The Savannah sparrow is a very variable species, with numerous subspecies, several of which have been split as separate species at various times. The different forms vary principally in the darkness of the plumage. The variation generally follows Gloger's rule, with Alaskan and interior races the palest, and southwestern coastal forms the darkest. There are some exceptions, though, most conspicuously in some island populations that presumably were strongly affected by founder effects. The general pattern of variation has a fairly clear divide, southwest of which the birds become notably darker; this agrees quite well with the limit between "P. sandwichensis" and "P. (s.) rostratus". Savannah sparrows show some variation in size across subspecies. The total length can range from , wingspan ranges from and body mass from . In the nominate subspecies, the body weight averages .