A wood thrush is about 7-8 inches long and has a brown back and white chest with dark brown spots. They tend to feed on the ground but sing when up in trees. It's ethereal sound is difficult to describe, with melodic rising and falling notes often followed by a rapid trill. This call had a profound effect on Henry David Thoreau, who once wrote, "The thrush alone declares the immortal wealth and vigor that is in the forest. Whenever a man hears it, he is young, and Nature is in her spring."
I've had them pointed out to me while visiting conservation land near the White Mountains in New Hampshire; they generally need large areas of forest for their habitat. But since the garden borders land designated as an urban wild and is located about two blocks away from Franklin Park (where a wood thrush was recently sighted) hearing one around dusk should not come as a complete surprise. Next time I'm in the garden, I'll keep my ears open and eyes on the trees.