Savannah Sparrows can be easily mistaken for song sparrows, save for a distinctive yellow "eyebrow." They're prevalent throughout North America in large numbers but I very rarely see them as feeder birds; all my sights of them have been in migratory tracts such Dead Creek, Parker River and Herrick's Cove. They're gorgeous birds, with a great deal of variety between them. I'd never, for example, seen one as dark as the one shown here before.
Cornell reports that there are seventeen different subspecies of the Savannah, some of which are much heavier and paler than other varieties. This can lead to confusion in the field, but generally speaking, if you see a bird which bears close resemblance to a song sparrow but has a clear yellow eyebrow, you're probably dealing with the Savannah. White-throated sparrows often also have a yellow eyebrow, but the white throat is prominent and clear and seaside sparrows have a lot more greyish hues in the head.