Sunday, May 31, 2015

Swainson’s Thrushes, Swainson’s Thrush Pictures, Swainson’s Thrush Facts

Swainson’s Thrushes, Swainson’s Thrush Pictures, Swainson’s Thrush Facts

This species is the most common migrant Catharus in much of North America. Polytypic (2 ssp. groups: olive-backed and russet-backed). Length 7" (17 cm). Identification Typically bold, buffy eye ring and supraloral patch of most Swainson’s thrushes distinctive. Adult: in “olive-backed” group, upperparts olive-brown with slightly more reddish uppertail coverts. Strong, usually dark brown, lateral throat stripes contrast with buffy throat and malar stripes; ear surround usually buffy and obvious, but varies in distinctness and color. Chest spots dark brown and round, often somewhat pointed on top, and usually entirely within buffy wash. Sides and flanks olive-gray. Tail similar in color to upperparts in eastern subspecies, more reddish in western races, often contrasting slightly with upperparts. Typically lacks contrasting wing panel. Bill dull yellow, with dark culmen and tip, not contrasting with face color. Juvenile: spotted buff above, dark gray-brown below, with white throat; some older immatures distinguishable by presence of retained juvenal wing coverts with buffy shaft streak. Geographic Variation Six subspecies, some of questionable validity; subspecific identification in field problematic due to slight differences and individual variation. Olive-backed group: widespread swainsoni described; appalachiensis upperparts slightly darker and same color as uppertail coverts; incanus of western Alaska to northern British Columbia and north-central Alberta has upperparts paler and grayer, underparts whiter, and chest spotting nearly black. Russet-backed group: Northwestern coastal ustulatus (southeastern Alaska to northwestern California) and phillipsi (Queen Charlotte Islands) upperparts brown, tinged reddish, with breast washed brownish with indistinct spots, and flanks warm brown; and oedicus of rest of California paler brown above with almost no reddish tinge; eye rings of western subspecies usually thinner, sometimes less buffy. Similar Species Russet-backed subspecies is regularly confused with western salicicola Veery, but note differences in breast spotting, flank color, and facial pattern. Voice Call: russet-backed gives a liquid dwip, olive-backed a sharper quirt; also a rough, nasal chatter introduced by call note. Flight note: clear, spring peeper-like queep. Song: flutelike, similar to Veery in spiraling pattern, but spiraling upward. Status and Distribution Common, but in West restricted by limited suitable habitat. Breeding: mostly in deciduous and/or mixed forest; birds in western U.S. primarily nest in riparian habitat with combination of shrubby willow understory and deciduous or coniferous overstory. Migration: olive-backed group are trans-Gulf migrants; russet-backed birds migrate along Pacific coast and California deserts. Peaks on Gulf Coast ±25 April, southern Great Lakes ±15 May; arrival in montane Wyoming breeding areas ±25 May; spring migration continues into early June as far south as Kansas. In fall, most migrate September–mid-October, arrival on Colorado plains ±25 August. Russet-backed winters from western Mexico to Panama; olive-backed birds winter in South America. Vagrant: casual to accidental to northern Alaska and Northwest Territories; casual to Europe. Population Like other brown thrushes, forest fragmentation may negatively impact populations. —From the book Complete Birds of North America, 2006.............................................................................................................
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.

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