Black-tailed Gnatcatcher

Early on while walking the trails at the Henderson Bird Viewing Preserve, I started seeing these small blue-gray birds. They hung out in the heavy brush, but came out in the open regularly and stayed reasonably still for small birds.
The preserve helped out with their identification, as many do, by providing a checklist of birds seen, both regularly and rarely. I had a hunch that they were Gnatcatchers, but I had never seen any and the preserve listed two varieties as resident; the Blue-gray and the Black-tailed. There is not a lot of difference between them, especially in the winter.
One field mark that appears to be a sure thing is the amount of white under its tail. It you look closely at the white spots under the tail of Gnatchatcher in the bottom image, you can see that this Gnatcatcher is a Black-tailed. The Blue-gray's are almost entirely white. While most of these were seen together in a limited area, I assume they were all Black-taileds.