Three nestlings in an osprey nest in the channel between Hart… (Amy Davis/Baltimore Sun )
Perched atop a weathered navigational marker near Rocky Point in Back River, the osprey shifted nervously, screeched and flew off as a boat full of people approached. With the raptor circling overhead, Rebecca Lazarus climbed onto the marker and peered into its nest, a tangled heap of tree branches and scraps of plastic.
"She's got one chick in here," called out Lazarus, a doctoral student at the University of Maryland, College Park. The osprey had laid two eggs, but only one hatched. The 3-week-old chick hunkered down to avoid detection, camouflaged by its still-developing brown-and-white plumage.
Lazarus and Barnett A. Rattner, a veteran scientist with the U.S. Geological Survey's Patuxent Wildlife Research Center, have been paying "nest calls" every 10 days or so since March on all the ospreys in Back River. They're trying to figure out why the "fish hawks," as they're sometimes called, aren't doing so well there.
Ospreys thrive across much of the Chesapeake Bay region. They have rebounded since the banning in the 1970s of DDT and other pesticides blamed for hurting reproduction of many raptors, including bald eagles. The last baywide osprey survey, in the 1990s, put the population at 3,500 nesting pairs, but that number has likely grown to 6,000 or 8,000, said Bryan D. Watts, director of the Center for Conservation Biology at the College of William & Mary.
"They've really exploded," Watts said, "and their distribution has changed. They've moved more inland than they were historically."
But even as ospreys spread up other bay tributaries, there seem to be relatively few nesting on Back River and in heavily urbanized waterways such as Baltimore's Inner Harbor and Washington's Anacostia River.
The scientists' Back River visits are part of a larger study, now in its second year, investigating whether ospreys' health and reproduction are being affected by past and continuing pollution. The team also is checking nests in the Susquehanna, Potomac and James rivers, as well as in Baltimore harbor and the Elizabeth River in Norfolk, Va. — two of the bay's toxin-laced "regions of concern." The researchers monitor the nests to see how many eggs are laid and how many hatch. They also take blood samples from the chicks, and the occasional egg, to check for chemical contaminants.
Because ospreys spend so much time on and around water, and subsist by catching fish, the birds are great sentinels for monitoring the health of the Chesapeake's living resources, the researchers said. Ospreys mate for life at the age of 3 and return to the same nest every March from wintering grounds in Central and South America. Females lay three or four brown mottled eggs, and the chicks that hatch in late May or early June remain nestbound for about eight weeks, eating fish brought to them by their parents until they are big enough to fly.
With USGS biologist Dan Day guiding the boat, Lazarus and Rattner went from channel marker to marker in Back River, checking nests as the adult ospreys soared noisily above. Lazarus clambered up metal ladders or hoisted herself onto platforms holding the nests to take a look. She pulled out bits of plastic the birds acquired for their nests that could choke or injure a chick. At a couple of markers that lacked ready access, they held a mirror on a long pole up to the nest to see what's inside.
"There's a full house here," Lazarus said after finding three chicks huddled in a nest. "This one looks like it's going to be a female," she said, holding up one with the neck coloration and yellowish eyes characteristic of females.
At another marker, this one without a platform, only a few branches remain of an unsuccessful nest, possibly damaged by stormy weather.
Just outside the river's mouth, by Hart-Miller and Pleasure islands, practically every channel marker has an osprey nest, and the team visited five with chicks. Upriver, the researchers have spotted seven nesting attempts by ospreys, but only three have yielded chicks or eggs.
The relative lack of osprey nests in the river could stem from its murky green water, Rattner suggested, which might not be clear enough for the birds to spot and catch fish.
It's also possible that chemical pollutants in the river are affecting the birds. It's taken decades to diminish since its banning 40 years ago, but DDT, the pesticide blamed for thinning raptor eggs, is no longer a threat. Likewise, traces of compounds once used as flame retardants — which showed up in osprey eggs sampled in a previous survey in 2000 — were found to have declined slightly in samples taken last year, Rattner said. But polychlorinated biphenyls, or PCBs, widely used as insulators and coolants until they were outlawed in 1979, have yet to decline in egg samples.