Bird of the Month: Yellow-rumped Warbler
By Andy McCormick
PC: Mick Thompson (Yellow-rumped Warbler)
Scientific Name: Setophaga coronata
Length 5.5 in
Wingspan 9.25 in
Weight 0.43 oz
AOU Band code YRWA
Formerly listed as two separate species, Myrtle Warbler S.c.coronata in Eastern North America and Audubon’s Warbler S. c. auduboni in the west, the Yellow-rumped Warbler is the most widespread and most common warbler in North America. Both forms are seen in Washington with Audubon’s more common and Myrtle more often seen in spring and fall migration. A zone of interbreeding between the two forms exists along a diagonal following the northern Rockies at the border of British Columbia and Alberta (Dunn and Garrett). Intergrades of the species exhibit a combination of appearance characteristics.
The Yellow-rumped Warbler is now part of the genus Setophaga, moth eater, from the Greek setos, moth, referring to warblers in this genus that will hawk for insects. The species name coronata from the Latin for wreath or crown refers to the bird’s yellow crown (Holloway). This warbler is quite hardy and due to its specialization in being able to digest waxy berries such as wax myrtle, bayberries, poison ivy and juniper berries, it is able to winter farther north than any other warbler. In some areas it is the only warbler present in the winter months (Dunn and Garrett). When they can, these warblers will fly out to catch insects which dominate their diet in spring and summer.
The two forms are distinguishable by plumage differences. Briefly, Audubon’s form has a yellow throat patch, with a dark but plain face. The Myrtle form has a white to pale throat which curves up behind a darker ear patch. The Myrtle also has a thin eyebrow stripe which is missing on Audubon’s. Both forms have a yellow rump patch which has elicited the species’ affectionate Northwest nickname of “Butterbutt.” They also have yellow patches just below the wings. The yellow crown patch can be concealed much of the time. Adult males are gray and females and juveniles are more brown.
Yellow-rumps breed in the boreal forest. They migrate earlier in April than other warblers do. The female builds an open cup nest made of bark fibers, weeds, and twigs. Usually four creamy white eggs with brown markings are deposited. Incubation lasts about 12 days and first flight can occur about two weeks later. Some pairs have two broods per year (Kaufman).
The Yellow-rump is a Neotropical species that is so widespread that it will migrate along all four North American flyways. Birds in Western North America make full use of the Pacific Flyway wintering in Western Mexico and Central America in a wide variety of habitats from open forest areas, pine forests, mangroves and shade coffee areas. In Mexico they prefer thorn and pine-oak-fir forests (Hunt and Flaspohier).
The species is very adaptable and has a stable population. Breeding Bird Surveys between 1966 and 1994 indicate increases in numbers (Hunt and Flaspohier).