Butter-butt. Yep, that’s the nickname of this little warbler, and how appropriate it is. A Yellow-rumped Warbler looks like somebody just spread a little bit of butter on its butt.
We only see these little guys here in the winter. In the summer months they head north up into Canada and Alaska.  When they are here, you see them everywhere. On the Georgia coast one January it seemed that anytime a small bird moved in a thicket or group of small trees, you could bet it was a Yellow-rump.
During winter, Yellow-rumped Warblers find open areas with fruiting shrubs or scattered trees, such as parks, stream-side woodlands, open pine and pine-oak forest, dunes (where bayberries are common), and residential areas. On their tropical wintering grounds they live in mangroves, thorn scrub, pine-oak-fir forests, and shade coffee plantations. 
They are primarily insectivorous. The species is perhaps the most versatile foragers of all warblers. Beyond gleaning from leaves like other New World warblers, they often flit, flycatcher-like, out from their perches in short loops, to catch flying insects. 
References and Additional Information
 Wikipedia: Yellow-rumped Warbler
 The Sibley Guide to Birds, second edition – David Allen Sibley
 Cornell – All About Birds: Yellow-rumped Warbler
 Cornell – All About Birds: Yellow-rumped Warbler – sound