Ruby-crowned Kinglet
Regulus calendula
Photo original hosted at Wikimedia - This photo was taken by United States Fish & Wildlife Service employee Donna Dewhurst.
Kinglets are very small (barely 10 cm long, about the size of a common hummingbird), very energetic birds that are quite common in the Sonoran Desert during the winter months. The red crown is concealed more often then not so an olive-gray bird marked with white and black is what is normally seen. They move and flit constantly as they must consume great quantities of tiny insects as fuel. Notice the fine beak for handling small insect prey. Look for kinglets mostly in riparian areas among verdant vegetation, but occasionally they can be found in more typical desert areas favoring leafy mesquites and palo verdes.