A different sort of Clay-colored Sparrow

While counting songbird migrants at the Higbee Dike this morning, I was distracted for a while by a small sparrow feeding along the edge of the dirt track. I was especially puzzled by the fairly heavy, blackish flank streaking and richly saturated upperparts on what otherwise looked and sounded (thin, Tennessee Warbler-like "tsip") like a Clay-colored Sparrow, and figured that the bird was most likely a hybrid, perhaps with a member of the genus Melospiza (like a Swamp or Song Sparrow). A bit of digging in Pyle and the Birds of North America account revealed that some young-of-the-year Clay-colored Sparrows don't finish their post-juvenal molt until after migration, and so can retain some of the streaking on their bodies where juvenal feathers haven't yet been replaced. Though the overall color of the upperparts still strikes me as a bit odd, I now think this is most likely a young Clay-colored Sparrow with some vestiges of juvenal plumage, something that we don't often see on the East Coast in fall. Thanks to Michael O'Brien for his comments and help with the identification.