Old Lyme — Soaring in a wide arc over the marsh, talons gripping a silvery menhaden, the osprey dove toward its stick-pile nest atop a wooden platform, where three hungry chicks awaited dinner.

“It’s a good time of day for a fish delivery,” said Paul Spitzer, peering through binoculars in the late afternoon Tuesday as his motor boat bobbed in the Connecticut River around Great Island marsh at high tide. “They’ll eat the whole fish, except for the gill cover, which is pure cartilage.”

Spitzer, an independent ecologist working for the Connecticut Audubon Society, has witnessed many such deliveries this spring and summer, sometimes even seeing a trail of menhaden blood dripping through the air after the big raptors have plucked the fish from the river.

“I call this the osprey garden,” said Spitzer, an Old Lyme resident whose vocation and personal passion are clearly one and the same.

Now in the third year of a three-year study funded by grants from Millstone Power Station owner Dominion Resources and private sources, Spitzer has been visiting ospreys throughout the 500-acre salt marsh at the mouth of the river two to three times a week since the end of May, following the progress of the nesting pairs and their offspring.

This year, he’s tracking 25 pairs in the marsh, with about 50 chicks among them, often using a long pole with a mirror on the end to see inside the nests, and sometimes even using a ladder to get a better look.

“Ospreys sort of force people to appreciate nature,” he said, noting that the big birds — once nearly extinct because of DDT — coexist well with the many kayakers, canoeists and other boaters who ply the waters around Great Island. “The ospreys tame down, and the people tame down.”

The purpose of his research, he said, is to track the success of breeding ospreys as a way to evaluate the menhaden population in Long Island Sound.

These bottom-of-the-food-chain fish, which migrate into the Sound from southern waters in late spring and summer, play a vital role in keeping the whole estuary healthy, so ensuring they are present in sufficient numbers is key to determining whether current management of the commercial menhaden fishery is working.

“There’s enormous benefit to us when these fish are left in the water,” Spitzer said.

Menhaden, he explained, consume large amounts of plankton that helps maintain water quality, and are in turn the main food for important species in the Sound, including striped bass, bluefish and ospreys.

Adult ospreys and their young can’t thrive without ample supplies of the menhaden, a member of the herring family also called bunker.

His preliminary conclusion, Spitzer said, is that the growing osprey population indicates that the menhaden population in the Sound is healthy.

Fisheries regulations enacted in recent years to limit the harvest of menhaden in southern waters have been working to restore populations, he said.

“What we don’t want to see is a rollback to the days of heavy harvesting,” he said. “This is a demonstration study that can plug into advocacy to maintain the steps taken to reduce the harvest.”

While ospreys are the main attraction for Spitzer, the many other birds that nest and feed at Great Island also spark his enthusiasm.

He noted mixed flocks of great egrets, snowy egrets and cormorants fishing at the southern shore of the island, and squished his way across the spongy marsh to check on two nests he’d discovered hidden in the grass on earlier visits.

At the first, a tiny bowl of dried grass, he found three tiny pink sharp-tailed sparrows that had hatched a few days earlier, mouths agape.

On his way to another osprey platform, he parted the grass to find a newly hatched downy willet chick in a nest with two other brown-speckled eggs, one of them starting to crack.

On an earlier visit, the nest had been partially flooded at high tide, so he wasn’t sure the chicks would survive.

He’ll share the information with Chris Elphick, an associate professor of ecology and evolutionary biology at the University of Connecticut who’s studying how marsh-nesting birds are faring with rising sea levels.

“This shows the eggs can still be viable after they're flooded,” he said. “This is really a breakthrough day, in that I very much wanted to find out what was happening to those nests.”

j.benson@theday.com