Ground zero for ospreys

Around 20,000 ospreys are attracted to Chesapeake Bay annually, making it the world's most important nesting grounds for the species. But researchers believe that the decline of one little fish might play a significant part in the decreasing breeding success in recent years.

Words by Cheryl Lyn Dybas
Photographs by Jay Fleming

Rosy orange fingers of first light shimmer across a marshy inlet on the Ware River, a tributary of Virginia’s Mobjack Bay. Mobjack lies along Chesapeake Bay’s western shore, not far from where the Chesapeake meets the Atlantic Ocean. Tracking the next generation of ospreys, or fish hawks, so-called as the birds only eat fish, is today’s objective for biologist Bryan Watts, director of the Center for Conservation Biology in Williamsburg, Virginia. On this June day high tide is at 6:00 a.m. Checking on young ospreys in Mobjack’s nests depends on deep enough water to manoeuvre in shoal-laden creeks, so researchers need to get underway early. Watts and his colleague Michael Academia, also of the Center for Conservation Biology, will census young ospreys in three of Mobjack’s tributaries: the Ware, North and East rivers. A boat ramp leads into the Ware from a landing just outside the community of Gloucester, Virginia. At the ramp’s top, it’s all aboard as Watts backs a skiff into the inlet. 

Although ospreys are found in many parts of the world, they’re attracted in large numbers to shallow bays and estuaries along the U.S. East Coast, especially the Chesapeake. The bay and its tributaries are now home to some 20,000 ospreys, making it the most important nesting grounds in the world for these raptors. Each spring, ospreys fly north to breed along the Chesapeake from wintering areas in Central and South America. The birds spend the summer feasting on schools of fish such as menhaden. 

Chesapeake Bay is the largest estuary in the U.S. and is among the most productive waterbodies in the world. Submerged, salt-tolerant grasses offer shelter and food for fish seeking a place to hide. But it’s not easy to elude the osprey, master of the splash-and-grab. During most of the breeding season, the male does the fishing. Hovering above the waves, the bird suddenly folds its wings and plunges, bulletlike, into the water. It hits feet first in a burst of spray and seizes its prey with formidable talons. Flapping heavily, it turns the fish head first for aerodynamic flying, then carries its prize to a perch and helps itself before bringing home the remainder.

Watts, Academia and I motor downriver on the Ware, stopping at every nest we see to check for young birds. The nests are on channel markers, docks, duck blinds, platforms built for the birds by waterfront landowners, and any other structure ospreys can land on. Ospreys set up housekeeping wherever they find good fishing and protection from raccoons and other land-based predators. As we slowly float up to Ware nest number 4, there’s one young bird visible, but, as Watts says, “it’s not in great shape”. Female ospreys usually lay between two and four eggs. To see into the nest, Academia holds a long-handled mirror high above, allowing us a glimpse of its lone resident.

We zoom on to nest numbers 5, 6 and 7 through 10. Each time, there are no young. Finally we reach the last Ware River nest, number 29: 1 chick!  By now, it’s a celebration to find a single young osprey. In 29 nests on the Ware, two chicks. Next we head to the North River, where out of 33 nests, there’s just one chick. On to the East River, where, out of 21 nests, not even one chick. Out of 83 osprey nests in Mobjack Bay and its tributaries, there are three young, all in different nests. “That means that the success rate is 0.036, the lowest reproductive rate I can remember anywhere for ospreys,” says Watts. “Maintenance rate is 1.15 young per nest.”

Sadly, many of the adults are still perched on failed nests, seeming to wonder what could have happened to their chicks. In Mobjack Bay, the halcyon days for ospreys seem to be gone. Watts suspects that menhaden in this bay, the ospreys’ main food here, are also largely gone. A few days later, Watts surveyed the lower York River south of Mobjack Bay, checking 44 nests. Only three nests had chicks, including one one-chick nest and two two-chick nests. “The birds in the two-chick nests were only 3.5 weeks old, so it’s unlikely they will all make it to fledging,” says Watts. “A waterman who works nearby says ospreys haven’t successfully produced chicks in three or four years.” Some of the structures, Watts says, “have been used by ospreys for decades and have been consistently productive. From some, we used to send chicks to other states that needed them. It’s striking to see them empty.” Watts and Academia found better news in the upper Rappahannock River. The Rappahannock is north of Mobjack Bay and its tributaries. ‘Up river’, the researchers checked 37 active nests; 28 were successful, with 56 young, a 76% success rate or 1.51 young per nest. 

“The productivity there,” says Watts, “is in line with that of the upper James River, where there are a lot of adults with fish.” The James lies south of the Rappahannock. There the scientist found 45 of 54 nests with young, an 83% success rate. Ospreys in the upper James produced 77 young, for a reproductive rate of 1.42 young per nest.

In a place that might be ground zero for ospreys, Watts’ findings are unexpected. He calls the Chesapeake a tale of two bays: one where ospreys are abundant, primarily in its upper tributaries, and another, in the lower mainstem, where so few of the birds’ young survive the population can’t break even. “It’s clear that the tidal freshwater reaches [of Chesapeake tributaries] – where the birds are feeding on catfish, gizzard shad and other fish – are producing normally and above the break-even rate,” Watts says. “That’s in stark contrast to saltier areas in the mainstem of the Chesapeake, where ospreys depend on menhaden.” In his 2007 book on menhaden, H. Bruce Franklin called them “the most important fish in the sea”. Known as forage fish, menhaden are crucial to the diets of striped bass, bluefish, summer flounder and other fish. They’re also critical to countless other species, including ospreys. 

Photographs by Jay Fleming

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This feature appears in ISSUE 32: SENTINELS OF CHANGE of Oceanographic Magazine

Issue 32
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