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Swainson's Thrush

Catharus ustulatus ORDER: PASSERIFORMES FAMILY: TURDIDAE

IUCN Conservation Status: Least Concern

More likely to be heard than seen, Swainson’s Thrushes enliven summer mornings and evenings with their upward-spiraling, flutelike songs. During fall and spring migration, their soft, bell-like overhead “peeps” may be mistaken for the calls of frogs. These largely arboreal foragers pluck berries, glean bugs from leaves, or perch on branches and stumps. They also bound across the forest floor to catch insect prey. They breed in the north and the mountainous West, but they become very widespread during migration.

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At a GlanceHelp

Measurements
Both Sexes
Length
6.3–7.5 in
16–19 cm
Wingspan
11.4–12.2 in
29–31 cm
Weight
0.8–1.6 oz
23–45 g
Relative Size
Smaller than an American Robin; larger than a White-throated Sparrow.
Other Names
  • Grive à dos olive (French)
  • Zorzal de Swainson (Spanish)

Cool Facts

  • The "russet-backed" Pacific population has more reddish plumage and a slightly different song than the "olive-backed" birds in the rest of the range. The russet-backed birds winter in Central America while the olive-backed ones winter in South America.
  • The Swainson’s Thrush’s whirling song has a ventriloqual quality that can make it difficult to track. This may happen as the singer moves quickly from one perch to another between songs. It may also have to do with the sounds’ reverberation in dense foliage. Swainson’s Thrushes also sometimes sing quiet songs that create the illusion that its song emanates from a more distant location.
  • Territorial disputes between males often take the form of singing duels, with song volume and frequency escalating until the combatants’ physical exertions are clearly evident. During these contests, rivals very occasionally invert their customary melody so that it spirals downward in pitch. A sing-off can lead to a chase and even occasionally to physical attack.
  • Swainson’s Thrushes have been called “mosquito thrushes” for their flycatching habit of going after flying insects while feeding on their breeding grounds.
  • In New England spruce-fir forests, the nests of Swainson’s Thrushes are often lined with root-like cords of horsehair fungus. The fungal filaments can have antibiotic effects and may help deter nest pathogens.
  • The longest-lived Swainson’s Thrush on record was at least 12 years, 1 month old when it was caught and re-released in Montana in 2006.

Habitat


Forest

Swainson’s Thrushes breed mainly in coniferous forests, except in coastal California where they are found primarily in deciduous streamside woodlands, alder or willow thickets, and occasionally in coastal scrub. These birds range from sea level up to about 8,500 feet in elevation. In coastal California, where their habitat may depend on the presence of fog, they tend to stay below about 500 feet. During migration, Swainson’s Thrushes occupy a wide variety of habitats, seeking mainly areas with dense undergrowth. Look for migrants especially in forests, canyon bottoms, young woodland, swamp forests, lake edges, and parks. Winter habitat includes primary and old second-growth tropical forest and forest-pasture edges.

Food


Insects

Swainson’s Thrushes eat largely insects and arthropods during the breeding season; they also eat fruits, particularly in fall and winter. They tend to reject yellow fruits and favor red ones, going after elderberries, blackberries, raspberries, twinberries, huckleberries, and other wild fruits including those of brier, false Solomon’s seal, and sumac. Insect prey items include beetles, caterpillars, flies, grasshoppers, and bugs. Swainson’s Thrushes also feed on ants—a dietary item more commonly associated with some woodpeckers and unusual among temperate songbirds. Nestlings are fed mostly insects, including especially caterpillars, beetles, moths, and flies.

Nesting

Nesting Facts
Clutch Size
1–5 eggs
Number of Broods
1 broods
Egg Length
0.8–1 in
2–2.6 cm
Egg Width
0.6–0.7 in
1.5–1.8 cm
Incubation Period
10–14 days
Nestling Period
10–14 days
Egg Description
Blue to greenish blue speckled with reddish or brown.
Condition at Hatching
Blind, partially covered with natal down, eyes closed.
Nest Description

The nest is a compact, open cup, 1.5-4 inches high, with outside and inside diameters of about 3.5-6 inches and 2-3.3 inches respectively and a depth of 1.0-2.5 inches. It’s constructed of fine twigs, stems, grass, moss, bark shreds, and decayed leaves, and lined inside with skeletonized leaves, rootlets, lichens, or moss. The female builds the nest by herself over a period of about four days. Like other thrushes, she probably shapes the nest by snuggling low into the cup and thrusting with breast and feet. She builds a new nest each season.

Nest Placement

Shrub

Swainson’s Thrushes nest in shady sites in the forest understory—especially in thickets of deciduous shrubs or conifer saplings, mostly 3–10 feet off the ground. They build their nests on plants such as willow, fir, spruce, blackberry, alder, aspen, birch, maple, oak, briers, gooseberry, rose, and sumac.

Behavior


Foliage Gleaner

Swainson’s Thrushes feed at higher levels than their relatives. They move in short hops along branches looking for food, gleaning from leaves of broad-leaved and coniferous trees. Going after insects, they also lunge, hover, and flycatch. Swainson’s Thrushes often perch on low twigs or branches to survey the litter below, then dive for prey. On the forest floor, they take long, springy hops from one hunting stop to the next. In addition to the male’s haunting song, a peeping flight call, and other vocalizations, Swainson’s Thrushes communicate aggression and other attitudes with a variety of silent body poses and displays, such as wing-flitting, leaf-tossing, and foot-quivering. On migration stopovers, Swainson’s Thrushes may join multispecies foraging flocks. On breeding grounds, mating begins with the male chasing a fleeing female. As the courtship warms up, the pair progresses to slow flights and perching together.

Conservation

status via IUCN

Least Concern

Swainson’s Thrush is a common species, but has been gradually declining across its range; experiencing a loss of about 30 percent between 1966 and 2010, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 100 million, with 28 percent spending some part of the year in the U.S., 72 percent in Canada, and 7 percent in Mexico. They rate a 10 out of 20 on the Continental Concern Score and are not on the 2012 Watch List. This species' short breeding season may render it sensitive to disturbance on nesting grounds. Problems on breeding grounds include grazing, development, human activity, and invasions of nonnative plants. During spring and fall migration, significant numbers of Swainson’s Thrushes die from collisions with windows, radio and cell-phone towers, and tall buildings. (For more on the dangers of lights to migrating birds, visit the Fatal Light Awareness Program.) Studies of bird deaths at communications towers in Minnesota, Illinois, and West Virginia revealed that Swainson’s Thrushes were killed in greater numbers than any other bird species.

Credits

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Migration

Long-distance migrant. Usually migrates at night. Migration routes vary between eastern and western populations but are very widespread: they extend over the west coast of Mexico as well as most of the continental U.S.

Backyard Tips

If you live within the Swainson’s Thrush’s range, you can make your yard more enticing to this bird by providing tree and shrub cover and ground-level bird baths, avoiding chemical pesticides, and letting leaf litter accumulate undisturbed.

Find This Bird

During summer, look—and especially listen—for the Swainson’s Thrush and its distinctive, spiraling song in closed forests of northern North America and the West. Swainson’s Thrushes become numerous across most of forested North America during migration in spring and fall. Though these birds can be hard to spot on the ground in a dim forest understory, they sing frequently in summer and call frequently during migration. In the breeding season, listen for the species’ beautiful, flutelike song coming from rich forest. (Just remember that Hermit Thrushes have a similar song, though it usually includes a clear, level introductory note.) Swainson’s Thrush also gives its distinctive water-drip call quite frequently. Once you get eyes on a candidate, check the face for that distinctive buffy-spectacled look. On winter grounds in Central and northern South America, the species inhabits closed-canopy forest and can often be found attending army-ant swarms.