From Alaska to Argentina and back: migration routes, migration timing, and wintering areas of adult male Swainson’s Thrushes ( Catharus ustulatus ) from breeding areas in interior Alaska

The Swainson’s Thrush (Catharus ustulatus) is one of the most common migratory passerine birds nesting in the central biogeographic region of Alaska, USA, and exhibits a Nearctic-Neotropical migration. Despite its common status, there is little published about the migration phenology, migration routes, and wintering area(s) of Swainson’s Thrushes from Alaska. Using archival light-level geolocators and archival GPS loggers, we provide the first documentation of migration routes, wintering areas, and the timing of autumn and spring migration for 16 adult male Swainson’s Thrushes from study areas in Denali National Park and Preserve and Wrangell-St. Elias National Park and Preserve, Alaska. Individuals initiated autumn migration by early September, exhibited a cross-continental migration pattern across western and central Canada, then a strong latitudinal southward migration after they reached the Great Lakes region. All birds arrived on their wintering areas in South America by mid-November. Study birds exhibited a leapfrog migration pattern, wintering farther south than birds from breeding populations at more southern latitudes. Birds initiated spring migration by late February and arrived back on their breeding grounds by late May. GPS data indicated a weak loop migration pattern during part of spring migration, with spring migration routes between 15°N and 50°N latitude being slightly west of the autumn migration routes. GPS data showed that birds made a minimum of one to three stopovers during autumn migration and one to five stopovers during spring migration. Six birds carrying GPS loggers spent five to 13 days in Colombia between 3–24 March 2019, near areas where individuals from other breeding populations have wintered, suggesting the potential importance of this area to Swainson’s Thrushes from multiple breeding populations. Our findings demonstrate that adult male Swainson’s Thrushes from our study areas complete among the longest round-trip migrations of any North American migratory passerine and indicate the importance of forested landscapes across the annual range of this species.