I introduced the genus Catharus two days ago. Here it is again, in the person of a Swainson's Thrush who shared
some land with us outside Eugene, Oregon. His songs spiral upward, while the Veery's spiral downward. "Spiral upward"
evokes a visual image, so let's look at the sonogram to see what it's talking about. It's easy enough to see the roller coaster
ascent of the traces as a spiral, but I see something else. A Swainson's song looks like a Chinese pen-and-ink drawing of
mountains. The foothills are on the left, major peaks in the middle, and distant crags peak dimly through the mists
on the right. Acoustic phenomena called sidebands conveniently give us more foothills below the peaks. True
harmonics add depth with another ridge in the distance. The Veery's sonogram looks very different up close.
My early birdbooks contained no mention of "Swainson's Thrush," but they did have an "Olive-backed Thrush." In those days
(the 1950s) birdbooks treated each subspecies of bird separately, and the subspecies all had official English names.
The eastern populations of Catharus ustulatus, breeding all the way to the east slope of the Cascades, were "olive-backed;"
the populations of the humid Pacific coastal forests were "russet-backed." Eventually, official English names for
subspecies were ditched and both groups were subsumed in Swainson's. But this did not change their biology. Though
not quite separate species, russet-backed and olive-backed populations have subtle differences in coloration, differ in their migration
routes and winter ranges, and have slightly different vocalizations. One acoustic difference is the form of the first note of the song (the first
foothill in our mountainscape). According to Nathan Pieplow it starts with a clear upslur. It's a faint buzz in the olive-backed. We'll
get to see the difference when we find an olive-backed recording.