The syrinx sits where the trachea splits, and each branch carries its own vibrating membranes and muscles. Most songbirds alternate the two sides or use them for different frequency ranges. A smaller set can drive both sides independently and simultaneously, which is why some thrush song sounds impossibly rich to a human ear expecting one voice.
Spectrograms of wood thrush song show two independent harmonic series overlapping in the final flourish, with pitches that are not simple harmonics of one another. The bird is not producing a single note with overtones; it is producing two notes. Recordings played at reduced speed make the two voices audible directly.
The wood thrush is also a case study in decline. Continental survey data show a sustained long-term drop, and the causes are split between breeding-ground forest fragmentation, which raises cowbird parasitism and nest predation, and loss of Central American wintering forest. Acid deposition reducing calcium availability for eggshells has also been implicated in parts of the range.