I recently banded this juvenile Song Sparrow. Young are born naked and molt into this juvenile plumage, which lasts until the first fall, when birds obtain their basic (winter) plumage. Sibley's Birds of North America is one of the few field guides to illustrate this plumage. The underparts of these young birds have brown stripes against a buffy background (unlike the white background of older birds). Often they are so buffy that the beginning birder might mistake them for Lincoln's Sparrows.
Note also the radius and ulna bones visible on the underside of this bird's wing. The outer end of these two bones are the bird's wrist. The secondary flight feathers attach to the ulna. The outer bones on the wing are the bird's carpels and digits (modern birds have but three). The primary feathers attach here. Thus birds regulate their flight with their hands.