rehabilitated by Tristate Bird Rescue in Newark, DE. The number of annually viable nests on the island has risen from five to 28 in that span, said Peter McGowan, who heads the program for the agency.

“The ospreys are a great success story,” he said. “It shows you how impacted those guys were during the DDT era.”

During the USGS’s most recent osprey study, which took place 2011–2013, Rattner and his team found lingering traces of DDT and PCBs in fish and young ospreys. But the concentrations were much lower than the levels that caused the 20th-century decline, he said.

Although ospreys appear to be thriving, scientists and wildlife officials need to remain vigilant, Rattner said. He is concerned about the impact of hunting in parts of the birds’ Southern Hemisphere territory, as well as signs of reproductive troubles in heavily industrialized areas, such as near Baltimore’s Back River wastewater treatment plant.

Overall, though, the study showed that the long-range improvements have continued. “Things seemed to have been improving,” he said, “and that was good news.”