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Swainson’s Thrush

A SONG ABOVE OTHERS

By Kathy Ross

Mid-spring is filled with anticipation for the enchanting, heavenward spiraling song of the male Swainson’s Thrush (Catharus ustulatus) returning each year to our mountain world to breed. This secretive, not often seen, medium-sized Thrush (6″-8″ long, 12″ wingspan), entertains our sense of hearing during the summer months with its distinctive flute-like song. It cares not if other species enjoy its melodic song, but does hope it will find another of its species somewhere amidst the thick forest canopy it claims as home during the summer months.

Swainson’s Thrushes nest in northern and western coniferous forests, preferably in dense thickets along streams. Here the female builds a cup-shaped nest of twigs, mosses and mud where for 10 to 13 days she will care for 3-4 eggs. After the hatching, both parents care for the young until they leave the nest at 10-12 days. Fledged young and adults will spend the rest of their summer in the northern forests of the U.S. and Canada, foraging the understory litter or catching insects on the fly. Their diet includes caterpillars, spruce budworm moths, and worms, plus an assortment of small berries and other fruit, which are a necessary food source later in the summer during a long migration south. Even though the haunting breeding song may not be heard during their trip south, their illusive presence is still made known by two very distinct, single-note calls, a ‘whit’ and a ‘peep.’

Being very much a shy forest bird, the Swainson’s Thrush is rarely found far from a closed canopy of evergreen or deciduous woodlands. In these settings they can easily be mistaken for two of the other mountain-breeding spotted thrush, Hermit Thrush and Veery, which are similar in appearance and song. The Swainson’s is a little more slender-bodied, longer-winged thrush and has large buffy eye-rings extending to the front of the eye, which give it the appearance of wearing “spectacles.” Along with vocal differences, these “spectacles” are a useful field mark for identification.

Besides similar thrush species, research has revealed four subspecies of Swainson’s Thrush. The two most discussed are the Russet-backed, found mostly breeding in the Pacific coastal areas, and the more widely distributed eastern/northern continental birds called Olive-backed Swainson’s Thrush, more commonly known to breed east of the Cascades/Sierra Nevada. These two genetically distinct subspecies vary slightly in appearance–Russet-backed with medium-brown upperparts and Olive-backed with a dull olive-gray back. Both populations have pale underparts with spotted chests and always seem to wear their “spectacles.”

The following discussion from recent research by Kristen Ruegg and colleagues, noted in the Wikipedia article on Swainson’s, suggests how these subspecies might have evolved and how these changes might be affecting current impacts on this Nearctic-Neotropical migrant.

“The genetic differences between the subspecies, and the circuitous migratory route of the continental birds, strongly suggest that these species underwent a rapid range expansion following the end of the last ice age, with populations originally summering in the southeast of North America expanding their ranges northwards and westwards as the ice retreated. Details of the molecular genetic analysis support the hypothesis of rapid expansion of both coastal and continental populations. Current migratory routes of the continental birds, especially the western populations, are not optimal in ecological terms, and presumably represent an inherited, historical route pattern that has not yet adapted to the birds’ modern population locations.”

 

These results notwithstanding, genetic testing suggests that “Swainson’s Thrush is the most ancient North American species of its genus; it is not closely related to other Catharus, and the outward similarities with the other North American species are due to convergent evolution.” (Winker &Pruett, 2006)

Our Swainson’s travel thousands of miles a year from far northern breeding grounds to forested habitats of Mexico and Central and South America. Traveling up to 200 miles at a time, mostly at night and often along with other migrating songbirds, they tend to fly low and may encounter many obstacles along the way. Sibley notes, “In Indiana, Swainson’s Thrushes are the most common migrants killed in collisions with windows and buildings.”

Although Swainson’s Thrush has the conservation status of Least Concern, many areas note a decline in their numbers from habitat loss as well as an increase in human-made obstacles across their migration routes. Those inherited route patterns take some populations across the entire continental U.S. and then south to Central and South America. Statistics notwithstanding, our NW Montana forests seem filled with their song and calls during spring and summer.

Recent research out of universities in Ohio and Pennsylvania suggests that during this long migration they “take numerous daytime ‘power naps’, lasting only a few seconds.” By sleeping with only one eye closed, it seems they can rest half their brain, keeping the other half alert for danger.

Again, I am awestruck by the feats of our long distance traveling songbirds, and, more than ever, I will appreciate and thrill to the haunting song of the male Swainson’s Thrush when it flits through the forest outside my back door after what is to me an epic journey.

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