The number of migrating loons stopping along Maryland’s Choptank River to eat has dropped sharply in the past 10 years.

Last fall, Paul Spitzer, a visiting scientist at the Cooperative Oxford Laboratory in Maryland, said he observed only about 300 loons. That was more than the roughly 200 counted in 1998, but three to four times below the number seen a decade ago.

“It was a little better than last year,” Spitzer said. “But not much. Also, they didn’t linger that much. It was clear there wasn’t the food there to hold the birds in this part of the Bay for an extended period.”

The reason for the decline, he said, appear to be a lack of small, young menhaden in the river, whose number is down dramatically from a decade ago.

It’s a relationship that Spitzer has seen elsewhere. While concerns have been growing that a sharp drop in the coastal menhaden population may be affecting the fish that eat them, such as striped bass, Spitzer believes that some fish-consuming birds are suffering, too.

The most notable example is Gardiners Island off Long Island, site of one of the longest-studied osprey nesting areas in the country, and also a subject of Spitzer’s research. From 1800 to 1950, the island was the site of 150 to 300 active osprey nests and was considered the world’s largest osprey colony.

DDT caused massive reproductive failure in the 1950s and ’60s, and osprey nests declined to 26 before beginning to rebound after DDT was banned in 1972.

Over the past five years, though, the number of active osprey nests counted by Spitzer’s colleague, Mike Scheibel of The Nature Conservancy, on the island has fallen from 71 to 47. “They lost 10 nests in just this past year,” Spitzer said. Osprey reproduction has also suffered.

The drop parallels the decline of menhaden in the area.

In near-shore areas, osprey can feed on a variety of fish species. But on isolated islands, such as Gardiners, osprey are often a “menhaden specialist” — depending on passing schools of the small, oily fish.

But the story is not clear. Large numbers of cormorants have taken up residence on Gardiners Island and could be competing with the osprey for food.

“It’s hard to sort out,” Spitzer said. But he notes that in the 1970s, when osprey numbers in most other places rebounded after the ban on DDT, the birds on Gardiner’s Island were slow to recover. The 1970s were also a time of low menhaden abundance.

Also, cormorants, which dive deeper than osprey, tend to eat slower-moving bottom dwelling fish. They also prefer smaller fish than the osprey.

Spitzer said he would like to see how the osprey fared if the cormorants were removed. If that were to happen, Spitzer said he believes the island’s osprey would prove to be a “bioindicator” of the health of the menhaden stock. “I don’t think that colony can exist in a viable form without proper menhaden management,” he said.