Fleischman is deeply interested in American history, and his books often focus on historical events. He is also interested in the natural history of birds. He combines these interests in Townsend's Warbler. The book describes both the 1834 expedition to the American northwest of Nathaniel Wyeth and the migrations of Sylvia townsendi, better known as Townsend's warbler. The paralleling of two separate journeys, which result in the crossing of paths of the naturalist John Kirk Townsend and the bird that will be named after him, makes for interesting reading. Townsend is driven to travel across North America out of sheer curiosity; the birds are driven by instinct, traveling to where their food will be abundant.