Nature notes: Get to know yellow-rumped warblers

I recently introduced the many autumn birds that have now returned to our latitudes, the shorebirds, ducks, raptors, sparrows and other songbirds that journey south each fall. There are many of these that are newly forming a regular part of my daily life: crowned sparrows scratching by the roadside, flickers trumpeting from a distant treetop and waxwings trooping by overhead in search of berries. But there is another winter bird that challenges even the white-crowned sparrow for abundance around town, and one to which I don’t think I have ever dedicated a full column: the yellow-rumped warbler.

The wood warblers are one of the great bird families of the Americas, with dozens of colorful, musical migrants that make a major contribution to the thrill of spring and autumn in the mind of most birdwatchers. Small, mostly insectivorous birds frequently adorned with yellow plumage in varying quantities, warblers as we know them in the United States are primarily classed with the Neotropical migrants, birds that mostly winter in Central and South America, although they may breed in Canada and the United States. There are a few exceptions: the common yellowthroat is an unusual wetland-dwelling warbler that is here year-round, and the lovely Townsend’s warbler has a small population that winters along the California coast, in addition to its larger winter population in Central America. But the family’s biggest exception to the “leave the United States in winter” trend is undoubtedly the yellow-rumped warbler.

These birds are common, and in fact are common across much of the country in winter, with an unusual ability to digest waxy berries such as those of wax myrtle and poison oak, expanding their range compared to that of more typical, more strictly insect-eating warblers. Now they are all over the place, moving through open woodlands and neighborhoods in flocks that can contain dozens of birds moving through the canopy of trees in search of berries, surface-dwelling invertebrates and flying insects, which they can pursue on the wing in short sallies from the branch tips.

In many cases, yellow-rumped warblers are easy to identify: They are the ones with the yellow rumps. Yellow under the wings and on the throat, although somewhat more subtle than that of the rump and not always obvious, is also unique to this species. Beyond details of plumage, these birds are fairly easy to identify by expectation, habits and sound. They are by far our most numerous warbler and our most gregarious, often foraging in loose flocks that maintain steady contact with an upwards-slurred call note that sounds like “chwit.”While this sound is difficult to transcribe effectively (“its peculiar quality is indescribable,” noted ornithologist and author
William Leon Dawson, “save for its manifest content of good cheer”), it has a distinctive tone that now alerts me to the yellow-rumps’ presence every single day.

In detail, however, they can often be confusing, with several different plumage variations depending on age, sex, season and subspecies. At one extreme are young females in winter, which can appear mostly dull gray-brown, with the yellow patches often reduced in size and intensity. If you don’t see the rump, it’s easy for these birds to seem simply anonymous and pale, somewhat streaky brown. At the opposite extreme are males in spring, which offer a radically different appearance of strongly contrasting gray on the back, bright yellow on their throat (and cap, flanks and rump) and black on their breast. A final complicating quirk: the West is mostly home to the Audubon’s subspecies, but we also see a lower number of the predominately eastern Myrtle subspecies, members of which have white rather than yellow throats.

Yellow-rumped warblers are one of those birds that seem to fly under the radar of most normal people, and yet they are one of the most indispensable elements in bringing life and animation into the quieter days of winter. Fall is not the season for song, not the season for brilliant orioles in the palms or swallows careening overhead and shimmering in the sunlight. It is a season of grayer skies and trees that lose their leaves. But those trees are not yet empty, and the sky is not all gray, as long as these boldest of the warblers dot both the air and branches with their gold.

Jack Gedney’s On the Wing runs every other Monday. He is a co-owner of Wild Birds Unlimited in Novato and author of “The Birds in the Oaks: Secret Voices of the Western Woods.” You can reach him at jack@natureinnovato.com.