The Yellow-rumped Warbler has two subspecies, Audubon's most common in the west, and shown here, and the Myrtle, common on the East Coast but present nationally. For a nice description of the two and outstanding photographs, see this page on Tom Grey's website, Yellow-Rumped Warbler. The most common distinction between the two is a yellow throat in the Audubon's and a white throat in the Myrtle's. The two images above were captured along the San Pedro river in Hereford in January of 2017. The image below caught at Agua Caliente in November 2016. The images above show the yellow rump in all its glory. One can clearly understand the common nickname for this species on bird walks, "butterbutt."