The song sparrow occupies an enormous range across climates from Aleutian coastline to southwestern desert, and populations differ accordingly. Large, dark, heavy-billed birds occupy the wet, cool Pacific northwest and Alaskan islands; small, pale birds occupy arid interior southwest habitats. Dozens of subspecies have been described, and the exact number depends on the authority consulted.
The pattern follows well-known ecogeographic rules: larger body size in colder climates, darker pigmentation in humid ones. Because the variation is broadly clinal, drawing subspecies boundaries is partly arbitrary, and much of the named diversity reflects the ambition of early describers as much as biological discontinuity.
The song is a separate story. A male learns several distinct song types, typically around eight or nine, usually from neighbouring adult males during his first year, then keeps that set. Neighbouring males share types and match each other during territorial exchanges, so the song sparrow provides both a case study in geographic variation of appearance and one in local cultural transmission of sound.