For decades, enormous groups of great blue herons returned every spring to the small island off the southeastern shore of West Rush Lake near Rush City, Minn. The deep, throaty croaks of adults and chirping of hungry chicks audible for anyone nearby, the birds’ broad wingspans likely casting shadows on the water as they flew overhead.

The number of nesting pairs hovered above 500 in the 1970s, making it one of the largest colonial bird nest sites in the state. So in 1974, with the herons’ haven potentially threatened by human development, the Minnesota Department of Natural Resources moved to protect it. The agency took advantage of recently passed legislation to designate Rush Lake Island the first Scientific and Natural Area (SNA).

“Their key habitat is their reproduction area,” said Gerald Jensen, now 81, of the species. “So if you didn’t have that you’d lose the population, or they’d have to find another home.”

Jensen, the first SNA program supervisor, said Rush Lake Island wasn’t the DNR’s top priority for the program. It was, however, relatively simple to move through the regulatory process. On Aug. 8, 1974, Rush Lake Island became an SNA, public land protected from human interference and disturbance.

“There was always plenty of activity up in the top of the trees because that’s where the herons nest, right at the very top,” said Larry Steeves, 77, who has owned property on the lake for 33 years. “So there was quite a bit of chatter going on of course with the little ones and the old ones. Other than that, pretty peaceful island. Nobody bothered it.”

Nearly five decades later, that scene no longer exists. Nesting great blue herons, the primary reason the 21-acre island was protected, have abandoned all nesting on the island. Nest figures have dwindled. Unofficial surveys in recent years haven’t turned up a single nest. It’s been left to avian visitors, other wildlife, and whatever plants grow from the soil.

“We all think it’s too bad,” Steeves said, “but sometimes that’s the way nature does it.”

Which, in many ways, is exactly the goal of the SNA program.

“There’s kind of a natural evolution,” said Carrol Henderson, the now-retired former director of the DNR’s Nongame Wildlife Program. “And that’s one of the things the SNAs were designed to do, is to just allow nature to proceed, and we can see how it functions.”

When Bob Djupstrom took over as SNA program supervisor in 1981, the great blue heron population on Rush Lake Island still was flourishing. The DNR never had issues with people venturing to the island during the off-limits sanctuary period from April 1 to July 15, he said, nor were there problems with parties, drinking or other activities.

“There weren’t any major issues,” said Djupstrom, who ran the program until 2006. “It was pretty much an isolated tract off by itself, and nature was calling the shots on it.”

Steeves said locals simply let it be.

“As far as I know, nobody went on the island anytime,” he said. “I’ve never seen nobody on it. It was just left to the birds.”

Signs of a nesting decline came rapidly. In 2004, the estimated number of nests on the site fell to 175, according to the DNR. By 2013, that figure was down to maybe a dozen.

Recent informal surveys haven’t turned up a single heron nest, said SNA program outreach coordinator Kelly Randall, though the birds are still spotted regularly in the area.

The island, which had always been low maintenance, requires almost no upkeep. Russell Smith, regional management specialist with the SNA program, said he and his small team go out there every couple of years to remove buckthorn or replace damaged signs. Last time he went there was 2016. Otherwise it’s “pretty hands-off,” he said.

So what caused the great blue herons to gradually abandon what had been a thriving rookery, one set aside and left alone specifically for them? It might have been the birds’ own poo.

Great blue herons produce a lot of droppings, aka guano, and it is heavily acidic. When the guano builds up, it changes the acidity of the dirt and affects what types of vegetation can survive there, Henderson said. This acidity could have built up over time, and destroyed the herons’ tree habitat for nesting.

Henderson, who has never been to Rush Lake Island but has worked with other heron rookeries, said he’s seen it happen before. And it’s nothing that alarms him. The herons that nested there likely found a more suitable spot nearby, as they do, he said.

Other possibilities have been bandied about: predators, such as raptors or raccoons; human or boating traffic; Dutch elm disease destroying the preferred American elm trees; water levels.

Yet there are other rookeries in the state dealing with some of those same issues and doing fine, even thriving, Djupstrom and Henderson said.

“One thing that’s constant in life is change,” Djupstrom said. “It’s just that sometimes we have to figure out what’s causing or accelerating some of those changes. At Rush Lake Island, it could be a very, very complex issue. It might be a very simple issue.”

The DNR’s best guess is a combination of factors — but which ones, and to what extent, isn’t known.

Valuable space

So without the great blue herons, where does that leave Rush Lake Island? Just as valuable as ever, it appears.

For one, it and other SNAs can serve as valuable research sites, said Djupstrom. “It’s a private place where you can do an intensive study over many years to try to figure out what exactly is going on.”

That could include determining why the great blue herons left, though Rush Lake Island has been oft-ignored in the research regard. Mark Cleveland, SNA statewide management coordinator, said the agency hasn’t had a request for a permit on the island since 2001.

Regardless, Rush Lake Island remains a valuable natural space, said Smith, the SNA regional manager, citing the intact plant community and undisturbed shoreline on a developed lake. And there are still plenty of birds, amphibians and other wildlife that use the island.

“[The herons’ departure] is just an indication that the environment changed on the island, it’s not the same as it used to be,” Henderson said. “There will be other wildlife or nesting species that will take up use on the island over time.”

It could be terns or black-crowned night herons, he said. Ring-billed gulls also are possible. And in the future, maybe it becomes habitable again for the birds that led to its protection in the first place.

Djupstrom said there’s “no reason to doubt” the great blue herons will one day return to the island, calling it a “very protected environment.”

And if they don’t?

“That’s one of the things that’s fascinating about the SNA program,” said Henderson. “They want to have these areas that basically show how nature functions when we’re not intervening, trying to impose our standards on what nature does by itself. And so in this case we’re watching the demise of the past heron colony, and now we’re waiting to kind of see, well, how do the herons adapt, and how do other birds adapt to the island.

“It still provides habitat,” he continued, “it’s just a different kind of habitat.”

 

Shaymus McLaughlin is a freelance writer. He lives in Minneapolis.