Hermit and Swainson's Thrushes
LISTEN to the Swainson's Thrush (91 KB)
The Hermit and Swainson's Thrushes are easily confused yet have very different breeding habitats, migration schedules, and winter ranges. In San Diego County we are at the southern tip of the breeding distribution of each. Neither has yet been confirmed breeding in the county during our atlas effort, but the best time to confirm them is from mid-June to July, making these species appropriate for this issue of Wrenderings.
Because of their terrestrial and low-flying habits, both the Swainson's and Hermit Thrushes run a high risk of predation by cats and collision with windows. Consequently, both species are brought frequently to the Museum, and we have a large collection of each. I preserve specimens of both species, especially the Hermit, as often as possible because of the complex variation in each. These specimens also help document the birds' migration schedules, arrival and departure dates, when sight records of these similar species are not always reliable.
Farther west the Hermit Thrush's distribution, on the scale of the atlas grid, is probably continuous, except possibly for some squares in the treeless grassland east of Lake Henshaw (F18 and G18). Its abundance, however, varies noticeably from year to year and from place to place. Like many fruit-eating winter visitors, the Hermit Thrush moves opportunistically to take advantage of ephemeral food supplies and to avoid harsh weather. It was much more numerous in San Diego County's mountains in the winter of 1998-1999, with little snow, than in the winter of 1997-1998, with far more.
The Museum's collection documents the Hermit Thrush's occurrence in San Diego County from 30 September to 3 May, the species' primary season. A few late stragglers are seen later in May, even up to 4 June in 1998, when Pete Ginsburg had one on Point Loma (S7), and 6 June in 1984, when Richard Webster had one at the same place. The late migrants, however, are not likely to be birds that had spent the winter locally. The Hermit Thrush features interesting and complex geographical variation in plumage, that is, multiple subspecies. No fewer than five of these occur in San Diego County. Only three are regular winter visitors. Most common is subspecies guttatus, which breeds in southwestern Alaska. Next most numerous is nanus, which breeds in southeastern Alaska and differs only in the more rufous, less gray, shade of brown on the upperparts. Least common is vaccinius, which breeds in western British Columbia and winters mainly to the north of us. Besides being a darker, more chocolate brown above than guttatus, it has heavier, darker spots on the breast, more buff tinge on the breast, and dark gray flanks.
The Hermit Thrushes breeding in western Washington and Oregon, and down the California coast in the redwoods, suddenly reverse the trend. They are pale grayish brown above, have small, sparse spots on an almost white breast, and only slight gray on the flanks. Also, they are only about three quarters the size of the others. This is subspecies slevini, which passes through San Diego County in migration but doesn't spend the winter. Its winter habitat is not chaparral and temperate woodland like the last three but the tropical deciduous forest and thorn scrub of southern Baja California and western mainland Mexico. Nicole Perretta's sketches show both vaccinius and slevini, the extremes of the Hermit Thrush as seen in San Diego County.
In addition to wintering in and migrating through San Diego County, the Hermit Thrush occurs as a rare summer visitor in the shady undergrowth of coniferous forest on north-facing slopes of our highest mountains. Our atlas results so far show it only at two sites. On Palomar Mountain, along Chimney Creek, which empties into Doane Pond (E14), Karen Messer and Bob Turner followed up a report to find a single individual singing on 27 June 1997. Then Dan Cooper found three singing there on 19 July 1998, the largest number of summering Hermit Thrushes ever reported in San Diego County. Geoff Rogers had a single bird, not singing, at Deer Spring on Cuyamaca Peak (M20), on 23 May 1998, possibly still a migrant on this date but in possible breeding habitat. There are very few other records--mine from Hot Springs Mountain (E21) in 1980, Richard Webster's from the Cuyamaca Mountains (M20) in 1991, and Robert Patton's from Volcan Mt. (I20) in 1993. These summering Hermit Thrushes are presumably the subspecies sequoiensis, close to slevini but slightly larger, which breeds in the Sierra Nevada and San Bernardino Mountains and winters in central Mexico.
The Hermit Thrush has yet to be confirmed breeding in San Diego County--will we achieve this through our atlas effort? Its nest is a bulky cup usually 3 to 5 feet off the ground in a sapling or shrub. A study in Arizona found nests placed overwhelmingly in small White Fir trees. The juveniles are distinctive, with pale buff spots on the head, back, and median wing coverts. The spots on the underparts are shaped differently too, more like bars.
There is profound geographic variation in the Swainson's Thrush as well, but it doesn't concern us directly in San Diego. The subspecies in the Rocky Mountains and eastward, swainsoni and almae, have the upperparts olive brown (so are called Olive-backed Thrushes) but are not known to reach southern California. The subspecies we see here, ustulatus and oedicus, have the upperparts russet brown, so are called Russet-backed Thrushes. The Swainson's Thrushes that we see commonly in migration are the most intensely russet, ustulatus on its way between its breeding range in the Pacific Northwest and its winter range in southern Mexico and Central America. In fall this subspecies occurs in San Diego County from 26 August to 1 December, with one injured bird lingering in Coronado as late as 16 December in 1979. It is unknown in California later in the winter.
In spring, Swainson's Thrush has been reported as early as 1 April but doesn't become common until the last week of April. This year they have, like many migrants, been especially abundant and conspicuous. They are frequent through the end of May and recorded as late as 9 June, so the best opportunity for determining whether the species is breeding locally is from mid-June through July. The subspecies oedicus, slightly paler and duller than ustulatus, breeds in California south to San Diego County--we are at the southern tip of Swainson's Thrush's breeding range.
Here the species nests only in the most lush riparian woodland. Past breeding or summer records are scattered as far south as the Sweetwater River, but our summer records during the atlas period so far are from far northern San Diego County only. Lisa Ellis has found up to four singing birds along San Mateo Creek (C1), Robbie Fischer a pair along the Santa Margarita River in Ysidora Basin (F5), and Pete Ginsburg a territorial bird near O'Neill Lake (E6). Ken Weaver has found likely breeding birds along De Luz Creek (C6), the Santa Margarita River north of Fallbrook (C8), and, most interestingly, far to the east along Temecula Creek near Oak Grove (C16). The southern end of the species' breeding range may fluctuate naturally or loss and degradation of riparian woodland (invasion of Arundo donax) may be eroding it.
We have no confirmations yet of Swainson's Thrush breeding in San Diego County during the atlas period. Its nests are likely to be concealed in dense thickets of willows within a few feet of the ground. The juveniles are similar to those of the Hermit Thrush, with similar tawny spotting on the upperparts and wing coverts and the same tendency for the spots on the underparts to become bars.
Shy birds of dense undergrowth are more readily detected by their sounds than by sight. The Hermit Thrush frequently calls both "chup" and "zhreet." Its haunting, ethereal song, of varied phrases separated by long pauses, is uttered only rarely in winter. The bird can vary the song's volume at will, singing sotto voce to give the illusion it is still far away as it approaches to investigate a tape recording played to attract it. The call of Swainson's Thrush, a questioning "what?" or "whoit," resembles the call of a Phainopepla more than that of a Hermit. Swainson's song has the same resonant quality of the Hermit's but a fixed and characteristic pattern, spiraling upward in pitch. It sings often in spring migration, so don't be tricked into thinking you have a territorial Swainson's Thrush if you aren't in the species' preferred breeding habitat. But following the enchanting songs of the Hermit and Swainson's Thrushes to find a territorial male, then a pair, nest, or young of two of our rarest breeding birds would be one of the most thrilling experiences a San Diego birder could have.
Philip Unitt (with sketches by Nicole Perretta), from the summer 1999 issue of WRENDERINGS