Scientists divide the Swainson's Thrush into two subspecies, the "Russet-backed Thrush" of the western boreal region and Pacific coast, and the "Olive-backed Thrush" of the eastern boreal region. Our birds fall into the coastal subspecies that occurs west of the Coast, Cascade, and Sierra Nevada mountains from Juneau to Big Sur. The eastern subspecies occurs in eastern North America, and migrates eastwards, (a several thousand mile detour), before heading south via Florida to Panama and Bolivia and as far south as Argentina. Our coastal subspecies migrate down the Pacific coast and winter from Mexico to Costa Rica. Both groups fly at night, feeding and resting during the day. For more on this topic see K.C. Ruegg and T.B. Smith Evolution of migratory routes.
Swainson's Thrush occurs in deciduous (willow, alder, and aspen) riparian woodland and shrubby, wet meadows. In most areas, they avoid competition with other thrush species by utilizing different elevations, foraging strategies, or habitat. The birds maintain a very small territory, sometimes surrounding a birder with serenading male Swainson’s Thrushes in a single large yard or on a short creek-side path.
They breed along the Pacific coast nearly to Mexico and in the Cascades, northern Sierra Nevada, Rocky Mountains, and northern Appalachians. Seasonally monogamous, like most North American songbirds, Swainson's Thrush females show an unusually high degree of between-year fidelity to their nest sites, pairs often re-form in multiple seasons after repeating the pair-bonding process. This may facilitate rapid pairing, important in high-altitude, northern regions where the breeding season is short. Males, which arrive on the breeding grounds first, initially try to drive arriving females off their territories. After several days of female persistence, the male accepts the female and mating occurs.
The female builds a bulky, open-cup nest of twigs, bark strips, moss, grass, leaves, and mud and lines it with fine, soft materials including animal hair and lichen. She typically places it in dense understory cover and usually lays four speckled, blue eggs. She incubates them for 10 to 14 days. Both parents feed the young until they fledge at 10 to 14 days. Nest failure rates in this species are very high, sometimes exceeding 60 percent.
They forage on the forest floor, also in trees, eating mostly insects during spring and summer, and fruits in fall and winter. Beetles, caterpillars, and ants are among the principal insect prey; few temperate songbirds exploit ants to the extent that this and related species do. Typical fruits eaten include elderberries, blackberries, twinberries, and huckleberries. You will usually find them foraging on or near the ground but they spend more time foraging above ground than similar thrushes, sometimes even in the canopy. It uses a variety of techniques, especially pecking and gleaning, typical of thrushes, but also aerial-lunging, hovering, and fly-catching. In the tropics, it follows army-ant swarms like many resident tropical species, which typically displace it when encounters occur.
During the early years (1966-79) of the Breeding Bird Survey, overall trends were strongly positive, especially in Canada. From 1980 to 2002, however, the species declined almost everywhere, again especially in Canada but also strongly in Oregon and Idaho. Populations have disappeared from large areas of California, as a result of destruction of riparian habitat for development and agriculture and, perhaps loss of wintering habitat. Poor productivity seems to be the main limiting factor for this species.
Heard more often than seen, the shy Swainson's Thrush is one of our most beloved songsters. I look forward to hearing them next year, on their return from wintering in Central America. Until then, you can hear their voice at http://www.wildmusic.org/animals/thrush