The Yellow-rumped is a very common warbler. In fact, it is one of the most numerous songbirds of North America. It is so plentiful that by the end of the summer you'll often hear, "Oh, it's just another Yellow-rumped." But they're a very pretty bird, and I never tire of seeing them. The color variation across individuals and across sub-species is striking.
Also, they are the only warbler that regularly can be found over-wintering in the Chicago area (though I have to admit that I have never seen one in Nov-Feb). Their digestive system can adjust well to an insect diet in the warm months and a seed diet in winter. They're the only warbler that can digest really waxy seeds like bayberries.
The yellow rump—a patch on the lower back, just above the tail—is not always visible because it is often covered by the folded wings. But it is definitely there on every individual without exception, earning it a special nickname that I am not particularly fond of. If you need to know, Google it!
Dan's Feathursday Feature is a weekly contribution to the COS blog featuring the thoughts, insights and pictures of Chicago birder, Dan Lory on birds of the Chicago region.