The male Black-Tailed also sports a nice black cap when he's courting, though this leads to its own problems of taxonomical separation from the Black-Capped Gnatcatcher found in southeastern Arizona.
Gnatcatchers are warbler size, but nearly half of their body length is made up of the birds' tails, which they flick about and use to balance while hopping and running in dense foliage, as well as scare up insects.
I recently found a pretty good spot for observing both Blue-Grays and Black-Tails, so I'm hoping to follow up with more photos and behavioral observations. For some reason, this relatively dull birds hold a special fascination for me, perhaps only because it's so tricky to get a good long look at them.