Yellow-rumped Warbler

By far the most common warbler in Sonoma County from late autumn through late spring. Yellow-rumped Warblers are present year-round in the county, but their numbers increase dramatically in the winter months. The migrant population starts to arrive in mid- to late September. Numbers begin to thin again in mid-April through early May. Least common in the county in June, July, and August. According to Birds of Sonoma County, the Audubon's Warbler form (see below) greatly outnumbers the Myrtle Warbler form during migration and in winter. Usually solitary or in small groups, but, while staging to depart in the spring, may form flocks of 50 birds or more. Despite the bird's name, the yellow throat and flank patches of the Audubon's Warbler form are often much more conspicuous than the yellow rump patch. Most full adult plumages of both forms also have a yellow patch on the crown.

Is this one species or two? Yellow-rumped Warbler was the result of the lumping of forms formerly known as "Myrtle Warbler" and "Audubon's Warbler" in 1973 (according to Kaufmann, they were lumped because the two species were known to interbreed in a zone in southwestern Alberta, Canada). Both forms have a yellow rump (although, to complicate matters, the juvenile Myrtle form lacks the yellow rump patch), but the rump is not always the most conspicuous field mark, and, in most plumages, the two forms look quite different. Nearly all guide books show the two forms of Yellow-rumped Warbler separately, and it would not be terribly surprising if they were at some point split again into distinct species. As a general rule, forms with a white throat are Myrtle Warbler (as a mnemonic, think of W for "white" being an upside-down M for "Myrtle"), while those with a yellow throat are Audubon's Warbler--but that's an oversimplification; while distinctly yellow-throated birds will always be of the Audubon's form, female Audubon's Warblers in their first fall and first spring will often have a whitish or a creamy throat. In these plumages that cause confusion because of throat color, two features are useful; Myrtle Warbler will always have a supercilium (or "eyebrow") or at least a hint of one, and the shapes of the edges of the white throat patch are different in the two forms, with Myrtle Warbler showing the white wrapping up and slightly around the back of the auricular feathers (the ear patch). In the photo above, note the supercilium and the paler throat color that turns up behind the comparatively rich, brown ear patch--both indicating Myrtle Warbler. A picture is worth many words (see below). For more, I recommend the Peterson specialty guide on warblers or The Warbler Guide.

Note that as of July 2011, the genus Dendroica disappeared. These birds are now assigned to the genus Setophaga, but many older references will be around for a while, so expect to see this bird as Dendroica coronata, with newer sources using Setophaga coronata.