In the summer of 2015, a small group of researchers gathered in the shadow of Denali. Some had known each other for years, while others were meeting for the very first time. As Scott Weidensaul puts it, it was “instant family”.

Scott Weidensaul (left) and Dr. Iain Stenhouse (right, BRI) tagging a Swainson’s Thrush during the first field season in Alaska. Credit – George Gress

The days started at what most would call “night”. At 1:00 am, while the rest of the world slept, the crew was heading into the field. In the subarctic June, the sun is already up by 2:00 am, and the Swainson’s Thrushes are at their peak singing activity on their territories. Those early mornings were magic, watching the sunrise hit the slopes of Denali while trying to capture territorial males. The researchers lured territorial males to mist nets using audio lures and life-like wooden decoys placed near the nets.

Over A Century of Mystery: The role of the Critical Connections program

For over 100 years, scientists in Denali have asked: Where do these migratory birds go once they leave this area? We’ve long known that Denali, and Alaska at large, is a haven for migratory birds, from Arctic Terns to Blackpoll Warblers. The birds that breed in Alaska fan out over literally 75% of the Earth’s surface.

Answering that very simple question was the initial inspiration for starting the Critical Connections program. And by connecting visitors to Denali with migratory birds that may very well end up in their backyard, the scientists hoped to provide powerful reminders of the connections we have with the natural world all around us.

What started as a shared vision between Carol McIntyre, Scott Weidensaul, and Iain Stenhouse, launched into a collaborative effort to study some of Alaska’s migratory birds. Scott and Carol first dreamed up the project during a Raptor Research Foundation meeting in Duluth, Minnesota; and later, Carol and Iain discussed ideas over breakfast at the Copper Whale in Anchorage. Like the beginning of most projects, the initial conversations were casual, posed with curiosity, and often in unexpected places.

Unlocking the Secrets of the Swainson’s Thrush

As part of the Critical Connections program, the team tracked a diverse cohort of Denali’s seasonal residents, including Gray-cheeked and Hermit Thrushes, Blackpoll Warblers, Arctic Warblers, and Fox Sparrows. The Arctic Warbler migration tracks provide the first documentation of the 8,000 km + Nearctic–Paleotropical route these birds take, where they winter on the Pacific islands of the Philippines and Palau (Adams et al., 2022).

Swainson’s Thrush (Catharus ustulatus) was chosen as a focal species because, despite there being an estimated 30 million of them in Alaska, there was little knowledge about their wintering grounds and migration routes. Luckily, thanks to technological advances in 2015, that was about to change.

Thrushes were outfitted with tiny light-level geolocators tags in 2015 and then tiny GPS tags in 2018. When these birds returned the next year, scientists were able to recover the tags and obtain data that revealed a journey of staggering proportions.

This map shows the migratory routes of adult male Swainson’s Thrushes tagged in Denali and Wrangell-St. Elias.

These birds undertake a nearly 9,000 km journey, from Alaskan tundra to the Argentinian forests. What’s more, is that the population from Denali performs a leapfrog migration – flying past individuals from more southern breeding populations who may spend the winter in Central America and northern South America – all the way to the deep interior of southern Bolivia and northern Argentina.

Why does this happen? Migration is a phenomena that has happened for millions of years, and birds have to adapt to the changing world around them. As glaciers retreated 15,000 years ago, these birds adapted to a changing world, pushing further north to breed but “leaping” over established populations to find their own wintering niche. This ancient strategy is now facing modern threats, from climate-driven habitat shifts to the loss of native plants they rely on to refuel.

You can read the full, open-access paper here.

Full-Life Cycle Stewardship: Every stop matters

This research is incredibly important as it sets the baseline biological information needed for effective conservation and management of these birds, and the places they rely on. That’s where full-life cycle stewardship comes into play. It challenges us to look at the 12-month calendar of a bird’s migratory journey.

If a Swainson’s Thrush spends three months breeding in Alaska and three months wintering in South America, that leaves six months of high-stakes travel in between. If we protect the ends of the journey but neglect the stopover habitats where birds rest and refuel, such as local parks and forests, we’re missing a massive piece of the puzzle.

The Science of Awe: How you can act

• Voting for Conservation: Support policies that protect bird-friendly spaces and conservation efforts, locally and internationally!
• Protect the in-between spaces: Advocate for the conservation of stopover habitats – wetlands, forests, and local parks.
• Reduce human-made hazards: Put up bird-friendly tape on your windows to reduce collisions, and keep your cat indoors!
• Plant native: Grow native, fruit-bearing shrubs and pollinator-friendly plants to provide essential nutrients to migrants in your area.