For a couple of weeks at the end of April, I would see darts of flashing yellow in the mesquite trees. The little birds rarely stayed still enough for me to see what they were, but I finally saw the black cap that identified them as Wilson's Warblers. After several days of trying to photograph them and getting mostly in-focus mesquite branches and a yellow blur as they flitted from branch to branch hunting insects, I finally managed to get a couple of fairly clear shots.
These are migratory birds and were probably on their way north. I haven't seen them in the past week or two.