We observed a male sparrow in May 1999 in Stow, Massachusetts, that showed hybrid characteristics resembling both Grasshopper Sparrow (Ammodramus savannarum) and Savannah Sparrow (Passerculus sandwichensis). During 1999 and 2000, this male sang a simplified Song Sparrow (Melospiza melodia) song. However, in 2001 it also sang a typical Savannah Sparrow song and frequently alternated between these two songs. We recorded songs and captured this individual in an area with numerous Song Sparrows and about three pairs of Savannah Sparrows. We compared mitochondrial DNA, morphological measurements, behavior, and audio spectrograms of this hybrid with similar data from Grasshopper, Savannah, and Song sparrows. Genetic analysis indicated that the hybrid's mother was almost certainly a Grasshopper Sparrow. The hybrid showed morphological traits that were within the range of both Savannah Sparrow and Song Sparrow, but generally were more consistent for Savannah Sparrow. We observed this individual frequently associating with Savannah Sparrows, attempting to copulate with a female Savannah Sparrow, carrying Savannah Sparrow fecal sacs, and feeding young Savannah Sparrows, lending additional support to Savannah Sparrow paternity. We conclude that this individual was a Grasshopper × Savannah sparrow hybrid that learned to imitate the songs of Song Sparrows at an early stage of development. Interestingly, the hybrid's exposed culmen was considerably longer than culmen measurements of any of the three species of sparrows under consideration.