ISU research finds that lowering reservoir water levels aids shorebird migration

When Dean Hester was getting his undergraduate degree at the University of Iowa, he never thought of taking up birding as a hobby.

But while he was in Montana for field course he took while at UI, a friend encouraged him to join on a birding trip. Hester joined in on birding because he always loved animals. But he didn’t discover his passion for birding until he saw shorebird sandpipers and peregrines swooping down in the Montana wildness.

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“It was a pretty a pretty cool introduction to bird watching, to see two really cool species like that out in a really pristine prairie in Montana,” said Hester, 30, who lives in North Liberty. “After that, I was hooked. It's become a little bit of an obsession.”

Since then, Hester has traveled to nearly every state — and even internationally — to go birding.

“There's definitely a sliding scale of bird watching craziness … from people who just sit in the backyard and see what comes to their feeders, to people who travel all over the country to look for new species,” he said. “I've slowly fallen into the latter category.”

Hester said there are about 700 bird species nationwide that typical birders aspire to see in the outdoors. As of now, Hester has seen and documented 693.

When he is birding in Iowa — and not out working as a lab technician at Integrated DNA Technologies in Coralville — Hester said that one most typical shore birds spotted in Iowa is the pectoral sandpiper.

Covered in sleek feathers of brown and black with stripes down its back, pectoral sandpipers typically feed in grassy marshes while they are in Iowa. But their time in the Hawkeye State doesn’t typically last too long.

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The pectoral sandpiper, or the “calidris melanotos,” breeds in wet coastal tundra areas in the summer, which includes northern Canada and Alaska, and then migrates to South America for the winter. With thousands of miles flown each year, pectoral sandpipers make stops in their flight in wetlands and agricultural fields across the country.

“That’s one of the coolest things about shorebirds … they have some of the most impressive, like feats of athleticism and endurance of any animal in the world, like the pectoral sandpiper,” Hester said. “They go more than 15,000 miles during their migratory loop even though they're tiny.”

In fact, researchers have been studying the flight patterns of the pectoral sandpiper --- trying to see if just a few tweaks to their stops could result in the birds flying all the way from Iowa to South America without stopping.

Diving into the research

Stemming from a study from ISU, researchers found that if the water level is adjusted in reservoirs, pectoral sandpipers can feed better before taking flight again.

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Stephen Dinsmore, department chair and professor in the Natural Resource Ecology and Management at ISU, was one of the researchers involved with the study.

Over several weeks, Dinsmore, along with ISU graduate assistant Victoria Fasbender, worked with the Army Corps of Engineers to slowly lower the water level at Lake Red Rock in Central Iowa. They found that by lowing the water level by about a foot and a half, the sandpipers could feed more because more aquatic insects, worms and other invertebrates they eat would be exposed in the mudflats.

“By lowering that water level, we are opening up this incredible area for them to use, and it provides them access to those aquatic invertebrates,” Dinsmore said. “I think what surprised us a little bit was how useful the reservoir was to them when they started their journey again.”

Dinsmore said he is not aware of another study like the one he and Fasbender conducted at ISU.

The researchers were able to track the birds by attaching battery-powered GPS devices to their backs. From there, researchers were able to track their flight patterns south.

“When you step back from it, what's neat is we documented it. Yeah, we pegged it to a central island reservoir in Iowa, which was unknown,” said Dinsmore. By taking “a foot and half water off a reservoir in Central Iowa, it can attract thousands of pectoral sandpipers, and at least some of them are going to fly non-stop to northern South America.”

The research was conducted over two years, 2023 and 2024. In total, researchers tracked 25 sandpipers in 2023 and 41 in 2024 through GPS for the study.

In 2023, nine of the tracked birds migrated directly to Venezuela, Colombia, Panama and the Dominican Republic. In 2024, nine other birds left Iowa but made a stop in halfway points, like Haiti.

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Fasbender, who is working toward her master’s degree in wildlife ecology, said researchers don’t have data on a few birds they tagged because the batteries in the GPS trackers died during the flights. She said a few birds also stayed at Red Rock throughout the duration of the battery life.

Lowering the water level

For this study, the researchers partnered with the Army Corps to lower the water level at Lake Red Rock, situated on the Des Moines River.

Perry Thostenson works as the assistant operations manager with the Army Corps. He has worked on many projects, specifically with their Environmental Stewardship and Sustainable Rivers Program, which works to “identify, refine and implement environmental strategies” in water infrastructure.

“We draw down the reservoir when we're able to — mid-to-late summer — to provide better shorebird and water bird habitat, and that's what attracts the birds,” Thostenson said. That “upper end of the reservoir is really important stopover habitat for migrating shorebirds (and) those reservoir mudflats provide a great resting and feeding resource for them.”

Going forward, Dinsmore said the researchers will be shifting their focus to Coralville Lake in Eastern Iowa and one to two other reservoirs out of state.

“They're interested in sort of replicating this at other places to gauge what the birds are doing there,” he said.

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Thostenson said he and researchers knew the sandpipers were out in Red Rock from “casual observation,” leading them to start their research there.

“But that was part of the science question is how long were they here? What were they feeding on? And where did they go after they left this area? We've learned a lot,” he said. “Asking ‘is this pool of management practice important and valuable?’ I think we found that, in fact, very much so is very important.”

The water in Red Rock was lowered gradually by slowly releasing water through gates and spillways. Thostenson said the water was lowered about a tenth of a foot every day or every other day until it hit a foot and a half, exposing “hundreds of acres” of mudflats.

Thostenson said the Army Corps can lower and raise the water level at certain times of the year for various projects, if the Corps isn’t in what he called “flood risk management operating mode.” For example, when parts of Iowa experienced flooding in 2008 and 1993, adjusting the water level would not be possible for the Army Corps.

“But about 80 percent of the time, the reservoirs aren't in a flood risk management mode, and we can do different outflows or pool management to benefit the environment,” Thostenson said.

Going forward

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Anna Buckardt Thomas, a wildlife diversity biologist and avian ecologist with the Iowa Department of Natural Resources, said that Iowa serves as an important stopover site for sandpipers — along with other birds — even without reservoir adjustments. But with the researcher’s tweaks, Iowa is even more important for the birds.

“Iowa is kind of a midpoint between their wintering and summer breeding location and so our area can be really important,” Buckardt Thomas said. “Actually, a lot of the many pectoral sandpipers do come through Iowa each spring and fall. Our wetland habitats are really critical.”

Thostenson said he and his team intend to continue adjusting the water levels in Red Rock to aid the sandpiper migration each year.

“It doesn't cost the government any extra money, just common sense management that we could have, or should have been doing, perhaps for decades,” he said.

Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.

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Comments: olivia.cohen@thegazette.com