Species Profile: Varied Thrush

Photo: USFWS

Few sounds can be more closely associated with Pacific Northwest coniferous forests than the ethereal, haunting song of the Varied Thrush (Ixoreus naevius). Pioneering northwest ornithologists Ira Gabrielson and Stanelly Jewett described the vocalization as “a single, long-drawn-out high-pitched note that manages to bring with it some of the mystery of the virgin forests.”

Ralph Hoffman, in Birds of the Pacific States, described the notes as “having a meditative quality due to their deliberation.” The first encounter with this robust songbird has even resulted in appreciative verse. John Burroughs, while on the Harriman Expedition to Alaska in 1899 wrote:

O Varied Thrush! O Robin strange!
Behold my mute surprise.
Thy form and flight I have long known,
But not in this new disguise.

The species’ resemblance to the ubiquitous American Robin is expressed in several of its geographically appropriate common names, such as Oregon Robin, Alaska Robin, and Varied Robin. In measurements and proportions, the two species are in fact quite similar, and the color and pattern of the two could be reversed with little notice. One recent immigrant to Oregon from New York City, an illustrator and world traveler, described the Varied Thrush as “the grandest of western songbirds.”

During predawn hours Varied Thrushes crowd the roadsides in search of seeds, berries and a wide variety of invertebrates. As the sun rises, most return to their retiring habits, retreating quickly at the slightest disturbance. Males sing from shadowy perches above the ground, and the sound may carry far. Nests are usually placed against tree trunks within 20 feet of the forest floor, and are built and tended by the female alone, though males may feed incubating females. The altricial (needing parental care) nestlings are born after approximately 14 days.

Willard Ayers Elliot described the Varied Thrush in his 1923 guide, Birds of the Pacific Coast as “a frequenter of deep woods, of shady fern-clad hills and alder bottoms.” Other authors of the early part of the 20th century refer to “dark spruce and fir forests,” “Deep forests of fir…where the sun barely struggles through to fleck the lower branches,” and “dense stands of fir, dripping with moisture.” The species does in fact appear to prefer the wetter end of the moisture gradient in conifer forests. Though it may be found in good numbers throughout the continuum of age classes, field studies from Washington and Oregon Coast Ranges and Cascades indicate that the species reaches its greatest relative abundance in mature and older conifer forests.

During the coldest part of winter, many birds will abandon deep conifer forests to form small loose flocks in the company of Fox Sparrows, Song Sparrows, and robins, frequenting orchards and suburban backyards in search of fallen fruit, cracked corn and even suit. The species’ preference for apples, madrone berries, and in California, acorns as wintertime fare is well-known. Varied Thrush winter movements are not entirely driven by weather, however, and some wander far away from the breeding range, surprising and delighting birders as far away as New England. Observers who gather data on wintering bird populations for the Cornell Laboratory of Ornithology have noticed a regular biennial cycle, each year of relative abundance followed by a year of relative scarcity. Cornell researchers suspect that food availability may drive this pattern of cyclical abundance, much the same as with northern finches and Red-breasted Nuthatches.

It is somewhat difficult to assess the population status for Varied Thrush throughout the breeding range. Breeding Bird Survey trends for Varied Thrush vary between regions and states. Most noteworthy is recent (1980-1998) significantly declining trends in the Cascade Mountains and Coast Ranges of Oregon, Washington, and northern California. Significantly declining trends also have been noted for British Columbia and Alberta, but significantly increasing trends occur in Alaska, Montana, and the Yukon Territory.

This mixed bag of increasing and decreasing trends is not easily explained, though researchers suspect forest fragmentation and habitat simplification, especially in western Oregon and Washington, as possible culprits. Logging of older forests on public land peaked in the middle to late 1980s, and during this period, harvest units were often very large and few older forest legacy feature such as stream buffers, snags, or green wildlife trees were left. In lowland areas of mixed public and private land ownership, contiguous square miles were sometimes harvested more or less simultaneously, rendering large patches of habitat unsuitable for Varied Thrush and other older forest associates for years or even decades. In areas where short harvest rotations are implemented, forest stands may never provide for the numbers of birds prior to first harvest.

When place against the background rate of natural disturbance and replacement of older forest, this period of large scale conversion of old forest to young forest may be clearing the path of decline for Varied Thrush, though perhaps other still unknown factors are involve. Though still locally common to abundant in much of its range, it would be prudent to begin now to assess the reasons behind the recent declines of “the grandest of western songbirds.” No matter what other birds might still sing, the forests of the Pacific Northwest would be sadly quiet without the song of the Varied Thrush.