For years now the birds have been fighting it out, two old rivals trapped together on a tiny island slowly sinking into a busy harbor of Lake Superior. The prize: nesting space on a sandy mound free from people and predators.

On the one side are the ring-billed gulls, larger, clumsier, more aggressive and much better adapted to living around humans. On the other are the common terns, small, sleek, durable in the air yet delicate on the ground, natural marvels on the brink of extinction.

Over the past five years, rising water has drowned out half of Interstate Island. The fighting has become more fierce. The gulls, desperate for space, are taking over tern nests, eating their eggs and their chicks, driving the terns out of one of their last known colonies in all of Minnesota.

To save the terns, wildlife managers began work on a project this week — a gambit, really — to build more nesting space, not for them, but for the gulls. The $1.4 million project will double the size of the guano-caked island in the Duluth-Superior harbor under the expectation that with enough space, the two birds will get along.

It’s still unclear whether it will work. More space may only attract more gulls bent on taking over the terns’ territory, which is down to a small square on the highest ground near the center of the island.

But the biologists who have kept an eye on the island for decades are hopeful. In better years, when the shoreline was more plentiful, the tens of thousands of gulls that nested there were less aggressive and even provided a swarm of natural protection for the endangered shorebirds, said Gini Breidenbach, restoration program manager for the Minnesota Land Trust, which is managing the restoration project.

“It’s really this delicate balance where you get much less predatory pressure from the gulls when they have enough space,” Breidenbach said.

And as long as the gulls are not actively destroying nests or going after tern eggs, nothing else can really touch them on that island, she said.

“Other predators just don’t come to the island when there are 40,000 gulls out there,” she said.

Terns were once common throughout the Great Lakes but have lost most of their nesting lands to human development and to predators that have been forced into smaller and smaller squares. The birds have now been listed as either endangered or threatened in just about every Great Lakes state.

Biologists believe Minnesota has lost more than half its tern population since 1900, dropping from more than 4,000 birds to fewer than 1,800. Along with the Duluth island, a handful of colonies remain on some of the state’s largest inland lakes, including Mille Lacs and Leech Lake.

The terns undergo one of the greatest migrations in the Western Hemisphere every year, traveling from their nesting sites in northern Minnesota, Michigan, Wisconsin and Ontario to the Pacific coast of Peru, some 6,500 miles away.

“They’re really such a special bird,” Breidenbach said. “If you grew up in Duluth, like I did, you’ve been seeing them come back every spring for your whole life. The thought of them being extirpated from this area just isn’t acceptable to me.”

Tern numbers on the island have been falling rapidly over the past few years as more and more of it has been submerged.

In 2016, the island held 162 tern nests. By 2018, that was down to 131 nests. Last year there were just 113, according to data from the Department of Natural Resources.

The goal is to have at least 200 nests on the island every year, Breidenbach said.

While the entire island is being expanded, biologists and engineers will also rebuild the terns’ nesting area, raising it high enough to keep it safe from flooding and adding more permanent fences and obstacles, such as rope grids, which the terns can navigate while the gulls are kept away.

The project, which will pause from May to August while the terns are nesting, is expected to be finished by the end of the year and biologists hope it will end up helping more than just the terns.

Once the island is expanded, project engineers expect it to have enough space to provide stopover shelter for more than a dozen other species of migrating shorebirds, including the tiny and endangered piping plover, as they travel south each fall from northern Minnesota and Canada.

“This is exciting because there’s this window where most of the gulls are gone and other species are looking for places to rest and we just don’t know exactly how this island may be used,” Breidenbach said.