Is this yet another wren song?

Apparently it's not.  According to the very helpful people on SaskBirds, this is a Warbling Vireo. Now I just need to see the bird.  It is very elusive, at least in my yard, and prefers to forage in the tops of trees.

Learning to recognize birds by their sounds as well as by sight is very challenging, though it certainly increases the number of birds I can identify at any random time.  It would be a whole lot easier if they didn't have so many different songs.  Or if they would just come out into the open while they sang.