This bird was dancing in the Groundsel bush (Baccharis halimifolia), reaching for a morsel of seeds or perhaps an insect.
Around here they are called “Butter Butts” because of the bright yellow patch which peaks out of tail feathers.
They eat mainly insects in the summer. Their digestive systems are “uniquely suited among warblers to digest bayberry” (wax myrtle) (Myrica cerifera) where I often see them feeding. At this time of year they are also fond of the seeds of Bidens alba and the bluestem grasses which they flutter above to grab.