I was birding in Stiles and Hart Conservation area a couple weeks ago and didn't have much luck with migratories (they've all moved on), but I did see my first blue-gray gnatcatcher. It wasn't until I got home that I was able to identify it. I couldn't get any decent pictures since it stayed in the canopy of the trees. But I did get to hone my birding skills by paying attention to important details and looking it up in my Sibley Guide later. I need to get better at that- it seems I rely too much on my pictures.
What first grabbed my attention was its wheezing call. Here is a good sample: http://fsc.fernbank.edu/birding/birdID/bggnat.htm
I spent some time trying to get a good grasp of its features and was able to pick up on its size (small) and coloring (gray with a white belly).
Source: North American Birds Photo Gallery Photo by Peter LaTourrette.