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WEST ALTON • Against a horizon broken by transmission lines and the occasional smokestack, amid the steady buzz of 60-mph traffic along U.S. Highway 67, the bottomlands near the confluence of the Mississippi and Missouri rivers may not seem like a sought-after natural oasis — especially during gray, winter months.

But things look differently if you’re a trumpeter swan, or a devoted birdwatcher drawn to observe the species, beginning this time of year.

A record 1,089 of the birds have already returned to the area’s marshy expanses for the winter, migrating from their northern habitat ranges in places such as Wisconsin, Minnesota or even Canada, based on a count completed last week.

That tops last year’s record of 1,022 counted at the Riverlands Migratory Bird Sanctuary in West Alton, helping cement the site’s status as home of the largest number of wintering trumpeters in the interior of the United States. The numbers represent a striking turnaround for a species that was once nearly hunted to extinction across the U.S.

With wingspans approaching eight feet, trumpeters rank as North America’s largest waterfowl. That majestic size makes them a favorite of regional birders.

“The bigger the birds are, the more fun they are,” said Pat Lueders, a retiree who, since 2011, has led the Great Rivers Trumpeter Swan Watch, a volunteer group that counts the number of swans roosting at Riverlands throughout the winter. “You’re not looking for teeny sparrows in the grass.”

Lueders and other birders haven’t always had so many trumpeters to count, as the species’ recent population surge took a long time to materialize.

Decades ago, hunting nearly wiped out the species in the United States, save for some remote populations in the Rocky Mountains and Alaska, explained Charlie Deutsch, an Army Corps of Engineers wildlife biologist based at Riverlands. But by the 1970s and ’80s, reintroduction programs took root, including in a number of states in the Upper Midwest.

By the winter of 1991 and 1992, five of those reintroduced trumpeters found their way to Riverlands.

“I was here when the first group of five showed up,” Deutsch said. “The idea that we have thousands here on a given day (today), it’s a cool conservation story.”

It wasn’t a sure thing that the first reintroduced birds, raised as hatchlings without their biological parents, would know to head south in the winter.

“They raised them and crossed their fingers about what they would do,” said Jean Favara, a conservation manager at the Audubon Center at Riverlands.

Not all birds figured it out in the early years. Some perished up north.

But Favara says others showed an “innate ability” to learn to migrate — something they’ve done at Riverlands in growing numbers as their population has risen over time.

Only over the last decade or so did trumpeters’ winterly totals at Riverlands climb into the hundreds, and just in the last few years did it leap again to approach — and then exceed — a peak of about 1,000 swans.

But once they find suitable wintering habitat, such as that at the confluence, they’re typically return customers, demonstrating “site fidelity” throughout their lifespan, according to Favara. Trumpeters also mate for life and migrate as a family unit, bringing their offspring — known as cygnets — with them.

The marshland habitat at Riverlands, Favara and Deutsch say, provides the birds with both food and plenty of areas where water can shield them from potential predators.

Deutsch thinks the area has emerged as the swans’ preferred winter destination thanks to its location near the confluence of three major rivers — including the Illinois River — and because it offers several large tracts of restored habitat.

“Birds loosely use waterways as migration corridors,” Deutsch said. “They’d probably travel this corridor regardless, but would probably bypass us otherwise.”

Those attributes help draw many other birds to the area, as well, with Riverlands supporting the largest number of bird species of any place in Missouri, according to eBird, a site run by Cornell University that catalogs crowd-sourced observations by birdwatchers around the globe.

That local biodiversity peaks in the winter.

“We probably have the most variety of birds in the winter season because of the waterfowl that comes through,” Favara said.

Although migration tendencies for many kinds of birds are facing potential disruption thanks to forces such as climate change, local experts say there have not been clearly discernible shifts for trumpeters or other species at Riverlands — either in terms of timing or geographic distribution.

“There certainly are changes in species’ behavior, distribution (and) migration that are likely influenced by climate change,” said Tom Bonnot, a postdoctoral fellow at the University of Missouri’s School of Natural Resources, focused on how changes to climate and landscape affect wildlife populations. “We are starting to see pretty comprehensive evidence that, across species, distributions are starting to shift northward.”

The reams of “citizen science” data compiled on eBird and elsewhere by dedicated birders such as Lueders will be a valuable tracking and comparison tool to see if those shifts eventually affect trumpeters or other birds at Riverlands, as the National Audubon Society projects.

“We’ll find that out maybe as the years go by,” Lueders said.

“Most people are doing it for fun,” said Frank La Sorte, a research associate at the Cornell Lab of Ornithology, describing eBird. “But it’s providing this really rich scientific tool that has tremendous scientific potential.”

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Reporter covering energy and the environment for the St. Louis Post-Dispatch.