Mike Callahan scanned the surface of Jones Pond in Caledon State Park on Saturday morning, using binoculars to zero in on a small group of waterfowl.

Most sported dark green heads, red bills and black backs over distinctive long, white bellies.

“They’re common mergansers,” Callahan, a park interpreter, told participants in the park’s Winter Waterfowl Adventure program. “See how they stood up and did a lot of flapping before they took off?”

He had spent the first part of the program, which will be held again Feb. 2, giving a slide-show presentation on how to identify the waterfowl most commonly found this time of year at Caledon. It’s one of a number of nature programs offered each month at the 2,579-acre park in King George County.

Mergansers, like other diving ducks, need to flap their wings and run across the surface of a pond, lake or river in order to take off, Callahan said. Dabbling ducks such mallards, on the other hand, can take off right from the water.

Three types of mergansers call Caledon home. Besides the common merganser, they are the red-breasted merganser, which sports a mottled, reddish-brown breast and ragged crest; and the hooded merganser, which is the smallest and flashiest of the three. The male has a white crest with a black border, and can sometimes be confused with a bufflehead, a small diving duck with a wedge of white at the back of its black head, he said.

Bald eagles also nest in the park this time of year. Chris McClintock, who manages the Friends of Caledon State Park’s Facebook page, spotted an adult perched on a branch at Jones Pond during a stop on the tour that Callahan took participants on after the presentation. He used the iBird Pro app on his cellphone to play a recording of its call, a high-pitched, whistling cry.

“Hollywood uses the red-shouldered hawk’s,” he said. “They think the eagle’s is too wimpy.”

Callahan urged everyone to be quiet as they walked around the side of the pond facing the Potomac River to see what else they could spot. Just beyond a clump of grasses, a great blue heron was standing patiently in water up to its belly as it searched the pond for lunch. From the woods came the shrill, hoarse “tchur” call of a red-headed woodpecker, whose black and white wings Callahan said remind him of a tuxedo. And on the ground was a pile of scat.

“The river otters have been here,” he said. “You can see fish scales in the scat. Cool!”

Boyd’s Hole, another stop, got its name because it was a deep spot in the Potomac. Except for a few mallards and gulls flying overhead, there was little to see there Saturday. Callahan speculated that it may not have been cold enough yet for some waterfowl to migrate this far south. Usually such species as dadwalls, pintails and both muted and tundra Swans can be spotted in the park this time of year, and large flocks of greater and lesser scaup have been known to fly in.

But that didn’t mean that Boyd’s Hole didn’t yield anything of interest. Callahan gave an impromptu history lesson about the spot, saying that George Washington used to come there to fish and visit with the Alexander family, who owned Caledon. There are records of the United States’ first president spending the night at the plantation.

“We are probably the only state park that can say that he spent the night here,” he said.

A swing around Aulder Flats yielded a stocky, blue-gray belted kingfisher sitting on the branch of a tree, beady eyes alert for fish. The area had been drained and converted into farmland before Caledon became a state park, and has since been returned to its former state, Callahan said. Recent rains have given it the appearance of a small pond.

Heading back to the visitor center, he pointed out three of the six species of woodpeckers that live at the park either seasonally or year-round, including the yellow-bellied sapsucker, pileated woodpecker and the red-headed woodpecker. They’d be the focus of the Woodpecker ID program he was giving that afternoon.

Then Callahan abruptly stopped the van so he could get out and examine a large pile of scat containing hair and bone, and photograph it with his cellphone. Then he uploaded the photo into iNaturalist, an app that’s a joint initiative by the California Academy of Sciences and the National Geographic Society. It connects users with more than 400,000 scientists and naturalists to help identify plants, animals and birds.

The first suggestions that came up were for bobcat and lynx, but Callahan decided that it looked more like coyote scat.

“You share it,” Callahan said, “and then it becomes part of the citizen science.”

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Cathy Jett: 540/374-5407

cjett@freelancestar.com