Yellow-rumped Warbler

The yellow-rumped warbler has an average length of 14 cm and weight of 12.5 g, its appearance is different across its subspecies groups, especially the two major ones: the coronata group (myrtle warbler) and the auduboni group (Audubon's warbler); intra-group variations are also observed. In spite of varying appearances, the yellow rump (as suggested by its name) is present in all subspecies and thus characterizes the yellow-rumped warbler. Within the myrtle warbler group, adult males during the breeding season have gray backs with dark streaks, while females have brown backs; male and female myrtle warblers can also be distinguished by their different cheek colors, with the former's being black and the latter having brown or gray cheeks. The Audubon's subspecies group is not very dissimilar to the myrtle: in summers, males of both forms have streaked backs of black on slate blue, white wing patches, a streaked breast, and conspicuous yellow patches on the crown, flank, and rump (the latter giving rise to the species's nickname "butter butt" among birdwatchers). Yet the color of the coronata and auduboni groups' throat patches differs and distinguishes them, as the Audubon's warbler sports a yellow throat patch while the myrtle warbler has a white throat and eye stripe, and a contrasting black cheek patch. …

The yellow-rumped warbler breeds from eastern North America west to the Pacific, and southward from there into Western Mexico. Goldman's yellow-rumped warbler is a non-migratory endemic within the highlands of Guatemala, and the black-fronted warbler is a non-migratory Mexican endemic. The myrtle and Audubon's forms are migratory, wintering in the southern U.S., Mexico, Central America, and the Caribbean. The breeding area of the myrtle warbler ranges from Alaska across Canada to Newfoundland, and as far south as Massachusetts and mountains of Virginia, including throughout the northern Great Lakes region. Myrtle warblers migrate south for the winter, ranging from southern British Columbia all the way to Panama and throughout most of the southeastern United States. Audubon's warbler breeds throughout western North America, from British Columbia to California, across the Rocky Mountains and as far east as the Dakotas. Among warblers, the Audubon's is by far the most widespread in North America in winter, being among the last to leave in the fall and among the first to return in spring.

Migration. The yellow-rumped warbler's migratory behavior varies across different groups and subspecies. …

Audubon's and myrtle warblers are among North America's most abundant neotropical migrants. While 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. Other places yellow-rumped warblers have been spotted foraging include picking at insects on washed-up seaweed at the beach, skimming insects from the surface of rivers and the ocean, picking them out of spiderwebs, and grabbing them off piles of manure. Common foods include caterpillars and other larvae, leaf beetles, bark beetles, weevils, ants, scale insects, aphids, grasshoppers, caddisflies, craneflies, and gnats, as well as spiders. They also eat spruce budworm, a serious forest pest, during outbreaks. When bugs are scarce, the myrtle warbler also eats fruit, including the wax-myrtle berries which gave the myrtle subspecies its name. It is the only warbler able to digest such waxy material. The ability to use these fruits allows it to winter farther north than other warblers, sometimes as far north as Newfoundland. Other commonly eaten fruits may include juniper berries, poison ivy, poison oak, greenbrier, grapes, Virginia creeper and dogwood. …