Bird of the Month - September 2003
 

Swainson's Thrush
Catharus ustulatus

by David Fix

The song of Swainson’s Thrush is so appealing and so distinctive that bird-conscious people living near a nesting territory promptly fall under its murmurous thrall.  Within the Redwood Region, forests, streamsides, and woodlots offering consistent overhead cover, multi-storied broadleaf shrubbery, and a humid or otherwise damp setting attract these summer visitors.  Swainson’s Thrushes are comparatively late-arriving spring migrants.  They are typically not in evidence until the last few days of April, although they become commonplace within a week or so of reported ‘first arrival’ each year. 

Field work during the 1995-1999 Breeding Bird Atlas lent weight to a preexisting perception held by birders here that the great bulk of the Humboldt County population occurs within the near-coastal fog belt.  Where moist conifer forests and luxuriant red alder or willow regrowth is found, so also are found great numbers of these shy thrushes.  In this well-watered zone they occur in both riparian (streamside) and hillside situations.  East of the fog belt where tanoak and other evergreen hardwoods come into their own, Swainson’s Thrushes are quite confined to streamsides, pond margins, bogs, and seepy benches or springs.  Because of the tolerance the related Hermit Thrush (C. guttatus) has for dry, completely-drained ridges and featureless midslopes, that species appears to far outnumber Swainson’s across the county (and elsewhere in interior n.w. California) east of the zone in which vegetation is influenced by the ‘marine layer’.

Swainson’s Thrushes are birds of the forest or woodlot interior, only infrequently foraging in the open.  Large eyes and sturdy legs are adaptations allowing the birds to see and scratch out the varied invertebrates they consume during the breeding season.  The female builds a sturdy, mossy, cupped nest in the crotch of a shrub or small tree, in which 3 or 4 eggs are laid.  Incubation lasts 12-14 days and the young fledge about two weeks after hatching.  In late summer and early fall, evergreen huckleberry thickets near the outer coast attract great numbers of migrants, as do sheltered stands of cascara, whose ripening berries are a favored food item.  On nearly any September evening, the mellow, piping heep notes of night migrants may be heard coming from overhead.  These calls diminish rapidly after the first of October.  The latest I have heard a night call is November 2nd.  Nearly all of these birds retire to the tropics for the winter.  There are very few confirmed reports from November through March—so, should you believe you’ve spotted a Swainson’s Thrush during a Christmas Bird Count, think twice and check your field marks!

Swainson’s Thrush can be distinguished from our only other regularly-occurring spot-breasted Catharus thrush, the Hermit, by its narrow but apparent buffy eye-ring and thin loral extension—imparting a somewhat ‘spectacled’ look—and by lacking the foxy-reddish tail of the Hermit.  Unlike Hermit Thrush, Swainson’s doesn’t habitually flick its tail upward and slowly lower it.  The song of this species is a rich, fluty, upward-spiralling ‘cor-cor-cordelia, cordeelia, corDEE-lia’.  Among several common call-notes, an interrogative, upward-inflected ‘wit?’ is most frequently heard.