On quiet spring nights, if we stand outside and listen, we can hear the nocturnal calls of migrating songbirds drifting from the sky. Especially at the end of May, in eastern North America, one of the most characteristic sounds is the gentle bell-shaped whistle of the Swainson’s Thrush. On a major flight we may hear dozens per hour, but the next morning it will take some work to find a fraction of those numbers: after landing at dawn, the shy brown thrushes seem to blend into the thickets.
Identifying migrating brown thrushes is an annual challenge for birders. The Red-backed Thrush occurs throughout most of North America as a migrant, and it is widespread in the North and West in summer, so observers around the world have reason to study its points of identification.
During migration, the main challenge is separating the Gray-cheeked Thrush from the Gray-cheeked Thrush (and from Bicknell’s Thrush, which is so similar to the Gray-cheeked Thrush that the same field markings apply). Swainson generally has a much stronger facial pattern, with distinct buff “spectacles” consisting of a full eyering and a broad pale line above the darker lores. The narrow lines of buff across the brown cheeks are difficult to see, but they add to an overall warm appearance, and the buff usually appears on the side of the neck and the side of the upper chest.
The Grey-cheeked Thrush, in comparison, has a much “colder” appearance. On closer inspection, it shows narrow gray lines on the cheeks, instead of buff. Although the gray cheek lacks the obvious eye ring of Swainson’s, it does have a pale area surrounding the back half of the eye. This face pattern – solid gray in front of the eye, paler behind the eye – is quite distinctive and worth studying.
The Hermit Thrush is also sometimes confused with the Swainson’s Thrush. It also has a bold eyering (although it usually lacks the pale line above the lores that would create the “spectacled” pattern), and it often shows buff on the side of the neck. Its main field mark – a reddish-brown tail, contrasting with the duller brown back – is well known, but often difficult to see in difficult lighting conditions. The hermit thrush is the only brown thrush likely to be found north of the Mexican border in winter. During spring migration, its peak passage occurs a full month earlier than Swainson’s in the eastern part of its range. In the far West, where Swainson’s disease does not migrate so late in the spring, the difference in its timing is less obvious.
The most intriguing identification tip for the Swainson’s Thrush is this: They might actually be two species. A century ago, ornithologists called birds on the Pacific coast “red-backed thrushes” and those farther east “spoon-backed thrushes.” Judging by recent studies, these names could be making a comeback. Birders may soon have to separate another pair of difficult thrushes, as detailed on the next page.
What to look for
General color. The upperparts are smooth, olive-brown to medium brown, with little contrast between the back, wings and tail.
Eye ring. Complete buff eyering, usually connected to
a pale buff band above the darker lores to create a “spectacled” look.
Face and neck pattern. Fine lines or patches of buff on the cheeks, and usually a buff wash on the sides of the neck. This color is evident on the widespread olive-backed birds, less so on the warmer brown birds of the Far West.
Chest model. Variably marked with dark spots,
generally less visible on birds of the Far West.
Grey-cheeked Thrush, adult. April in Galveston County, Texas
Although the Grey-cheeked Thrush is sometimes described as lacking an eye ring, this is misleading. It usually presents a sort of semi-eye circle, a pale border mainly behind and below the eye, leaving only the area directly in front of the eye completely unmarked gray-brown. Often, the pale border behind the eye does not have a distinct edge, fading evenly toward the center of the face and contributing to a pale appearance. (This may even suggest the plain face of the Veery, not shown here. The Veery is a much brighter reddish-brown bird, but in poor lighting it can look strikingly similar.) Appropriately, the gray cheek has fine lines gray across the cheeks, where the Swainson’s Thrush is marked in buff.
Hermit Thrush, probably first year. October in Los Angeles County, California
Among North American brown thrushes, hermit thrushes are the most cold tolerant. They migrate earlier in the spring and later in the fall than others, and are common in winter in the southern states. Reports of Swainson’s Thrushes during the colder months are usually based on misidentified Hermit Thrushes. The Hermit’s eyering can be just as striking as Swainson’s, and the general color of its plumage varies with geography. If Swainson’s disease is suspected in winter, birders should always look for the contrasting reddish tail of the Hermit Thrush. Furthermore, the bird in this photo has buff spots at the tips of the greater wing coverts, typical of all brown thrushes in first winter plumage.
Swainson’s Thrush, “solive-backed” form. April in Galveston County, Texas
A few generations ago, when birders more frequently used English names for subspecies, it was common to refer to the Red-backed Thrush throughout most of its range as “Red-backed Thrush”. olive” and the Pacific Coast population as “Rufous-backed Thrush”. The bird in the photo is a classic example of the former, washed with olive drab brown on the crown, back, wings and tail, and with quite marked spots on the breast. Comprising at least three subspecies, it is the most widespread and abundant form. It breeds throughout Alaska and Canada, except for a narrow region along the Pacific coast and southward into the northeastern United States and the northern Rocky Mountains. Its wintering area is mainly in South America.
Red-backed thrush, “red-backed” form. May in Riverside County, California
Red-backed Thrushes that breed near the Pacific coast from southeast Alaska to southern California were once known as “Rufous-backed Thrushes” – and perhaps they will be. again in the future (see box below). True to their name, on average they have a more rufous or reddish-brown upperside than their olive-backed oriental cousins. Due to their overall warmer color, buff tones around the eyes and neck do not contrast as much and are generally less noticeable. The birds also tend to have paler, less obvious patches on the chest than the “olive-backed” form, although the bird in the photo is close to extreme pale. They also tend to migrate earlier in the spring, perhaps because most of their nesting range is in mild coastal climates.
A new emphasis on old names
Birders in general stopped referring to Swainson’s and Rufous-backed Thrushes several decades ago, but recently scientists have taken a closer look at the birds represented by these names. Both populations, currently classified under Swainson’s Thrush, may merit treatment as species in their own right.
The olive-backed group breeds from western Alaska to Maine and far south into the Rocky Mountains, while the red-backed group breeds from southeastern Alaska south to California . Potential contact in their breeding grounds occurs primarily in British Columbia, and research there has highlighted differences between groups.
Although red-backed and olive-backed types interbreed, the hybrid zone between them is narrow – only about 50 miles wide in the best-studied location. In addition to plumage differences, songs also tend to differ, with rufous-accompanied songs being on average longer and lower pitched. Red-backed birds also migrate earlier in spring, reaching breeding grounds earlier.
Recently, studies have looked into the genetic bases of different migrations. Olive-backed birds of British Columbia migrate east and then south in the fall, heading to South America. Red-backed birds move south and end up in Mexico and Central America. Hybrids apparently take an intermediate route through more hostile terrain, making it less likely that they will survive the journey. If hybrids are at a disadvantage, this leads to greater isolation between the two groups, contributing to their evolution into separate species status.