Well, I finally broke down and purchased a 400mm f/5.6 L lens. While I was taking pictures of squirrels enjoying a feast at the backyard squirrel feeder, I noticed all these tiny, flitty things in the trees.

I hadn't tried to take any pictures of small birds with my 70-200mm f/4 L because I could never get close enough to get a decent framing of these tiny critters. I figured I'd see what the 400mm could do... not too bad for my first attempts at bird photography.

I have absolutely no idea what kind of bird this is.

Update: I've since learned that this is a tufted titmouse. :)