During two recent summers, scientists studied osprey nests in some of the most polluted regions of the Chesapeake, hoping to learn whether birds in the sky could shed light on conditions in the murky water below.

What did ospreys, the giant “fish hawks” often seen nesting on buoys and platforms around the Bay, have to say about their surroundings?

Quite a lot, it seems. From an osprey’s point of view, things have generally gotten better in recent decades.

“Birds are reproducing pretty well, even in some of the worst places in the Bay.” said Barnett Rattner, a scientist at the U.S Geological Survey’s Patuxent Wildlife Research Center, who headed the study. “Things certainly look a lot better than they did 20 years ago.”

But while the overall story is good, the results also raised some concerns. For instance, concentrations of the banned toxic chemical PCB were still surprisingly high in eggs from what was a supposedly clean reference site. And reproduction in Baltimore Harbor and the middle Potomac near Washington D.C. was only marginal.

The study, which will be published soon in the Archives of Environmental Contamination and Toxicology, helps to shed light on an issue of concern raised by the Bay Program’s 2000 Toxics Strategy. That document said more research was needed to determine potential impacts of contaminants on aquatic-dependent wildlife.

In that regard, Rattner said, osprey are a particularly good “sentinel” for showing how contaminants may be affecting predatory species. They eat fish exclusively and are at the top of the food chain, so they build up concentrations of chemicals that are in the fish.

They are widely distributed around the Bay, but pairs of osprey, which mate for life, return to the same area to nest each year. That allows scientists to compare how birds in different places are faring over time. And, because osprey are highly tolerant of human activity, they often nest near highly contaminated areas.

Like other large raptors, osprey around the Bay suffered greatly from pollution from the 1950s through the early 1970s, largely stemming from the widespread use of the pesticide DDT, which caused osprey eggshells to thin by as much as 35 percent. As a result, eggs often broke before they were ready to hatch. In addition, studies suggest the chemicals sometimes killed the embryos directly.

Their population, once numbering in the thousands of pairs, plummeted by as much as 6 percent a year during the 1960s. By 1973, only 1,450 nesting pairs were reported around the Bay. Today, they have rebounded to more than 3,500 pairs Baywide.

Toxic pollution problems are no longer considered to be as widespread as they were 40 years ago. But several areas—the Bay Program’s three Regions of Concern—are considered to be “hot spots” for chemical contaminants.

During 2000 and 2001, Rattner and his colleagues examined osprey in the Regions of Concern, as well as a relatively clean control site, to see whether contaminants were still affecting the large raptors.

“We went to the very worst places in the Chesapeake Bay to see what the birds were being exposed to, and how they were faring from a reproductive standpoint,” Rattner said.

Four areas were examined:

  • The Elizabeth River in Virginia.
  • The Anacostia River in the District of Columbia and adjacent areas of the middle Potomac.
  • Baltimore Harbor and the adjacent Patapsco River.
  • The reference sites were the South, West and Rhode rivers in Maryland.

During the study, Rattner and colleagues studied roughly 20–25 nests in each location, taking one egg from 14–16 nests in each site to study for contaminants. The rest were monitored to determine the number of eggs that hatch and the number of birds that survived to “fledge”—fly from the nest.

The study showed that in all areas, the birds were doing far better than in the past. During the late 1960s, hardly any osprey could be found in the Elizabeth River or Baltimore Harbor areas. No information was available about nesting in the Anacostia and middle Potomac at that time.

In the new study, Rattner and colleagues from the U.S. Fish and Wildlife Service and area universities found many nests in all areas, except the Anacostia River, where only one nest was found in 2000 despite the presence of many suitable sites. (That number had grown to three nests by 2003). Many nests were found in the nearby Potomac, though.

The study found that the percentage of nests that successfully produced at least one young bird was the same for the Patapsco River/Baltimore Harbor and Anacostia/ Middle Potomac as it was for the reference site. But the number of birds produced per nest was slightly less than at the reference site.

The paper called production in those areas “marginally adequate to sustain the local population in these tributaries.” Rattner, though, cautioned that some of the problems may have not been pollution-related: Several nests on the Potomac, for instance, were on unstable buoys.

On the other hand, he noted, two of four nests failed to produce any young in the highly industrialized Curtis Bay area of Baltimore Harbor, and three of four nests failed near the Naval Research Laboratory and Reagan National Airport on the middle Potomac River.

Meanwhile, osprey production in the Elizabeth River rivaled that of the reference site. “That was a big surprise to us,” Rattner said. “We thought that was going to be the worst of the three areas.”

The story got murkier as the scientists examined chemical concentrations in eggs.

Overall, eggshell thickness approached that of levels observed in the 1940s, before the DDT era. But eggshells were slightly thinner in the Anacostia/Middle Potomac area than in other sites. Eggs from that area also had slightly higher concentrations of DDE—a chemical that remains as DDT breaks down in the environment.

Still, current DDE levels for those areas were less than half of those observed in the 1960s and 1970s, when the pesticide contamination was at its worst.

On the other hand, PCB levels in eggs at all sites had diminished only slightly from those observed in the 1970s. PCB levels were highest in the Baltimore Harbor/Patapsco River and Anacostia/Middle Potomac sites, but were also surprisingly high in the reference sites, especially the South River, where concentrations rivaled those seen in the Elizabeth River.

“The thing that surprised us the most is that the South River didn’t come out as clean as we thought it would,” Rattner said.

Significant levels of mercury were not observed at any sites.

While some chemicals of historic concern, such as DDT, have declined, the study raised concern about concentrations of some “emerging” contaminants which had not been looked for in the birds in the past.

For instance, they found the flame retardant polybrominated diphenyl ethers (PBDEs) widespread in osprey, with the highest concentrations found in the Anacostia/Middle Potomac. The chemical, which is similar to PCBs in structure, has been of growing concern among scientists because it has become widespread in fish, wildlife—and human breast milk—although its exact impacts remain uncertain.

Rob Hale, a researcher from Virginia Institute of Marine Science who has studied PBDEs extensively and cooperated with the osprey work, said the concentrations in osprey appeared relatively high compared to those usually seen in fish. “It looks like there is some biomagnification going on,” he said. “They certainly are not low by any standard. The numbers are not astronomical, though.”

The manufacturer of the form of PBDEs of most concern to wildlife has agreed to end production this year, but Hale said the chemical is still in such widespread use that it would likely linger in the environment for years, much like PCBs, which have been banned for two decades but remain ubiquitous.

Hale said studies like the osprey work are useful because it helps to show how animals fare when exposed to a “witches’ brew” of chemicals at once.

“The birds are really integrators of a lot of this stuff,” he said. “Unfortunately, we often tend to think of chemicals in isolation, rather than consider the additive effects of the pollutant mixtures present. Hence we may underestimate the total impacts.”

Rattner said continued studies of osprey in places like the Regions of Concern could help to provide an early warning of problems—before they reach the magnitude of DDT in the 1960s.

“Some people only want to get concerned when you have effects at the population level,” Rattner said. “That’s too late, in my opinion.”

He also said that finding higher-than-expected PCB concentrations in places such as the South River show that ospreys serve as a good sentinel, and that studies in more areas of the Bay may be warranted. “There are probably some similar areas out there lurking that it would be good to know about,” he said.