At the age of 58, his fame already assured by publication of his great Birds of America, John James Audubon set out in 1843 on one last expedition to look for new birds. Traveling up the Missouri River with a team of younger naturalists, he did find species new to him, including a few that were new to science. One such novelty was a vireo that he named for expedition member John G. Bell, “an excellent companion in our not unperilous rambles.”
Audubon’s crew first encountered the vireo near what is now St. Joseph, Missouri, and continued to find it as they traveled upriver into the Dakotas. Typically, it was near the river in “the bottom lands, overgrown with low shrubbery, or along the borders of ravines” in prairie country. The species still nests mainly near streamsides or in shrubbery along dense thickets.
Its habitat often makes Bell’s Vireo hard to see. Fortunately, it has a distinctive song — a fast, jumbled cheedledoo-cheedledeedle-dee? . . . cheedledoo-cheedledeedle-doo! This little chant makes it easy to identify when it’s singing. But a silent bird, especially in migration away from its breeding range, can be quite challenging, because this is an exceptionally plain bird with few obvious marks.
That plainness can be a field mark in itself. A first step is to rule out other birds, such as a juvenile Verdin or a really drab warbler, by noting its distinctive bill shape. But among North American vireos, its faint wingbars and weak face pattern are unique.
Bell’s Vireo is divided into four subspecies, with a curious distribution. The Midwestern race has a breeding range stretching from Texas all the way north to the Dakotas and sparingly east to Ohio. Another race breeds in western Texas and southern New Mexico. The Arizona race spills over into southeastern California and extreme southwestern New Mexico. Finally, there’s the “Least Bell’s Vireo” of southwestern California.
Midwestern birds look different from those of California and Arizona: more colorful, with a stronger wash of olive on the back and yellow below, and distinctly shorter-tailed. They often hold their tails still or bob them up and down gently, but California and Arizona birds often flip their tails around, from side to side or up and down, like a gnatcatcher. Birds of western Texas and southern New Mexico are most like those of the Midwest, although slightly duller and longer-tailed.
The California race is classified as endangered, and almost throughout their range, Bell’s Vireos are birds of conservation concern. They have been hit hard by cowbird parasitism — in some studies, up to 90 percent of vireo nests have contained Brown-headed Cowbird eggs — and by destruction of streamside habitat. Fortunately, cowbird control and habitat restoration projects have helped local populations bounce back.
What to look for
Overall color and pattern. Quite drab. Midwestern birds show olive on back and yellow wash on flanks, while southwestern birds are grayer.
Wing Pattern. Two very narrow whitish wingbars, the upper one often quite faint.
Face. Weak pattern with broken eye-ring, stronger above eye than below, and short, narrow eyebrow, mostly above the lores.
Tail. Varies with geography. Longer in California and Arizona, where birds often flip tail from side to side. Shorter in Midwest, where birds often hold tail still or bob it up and down.
Bell’s Vireo, California Race. The birds breeding in southwestern California and northern Baja represent an endangered subspecies often called “Least Bell’s Vireo.” Despite the name, this race is not noticeably smaller than the Arizona form, but it’s grayer, showing barely a hint of color in spring and summer. The most similar species is Gray Vireo, which also shows faint wingbars and a longish tail. However, Gray Vireo has a narrow, complete white eye-ring, not diffuse and broken like that of Bell’s, and it’s usually found in juniper woods, not streamside thickets. Other ID contenders like Plumbeous Vireo have stronger wingbars. Indeed, Bell’s Vireos in the West are more likely to be confused with non-vireos, such as gnatcatchers.
Bell’s Vireo, Midwestern Race. Widespread in the Midwest, this form (called “Eastern” in some books) looks notably unlike the subspecies in California and Arizona. Its shorter tail gives it a different overall shape, and on average, it’s more colorful, with olive on the back and yellow along the sides and flanks. However, the face pattern is the same everywhere, weak but distinctive. It shows a broken white eye-ring, stronger above the eye than below, and a short, pale eyebrow, mainly above the lores in front of the eye. While the California and Arizona races often twitch their tails around in the manner of a gnatcatcher or Bewick’s Wren, the Midwestern race tends to hold its tail still, or to bob it up and down gently.
Bell’s Vireo, Midwestern Race. Wingbars are important field marks of many birds, but we shouldn’t merely note that a bird does or doesn’t have them. There are many degrees of variation. In this April photo, most of the greater covert feathers have narrow whitish tips, creating a thin wingbar. Pale tips on the median coverts are barely visible, for only a hint of a second. If we saw this vireo in fresher plumage, in late fall, both wingbars would be more apparent; if we saw it in worn plumage in summer, it would appear to have one faint wingbar or none at all. On this Midwestern individual, compare the colors and tail length to the California bird in the photo below.
Bell’s Vireo, California Race. Photographed on the wintering grounds in southern Baja, this Bell’s Vireo is almost certainly of the California-breeding subspecies, or “Least Bell’s Vireo.” It shows a stronger tinge of color than we usually see on this race, but that’s probably a function of the December date. The species goes through a complete molt in late summer, so individuals are in their freshest plumage in fall and early winter. When they come back north in spring, and when they’re singing on the breeding grounds in early summer, their colors are faded and the California birds can look completely gray. We always have to consider time of year and the effects of molt, wear, and fading on subtle birds like these.
Two species of Bell’s Vireo?
Differences between Midwestern and Southwestern forms of Bell’s Vireo may signify more than just distinctive subspecies. In 2017, the American Ornithological Society’s Committee on Classification and Nomenclature — informally known as the Checklist Committee — received a proposal to split this vireo into two species.
This proposal was based on a detailed genetic analysis of specimens from most parts of the species’ range. The study showed clear genetic differences between the Midwestern and west Texas forms and the subspecies from Arizona and California. Two of the authors of the proposal, Carla Cicero and Kevin Burns, were actually members of the Checklist Committee, and the third author, Luke Klicka, had headed up genetic research on the vireo. Even so, the vote on the full committee was split, five to five, so it didn’t pass.
The sticking point involved questions about the area where these two forms approach each other most closely, in southern New Mexico. Specimens used in the analysis didn’t include any from the area between Arizona and western Texas. Committee members wanted to know what happens in that 200-mile gap. Do the two forms come in contact? Do they interbreed? Are there subtle song differences that haven’t been quantified? The general sense was that these are probably distinct species but that it hasn’t been proven — yet. Regardless, birders should pay close attention to these forms, in anticipation of a likely future split.