We watched this chestnut-sided warbler feeding for several minutes. It was bouncing around a branch checking each leaf bud for insects.
The Chestnut-sided Warbler sings two basic song types: one is accented at the end (the pleased-to-MEETCHA song), and the other is not. The accented songs are used primarily to attract a female and decrease in frequency once nesting is well under way. The unaccented songs are used mostly in territory defense and aggressive encounters with other males. Some males sing only unaccented songs, and they are less successful at securing mates than males that sing both songs.
source - Cornell Lab of ornithology.