My first encounter with the sweet little American Tree Sparrow, but perhaps not; I may have been looking right at one all along and thought it was a Song Sparrow or any one of a number of little brown songbirds. Now that I know what to look for, most notably its beautiful rusty brown cap and that the bottom half of its beak is yellow, it may be easier to notice.

Several were foraging alongside a mixed flock of songbirds. They were feeding on the ground but at the tiniest disturbance, flew into the camouflage provided by neighboring trees. As territorial as was the Dark-eyed Junco, the Tree Sparrows behaved just as badly to one of its own kind.
American Tree Sparrow range map, courtesy Cornell