Wild With Nature

The nest boxes haven’t been up for five minutes, and already a small cloud of tree swallows (Tachycineta bicolor) is swirling around them. A dusky, gray-backed female lands at the entrance of the box closest to us, her liquid chatter blending with the staccato calls of the western kingbirds (Tyrannus verticalis) on the electrical lines overhead. Beyond her, two more tree swallows are investigating another box.

Tree swallows are cavity-nesters, typically using abandoned woodpecker holes in old cottonwoods and other large trees to raise their young. They also readily use nest boxes as a reasonable substitute. But we certainly didn’t expect them to show up so soon.

This instantaneous response from the local swallows is immensely gratifying. But it also suggests to us that the birds in this area have been limited by a lack of nest sites.

“Apparently there is a cavity limitation here,” says Megan Fylling. 

Nesting habitat for declining swallows

Megan works as a Research Director and Avian Ecologist for the University of Montana’s Bird Ecology Lab (UMBEL). It’s the second week in May of 2023, and this morning I’m with her and five other wildlife researchers, pounding in metal T-posts and putting up cedar nest boxes. The project is a collaboration between UMBEL and the U.S. Fish and Wildlife Service, with funding from NorthWestern Energy. It’s an effort to create tree swallow habitat where these birds are in decline—and to learn what they can teach us about the places where they live.

We’ve just finished installing 10 new nest boxes along the heavily-fished tailwaters below Montana’s Holter Dam, where the Missouri River cuts a canyon through the red rocks of the Big Belt Mountains, between Helena and Great Falls. There’s something very inspiring about watching these sleek, athletic birds checking out their new homes here. And to see them so soon is a huge surprise. Often it takes a year or two before tree swallows will set up shop, says Brian Balmer. Brian, a biologist with the U.S. Fish and Wildlife Service, will be leading much of the monitoring to see how the tree swallows respond to these new nesting sites.

Waiting for tree swallows

There had been no such instant gratification at our first site of the morning. There, 16 miles farther upstream, our work was a matter of faith. No tree swallows appeared as we hammered T-posts into the shallow, rocky soil above the river, where larkspurs bloomed and Douglas-firs clung to the slopes. They still hadn’t arrived as we attached nest boxes to our last T-posts in that string of 10, on a ridge of loose cobbles overlooking the river.

But as we gazed out over the gray-green waters of the Missouri River and the bitter-herb smell of big sagebrush wafted through the gray, misty mid-May morning, it seemed like the sort of place where tree swallows could make themselves at home.

“I have a good feeling about these boxes,” said Tricia Rodriguez, UMBEL’s Project Manager.

Tree swallows and population declines

Tree swallows are familiar breeding birds across much of North America, equally at home in the baking heat of a central California summer or a somewhat-cooler July in Maine. Darting through the air, flashing their glossy blue backs, they twist and turn as they hunt aerial insects on the wing. But although they’re widespread and common, their overall numbers have been declining. Analyses of Breeding Bird Survey data since the mid-1960s point to particularly notable declines in the northern and eastern parts of their breeding range. 

Along the Missouri River in Montana, finer-scale surveys of breeding birds have also shown concerning declines in tree swallows. Systematic songbird surveys from 2004 to 2021, also funded by NorthWestern Energy and led by UMBEL researcher Anna Noson, point to a loss of around 25% of the river’s breeding tree swallows—an alarming downward trend. 

Critical cottonwoods

Why? Although the exact causes remain uncertain, habitat changes along the river have likely played a role. The cottonwoods (Populus spp.) along its banks—which provide nesting cavities for tree swallows as well as critical habitat for many other birds—are also declining. Two thirds of the cottonwood stands in this area of the Missouri River are at least 50 years old. The trees are aging, and relatively few young ones have been able to establish. Dams have limited the flooding that provides a seedbed for baby cottonwoods, while houses and farms have encroached into the once-unpredictable river’s floodplain. It’s a host of changes that seems to be impacting many of the river’s birds, which depend on these important trees for food (think insects), shelter, and nesting sites.

Nest boxes can’t replace the missing trees. Unlike cottonwoods, T-posts and cedar boards don’t host the caterpillars that yellow warblers (Setophaga petechia) and many other birds eat, for example. But by supplementing the nesting cavities typically found in old cottonwoods, nest boxes can help address the loss of the tree swallows. That’s why it’s especially exciting to see the swallows investigating the Holter Dam boxes so soon, just minutes after we’ve put them up. 

Tree swallows as sentinels

“I’m so obsessed with the birds just getting right into those boxes,” says Bridger Creel, a PhD student who works with UMBEL. Their work focuses on songbirds and heavy metal contaminants one mountain range to the west, along the Clark Fork River.

Tree swallow research projects like this one along the Missouri aren’t just helping provide new habitat for a declining insect-eater. Here and elsewhere, they can also teach us important lessons about the surrounding environment. Nesting tree swallows forage close to home, rarely traveling more than a third of a mile from their nests. Their local diet—made up of aerial insects such as midges, mayflies, and dragonflies—means that tree swallows can serve as sentinels for landscape health. If the swallows are thriving and fledging large, healthy clutches, it’s probably a sign that the food web they rely on is thriving, too. 

Meanwhile, by taking small blood samples from the swallows when they’re banding them, researchers can monitor for hidden environmental toxins—avoiding nasty, unforeseen surprises that could jeopardize wildlife and people. For the health of our rivers, tree swallows are like the canaries in the coal mine. But unlike the timeworn cliché of the canary in its cage, tree swallows live wild and free, twisting and gliding through the air in a high-speed insect hunt that no human can match.

Nest boxes, tree swallows, and waning cottonwoods

Our last stop of the day is along a west-facing hillside of skunkbush sumac, cheatgrass, and lupine that overlooks the reservoir above Holter Dam. Spotted towhees (Pipilo maculatus) mew as we install 10 more nest boxes. We stop to pick up the discarded beer cans and water bottles that litter the slope and look up to see a sharp-shinned hawk (Accipiter striatus) circling overhead. Below us, a common loon (Gavia immer) dives in the reservoir beyond a sparse scattering of aging cottonwoods, the trees crowded between a campground and the water.

The plight of the cottonwoods remains: a waning habitat that supports an astonishing, vulnerable bird diversity across the arid West. But today, I’m feeling hopeful. This morning, we put up 30 nest boxes along the river, providing homes for cavity-nesters where none existed last year. And over the years ahead, this team of biologists will be looking out for these birds, learning what these sentinels of the river have to teach us.

As we get back into the truck and drive away, a tree swallow is banking over the hillside, circling where a new nest box overlooks the dammed river. I hope that it will stay for the summer, and thrive.

Update: the tree swallows in 2023

Fast-forward a year now, to spring of 2024. Tree swallows have once again returned to Montana, and another nesting season is about to begin. In May and June last year, the tree swallow team installed 100 nest boxes along the Missouri River, grouped into sites consisting of ten nest boxes each. And even though they were installing the boxes relatively late, when the nesting season had already started, cavity-nesting birds still used 29 of them.

At several sites, the response of the birds was particularly impressive—and it wasn’t just tree swallows there. In the first area that I visited with the researchers, where we hammered T-posts into a rocky sagebrush slope, tree swallows used two nest boxes and violet-green swallows (Tachycineta thalassina) used two more. At Holter Dam, where we watched the birds find the boxes within minutes, it was pure tree swallows. Even in that first year, they occupied six of the ten boxes. At one box where a nest with six young fledged successfully, the tree swallows proceeded to re-nest in the same box, laying five more eggs. Meanwhile, on the dry hillside above the reservoir, where the spotted towhees had mewed, the nest box community was a mix of tree swallows and house wrens (Troglodytes aedon). Between the two species, they built nests in a whopping nine out of ten boxes.

Tree swallows in the years ahead

In the 2023 mid-season report that the tree swallow team shared with me, they wrote: “The relatively immediate high occupancy rates at some sites along the Upper Missouri River surprised even our most seasoned avian ecologists… we expect to see increased occupancy in 2024.”

I return on May 20, 2024, a sunny morning with a light frost coating the grasses, to check on the nest boxes below Holter Dam. I stand underneath the powerline and watch a tree swallow pair that seem to be claiming Box 32. Both are perching on top of it, vocalizing frequently over the background hum of the dam. In just a few minutes, I watch them copulate five times. When another female circles nearby, clearly interested in their box, they intensify their chirps. The female defending the box flies to the entrance hole, shielding it from the would-be usurper.

Every nest box in sight has two or more tree swallows perching on or near it. At Box 31, a cloud of five are swarming and calling, in the midst of an unsettled debate over who will get to nest here. Meanwhile, at Box 35, another pair is perching amiably together. If you close your eyes and listen, there’s a music here: a chorus of liquid chirps, a bustle of activity that wasn’t here until last spring. The pair on Box 32 continues calling. To my ears, it sounds a lot like they’re saying “thank you.”

Further reading

Farther downstream along the river, between Fort Benton and Judith Landing, a project led by Friends of the Missouri Breaks Monument has been attempting to mitigate cottonwood declines by planting young Plains cottonwoods (Populus deltoides). Read a 2019 report about that project here.