Swainson’s Thrush: What Geolocators Show About a Songbird’s Journey to the Tropics

Songbirds such as the Swainson's Thrush spend more of each year in the tropics than on their northern nesting grounds, so protecting them means knowing where they spend the winter. Laura Phillips of the National Park Service describes how geolocators light enough for a thrush to carry are now answering that, and what the answers mean for parks from Colorado to Alaska.

A light-level geolocator on a Swainson's Thrush records light levels against time, so researchers can calculate where the bird has been when they recapture it and recover the device the following summer. Image from NPS's page.

A songbird that links two forests

The Swainson's Thrush (Catharus ustulatus) is a medium-sized songbird known for a spiraling, flutelike song and a liking for mature dense forest. Twice a year it connects the boreal forests of North America with the tropical forests of Central and South America. About 200 bird species, mostly songbirds such as thrushes and warblers, are Nearctic-Neotropical migrants that fly thousands of miles a year between breeding and wintering areas. Long-distance migration exists, the article says, because it improves a bird's fitness, its ability to raise young over a lifetime.

Alaska lies far north at the edge of North America and the Pacific, and takes in much of historical Beringia, which gives it a wide variety of birds. Most of its 300 annually occurring bird species leave in the fall to spend the winter farther south. Glacial cycles during the Pleistocene are thought to have shaped northern birds' migratory pathways, and in Alaska the routes of many songbirds can be traced to their recolonization of northern forests as the ice sheets retreated. The thrush is a good model of that post-glacial colonization by birds that winter in the New World tropics.

Two groups, two routes

The "russet-backed" group (Catharus ustulatus ustulatus) breeds along the Pacific Coast, and the "olive-backed" inland group (C. u. swainsoni) breeds in boreal forests across Canada and the United States. They differ in plumage, migration routes, wintering areas, breeding habitat and song. Banding data and observations during migration suggested that inland birds flew a long, roundabout route east across North America before turning south, while their coastal cousins went directly south to wintering grounds in Central America.

Genetic work from the early 2000s found that the inland group's migratory pathway mirrors the post-glacial spread of boreal forests, and that the two groups probably diverged when ice sheets isolated their populations at the last glacial maximum. Where the two meet along the crest of the Cascade and Sierra Nevada ranges, hybrids show physical and genetic traits that shift along a gradient from coastal to interior habitats. The whole annual cycle stayed unknown, though, until tracking devices small enough for songbirds were developed.

Geolocators

Satellite and GPS transmitters were too heavy for small birds like thrushes, so the exact routes and wintering areas of particular breeding populations were unknown. A light-level geolocator weighs less than a gram. It is an archival device that records light levels against time, from which researchers calculate latitude and longitude using day length and the sun's angle. It has to be recovered to read the data, so the bird must be recaptured in a later year, and sample sizes are generally low. Combined with genetic and isotope data, the devices give strong support for defining where populations go each year.

What the tracking showed

Researchers have fitted geolocators to Swainson's Thrushes at Point Reyes National Seashore in California, at coastal and inland sites in British Columbia, in Rocky Mountain National Park in Colorado and, at the time of writing, in Denali National Park and Preserve in Alaska. The locations confirmed the direct route of coastal birds and the cross-continent route of inland birds, and showed intermediate routes for hybrid and Rocky Mountain birds. Wintering areas ranged from western Mexico to central South America, with strong migratory connectivity, meaning clear links between breeding and wintering places, for populations in California, coastal British Columbia and Alaska.

Migration routes of Swainson's Thrushes tracked with geolocators from sites in California, British Columbia, Colorado and Alaska. Image from NPS's page.

Why it matters for parks

Because these birds spend more of the year in the tropics than at their nesting grounds, knowing where each population winters has large implications for conservation. Widespread declines in Nearctic-Neotropical migrants have been reported since the 1980s, but current research shows that their population dynamics are complex, shaped by conditions on breeding and wintering grounds and along the way. For the thrush, the article says, what drives trends among birds nesting in Rocky Mountain National Park will likely differ from what affects Denali's birds, since the areas the two populations use overlap very little across the year. National Park Service managers will need to work with national and international partners responsible for the habitats each population uses through its annual cycle, and the author calls understanding where birds go when they leave park boundaries critical to their conservation.

Sources

• National Park Service, Alaska Park Science, Volume 17, Issue 1, Migration: On the Move in Alaska, Connecting Taiga to Tropics: Swainson's Thrush as a Model for Nearctic-Neotropical Migration in Alaska, by Laura Phillips, National Park Service; the acknowledgments are left out; rewritten in hubnx's own words.