The use of song by wood-warblers (Parulidae) with multiple song types has received little attention during migration. On the breeding area, the two song types serve to attract mates and defend territory, but the function(s) during migration, if any, are unclear. I studied the singing of Townsend's Warblers (Setophaga townsendi) during spring migration through southern Oregon and northern California. Males preferentially used Type II songs (79%) whether alone or in flocks of up to ∼20 individuals. Type I songs of wood-warblers tend to be acquired early in the hatch year while Type II songs appear to be subject to modification over an extended period of time. The preference for Type II songs in migration may reflect an important period in the development of Type II song. More than one dialect of Type I song was recorded within flocks indicating that flocks were composed of individuals from different breeding populations. Type II songs are recognizably similar in breeding populations from central Oregon to Alaska. The similarity of Type II songs across the range of the species may result from song refinement through interaction with other singing males while in transit.