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The Varied Thrush breeds from Alaska to California in forests “where spruce trees and alders and crowding ferns contend for a footing, and where a dank mist drenches the whole with a fructifying moisture” (Dawson 1923: 769). Its contrasting black-and-orange plumage and eerie, penetrating song make it one of the most distinctive birds of Northwestern forests. Most individuals winter along the Pacific Coast from Alaska to southern California, with occasional individuals seen throughout the western United States and, during irruption years (peaks every 2–5 years), across the United States and Canada.
Shy behavior, a propensity for dark, wet forests, and low density have made it difficult to study this species in its breeding habitat. Recent studies suggest that it breeds most commonly in mature and old-growth forests and may be sensitive to forest fragmentation. Its distinctive song—a slow series of single drawn out notes at different pitches—“is as perfectly the voice of the cool, dark, peaceful solitude which the bird chooses for its home as could be imagined” (L. A. Fuertes in Bent 1949: 92). Studies of song structure suggest that males vary the frequency and duration of their notes to reduce habituation by neighbors.
Other aspects of the breeding biology of this species remain poorly known. Most aspects of its migration are poorly studied, but there is some evidence that, in a leapfrog manner, northern breeding populations winter farther south than more southern- and coastal-breeding populations. During the breeding season, individuals forage for arthropods on the ground but switch to fruits, berries, and acorns during fall and winter. In winter, individuals congregate in loose flocks around food sources, where they often exhibit aggressive behavior toward conspecifics and other species.
The propensity for mature and old-growth forests, irruptive behavior, and distinctive song makes this species a good subject for conservation issues and behavioral studies. Studies in northern California have documented low abundance along forest edges (Brand 1998), which may explain the absence of this species from small (<16 hectares) forest patches during the breeding season (Hurt 1996). The aggressive behavior of this thrush in winter has been well described (Martin 1970), as have its winter abundance and irruptive patterns (Wells et al. 1996).