Montana Field Guide

Swainson's Thrush - Catharus ustulatus

Medium-sized thrush with olive-brownish upperparts, distinct buffy eye-ring, white underparts and brownish black spotting on the throat and breast. Overall length 16.1 - 19.3 cm; mass 23 - 45 g. Although wings and tail may be somewhat browner than body, upperparts appear fairly uniform in color. Swainson's Thrush is best distinguished from all other thrushes by presence of buffy eye-ring and lores. The distinctive song and call also distinguish Swainson's Thrush from others. The flutelike song spirals upward, differing from the descending or variably pitched songs of related thrushes (Evans Mack, Diane and Yong, Wang, The Birds of North America, No. 540, 2000).

Montana Range

In the Bozeman area, normal migration periods are from May 20 to June 15 and August 15 to September 15.

In general, most strongly associated with coniferous forests. In western states, at the southern end of its range, the species inhabits mountain deciduous riparian or aspen forests. Described as a bird of mature forests. Canopy closure, understory cover, tree density, and a conifer component are important habitat attributes (Evans Mack and Yong 2000).

Berries and insects. Breeding and spring migrating populations tend to be insectivorous; fall migrating and wintering populations more frugivorous (Evans Mack and Yong 2000).

Territory sizes of 1.7 to 3.3 acres in Douglas fir or lodgepole pine forests in western Montana have been recorded.

Most frequently nests in understory, particularly in thickets of deciduous shrubs or conifer saplings; less frequently but consistently found greater than 3 m high on top of a horizontal branch away from the bole of a larger diameter tree. Eggs are ovate, blue to greenish blue with reddish or brown speckles and smooth in texture. Clutch size is 1 to 5 eggs with 4 being most common (Evans Mack and Yong 2000). Near Fortine, egg dates range from June 12 to July 20.