Four closely related North American bird forms—the eastern myrtle warbler (ssp coronata), its western counterpart, Audubon’s warbler (ssp group auduboni), the northwest Mexican black-fronted warbler (ssp nigrifrons), and the Guatemalan Goldman’s warbler (ssp goldmani)—are periodically lumped as the yellow-rumped warbler (Setophaga coronata).
The Yellow-rumped Warbler is a mid-sized New World warbler, though it is one of the largest species in the Setophaga genus (formerly Dendroica) which comprises a lion’s share of the species in the family. In total length, the species of bird can range from 12 to 15 cm (4.7 to 5.9 in) long, with a wingspan of 19 to 24 cm (7.5 to 9.4 in). Body mass can vary from 9.9 to 17.7 g (0.35 to 0.62 oz), though averages between 11 and 14 g (0.39 and 0.49 oz). Among standard measurements, the wing chord is 6.3 to 8.4 cm (2.5 to 3.3 in), the tail is 5 to 6.6 cm (2.0 to 2.6 in), the bill is 0.8 to 1.1 cm (0.31 to 0.43 in) and the tarsus is 1.8 to 2.2 cm (0.71 to 0.87 in). 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. Audubon’s warbler also sports a yellow throat patch, while the myrtle warbler has a white throat and eye stripe, and a contrasting black cheek patch. Females of both forms are more dull, with brown streaking front and back, but still have noticeable yellow rumps. Goldman’s warbler, of Guatemala, resembles Audubon’s but has a white lower border to the yellow throat and otherwise darker plumage; males replace the slate blue of Audubon’s with black.
These birds are one of North America’s most abundant neotropical migrants. The Yellow-rumped Warbler is 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.