Kevin Marshall did not have a hobby before he stumbled upon the night herons nesting a block and a half from his rowhouse. Not really.

“Well, living in Fells Point, it’s easy to get into the ‘hobby’ of watching whatever sports are on TV while you’re drinking,” the 36-year old mortgage under-writer deadpans. “Growing up, I played soccer, so I watched that. I got into baseball once I moved here and started watching baseball. I was hitting the end of that cycle, kind of aging out, when I started getting into the birds, so it was a nice, smooth transition,” he adds, with a self-deprecating smile.

The colony of night herons at Thames Street Park, which Marshall first took interest in two years ago, are not the regular sort of birds you typically find in a dense urban environment. They’re squat and thick, more like the size and shape of an NFL football, but cloudy white, with a distinctive black crown and blue-black back feathers.

Which is not to say there isn’t a diversity of birds in the city. According to the Audubon Society, there are more than 40 resident species just in Patterson Park. A bald eagle was spotted there last year. Two peregrine falcons, “Boh” and “Barb,” famously nest atop the 100 Light Street skyscraper. Among other unique waterfowl, the odd great blue heron gets photographed from time to time at Stony Run and Herring Run Park.

Night herons settling in and starting families in a pocket park are different, however. They’re noisy and loud, for one thing, but certainly not songbirds. At first screech, the squawk of Thames Street night herons could be mistaken for that of a dog which has gotten its paw stepped on. The park is a popular destination for leashed local canines—as well as moms and dads and strollers and soccer balls. But it’s somehow worse than that.

“People will say they sound like monkeys or banshees,” Marshall says, looking up at the canopy of Japanese elms after one such outburst, not even requiring his binoculars to spot the culprit. Some 80 birds this spring—38 monogamous male and female pairs and two bachelors—have either rehabbed or built new nests.

“I’d describe it as ‘prehistoric,’” he says of their cacophony. Most of the chatter, Marshall notes, is between couples as they take turns gathering food, repairing the nest, incubating the eggs, etc.

Meanwhile, despite the dissonance—or maybe in a small way because of it—a flock of night heron admirers has sprouted around Thames Street. Marshall’s Instagram page (@thamesstreetnightherons), which he started last year, now has 700 followers.

Also, each spring, random Fells Point residents on their morning perambulations have rescued injured heron hatchlings who’ve fallen from their nest to the packed gravel path below—with the good Samaritans driving the fallen baby birds to Phoenix Wildlife Center for rehabilitation.

To that end, Marshall has been working with another night heron lover on a grant proposal to replace some of the park’s packed gravel that runs beneath many Thames Street trees with native plants—thereby softening the landing for plummeting hatchings.

This spring, he also co-hosted the first two Saturday morning tours of the Thames Street rookery with the Baltimore Bird Club and Phoenix Wildlife Center. Marshall, in fact, has become a citizen scientist himself. He has a notebook full of maps, data, sketches, and observations regarding the arrival of each pair of birds, the location of their nests, and the number of chicks.

Unfortunately, first-year mortality rate is high, 60-70 percent. However, if they survive past three years, adults can live for more than 20 years. Once the young ones learn to fly, the night herons generally decamp elsewhere around the southeast side of the harbor, typically in places with good trees and access to the water, like Canton’s Boston Street Pier Park.

On one recent, pleasant evening, Marshall showed another Thames Street night heron volunteer what he tracked and how, before he headed out of town for a week. (Full circle moment: Het met the volunteer after his birding talk at a Fells Point bar earlier this year.)

“I’m going on a trip this weekend to Wisconsin for a migration; I have a college friend who lives out there,” Marshall explains. “I’ve got a Canon R7 camera with a telephoto lens and all that, which I’m taking with me to hopefully get some sweet shots. Photography was something I was interested in, but never got into until the birds, but yeah, I fell in love with it, too, so…two hobbies now, I guess.”