The Swainson’s Thrush Is in a Rush
This long-distant migratory songbird is more often heard than seen during its quick stop on its breeding grounds in Alaska's boreal forest before heading all the way back to Argentina.
In May, Alaska is still breaking free of winter’s last grip, and it seems like summer will never come. However, a remarkable transformation happens almost in an instant. In the span of just a couple of weeks, the boreal forest unfurls into a sea of fresh green birch leaves, cow parsnip, devils club, and wildflowers. Along with this renovation comes the long-awaited return of songbirds to the forest.
First, come the songs of Ruby-crowned Kinglets and American Robins. As migration continues, the sounds of Northern Waterthrush and Wilson’s Warbler are added to the mix. At the tail end in early June the latecomer Alder Flycatcher finally adds its two buzzy notes to the mix. The sensory overload of daylong sunshine, fresh and heavily scented greenery, and rich birdsong marks the epitome of summer.
By far, most of these tiny singers are heard but not seen. The dueling sounds of dozens of birds singing over one another can be challenging for birders to identify and count, but one particular species truly stands out—the Swainson’s Thrush. Their loose, ascending, fluted trill is out of sight but surrounds visitors to the forest, with sometimes four, five, six, or more of them heard from a single location. Not unlike the beloved song of the Wood Thrush in eastern forests, the mechanical flute of the Swainson’s Thrush is the sound of summer in the boreal forest.
Summer also means it’s time again for the Breeding Bird Survey, a cornerstone of wildlife conservation and a cherished tradition among bird enthusiasts across North America. This invaluable survey effort, commonly known as BBS, is a summer-time event in which ornithologists volunteer their time to count birds along established routes throughout the United States and Canada. Counts are collected using a rigorous standardized protocol so that the data can be used to monitor the population status and trends of North American bird species. This cooperative effort between the US Geological Survey and Environment Canada began in 1966 and includes over 4100 survey locations. The result of this partnership is a long-standing and reliable dataset that has quietly powered bird conservation in North America for decades.
Along the lonely Richardson Highway in eastern interior Alaska is a BBS route called Sourdough, located in a classic boreal setting of birch and black spruce forest full of wetlands, rolling hills, with snowy volcanic peaks in the distance. Swainson’s Thrushes seem to appear in droves all at once, begging the questions of where they come from and why they are coming here. The Bird Genoscape project found that boreal Swainson’s Thrushes are a genetically distinct population from those in the Pacific Northwest and the Pacific coast. More specifically, tracking studies conducted in Denali and Wrangell St. Elias National Parks found that interior Alaska’s “olive-backed” Swainson’s Thrushes migrate south to northern Argentina via the midcontinent flyway, making them one of the longest-migrating songbirds in North America.
After these incredible fliers come thousands of miles to the boreal forest to breed, they rush out and head south again several weeks later. As if breeding, raising chicks, and fattening up for migration weren’t enough, fall migration is when the real work begins. These one-ounce titans dodge power lines and communication towers, avoid disorienting nighttime lights and window glass, and find food and safe spaces in landscapes altered by forestry and livestock management.
As capable as they are, they are not infallible. According to the Three Billion Birds report, this species has lost nearly 30 percent of its population in the last 50 years. These population trends come from the BBS itself, along with its winter counterpart, the Christmas Bird Count. These valuable datasets allow conservationists to track change over time, identify declining populations, and seek solutions to the conservation challenges they face.
Unassuming and highly respectable little brown birds, the Swainson’s Thrushes are much more often heard than seen, but they demand to not be overlooked. The return of their song brings richness to the boreal summer, woven into the fabric of a landscape that is quietly and altogether in a rush to get it all done before the short summer ends.