Sometimes, to enjoy a feast for the eyes, you have to hold your nose.

Chesapeake Bay birders have long known that the best place to see dazzling winter waterfowl and rare arctic birds is at the local sewage treatment plant.

There, among aerated ponds and concrete barriers, the geese and swans roost after a long day feeding in nearby fields. Come in the morning and watch a clear blue sky turn white with tundra swans on the wing. Stay for the evening concert as the birds “ou-oh” before they settle in to the settling tanks for the night.

Hurlock, on Maryland’s Eastern Shore, has a sewage treatment lagoon prized — among birders, anyway — for hosting thousands of roosting tundra swans. Waste facilities in Chestertown and Easton have also become regular spots for birders to check for geese and rare birds, like the Franklin’s gull. The Back River Wastewater Treatment Plant in Baltimore even hosts a monthly bird count.

Matt Whitbeck, supervisory wildlife biologist at Blackwater National Wildlife Refuge, said he’s not surprised that birds, particularly waterfowl, enjoy the ambience at sewage treatment plants. Many swans and geese at Blackwater also prefer the lagoons created as wildlife habitat at the refuge. Sewage plants often have manmade ponds to hold treated wastewater before discharging it to rivers or streams. The plants aerate the water, adding more oxygen and making it healthier and cleaner than the nearby river water in many cases.

“I can’t help but wonder if you sometimes get similar conditions at these wastewater plants,” Whitbeck said of the Blackwater impoundments. “You have a good freshwater wetland. They are conducive to a suite of waterfowl. You get a good plant community. You get good water levels. You have nice foraging depths. It’s kind of a self-sustaining habitat.”

Of nearly 14,000 tundra swans that winter in the Chesapeake Bay, about a seventh spend their nights at the Hurlock Wastewater Treatment Plant, according to Bill Harvey, the game bird section leader at the Maryland Department of Natural Resources. The department conducts an aerial bird survey every January; Hurlock is on the flight plan.

“There’s nothing that compares to Hurlock for swans,” Harvey said. In the morning, “they fly out in small groups, 20 or 30 at a time. “

Hurlock’s plant has even earned a spot on the Maryland Ornithological Society’s county-by-county list of places to view shorebirds. Birders must stop by the office and ask for permission to enter; it’s almost always granted. It’s open from 6:30 a.m. to 2:30 p.m. Monday through Friday.

The best time to see the swans, said plant manager Eric Barnhart, is about 7 a.m., before they leave to forage in the nearby farm fields.

Asked if he’s excited to see the swans roosting, Barnhart said: “I guess. I see it every day, every year. I see it all the time.”

Birders are a bit more animated about the sewage plant’s avian inhabitants.

“You can keep Mozart and Beethoven and Brahms, and just listen to the swans,” said longtime Shore birder and writer Henry “Harry” T. Armistead, who splits his time between Talbot County and Philadelphia.

Armistead, who has been organizing bird counts on the Shore for nearly 65 years, said he always tries to send someone to Hurlock not just for the swans, but to see rare birds hailing from places as disparate as the Arctic, Prairie states, California and Argentina. Armistead said he’s spotted a Ross’s goose and brant there, among others.

Less exotic wildlife at Hurlock include diving ducks, pectoral sandpipers and red-bellied cooters, aquatic turtles with high-domed shells.

Birders acknowledge that the rotten-egg smell at sewage facilities can be a bit off-putting.

“It can be pretty foul,” said Jim Brighton, a Shore resident who frequents Hurlock. Even so, he said he has seen several exotic birds there, among them the Marbled godwit, a large shorebird, and the Wilson's phalarope, a small wader.

While perhaps not fit for drinking, the water in treatment plant lagoons doesn’t appear to hamper the birds.

“I’ve never heard of a whole lot of bird kills at a sewage treatment plant,” said Jared Parks, a land protection specialist at the Eastern Shore Land Conservancy, who frequents the Chestertown wastewater plant to watch cackling geese. “It’s mostly water in those ponds — and lots of food growing in there.”

The Back River sewage plant, which treats a whopping 180 million gallons of wastewater a day, draws a comparably large number of birds and wildlife.

There was a time, plant manager Nick Frankos acknowledged, that the plant “had a reputation for stinking up the world, and it did.” But upgrades to the plant, which sits on 466 acres, have not only reduced the nitrogen and phosphorus in the effluent but have made the place a much better animal habitat, according to Frankos. In addition to deer, fox and turkey, Back River has a thriving population of Canada geese. (Maybe too many, according to the plant manager.)

In the last two decades, more than 170 winged species, 20 of them waterfowl, have been spotted at the eastern Baltimore County plant, according to birder Kevin Graff.

The birds avoid the primary and secondary tanks that contain dirty wastewater and go for the settling tanks, which Frankos said are warm and protected from predators.

“It’s pretty much clean water,” he said. “If you put a glass in there, it’s pretty clear. It’s cleaner than the river, I’ll put it that way.”

As with Hurlock, Back River requires permission to enter. Most birders e-mail Frankos, who notifies the guard at the facility’s gate.

Besides Graff, another five or 10 birders come regularly, Frankos estimates. The plant was mobbed in 1990, though, when a Ross’s gull showed up. The pink-hued, dove-like gull is rarely seen outside the Arctic, and many birders wanted to add it to their lists of species seen without taking a flight to the cold climate themselves. It drew 2,300 visitors from 16 states.

“It was hanging out at Back River with the rest of the locals, and man, you would have thought it was a Martian or something. People came from everywhere, and I mean everywhere,” Frankos said.

For enthusiasts like Armistead, birding at the sewage treatment plant isn’t odd. He also looks for unusual gulls at the landfill in Oyster, VA, and near the dredged material repository at Craney Island in Norfolk. He’s always mindful that the management doesn’t have to let the birders in, and that he’s lucky to be there.

“Some of these places are closed to birders,” he said. “It’s a privilege to be able to go into them and see the birds.”