I suggested that your mystery bird might be a gnatcatcher only because it is common at this time of year due to it being able to nest farther south than many other species. Juncos nest around Hudson Bay when they are not shopping elsewhere. Another bird that is sparrow-sized, because it's a sparrow, and may also flash white tail feathers in flight is the Vesper sparrow. With that supposition in mind there is the unlikelihood of seeing a Vesper near houses ... in a subdivision. Vesper sparrows inhabit open countryside such as grasslands or large farmer's fields. Now I know that you have an impressive garden but I hardly think that it qualifies as Vesper territory. Still, stranger things have been seen. I once saw a Vesper sparrow in a partially wooded parkette situated adjacent to an open field on an incline with a winding path leading down to a creek and there was this lion walking with a scarecrow and a tin-clad man ... but I digress, anyway that's my second guess.