Hi all: I've been having a discussion with Steve Mlodinow about Hermit Thrushes in the state, and I thought that some on this venue would be interested in a recap of that discussion. I also have a request to Colorado's birders for more information about Hermit Thrushes, particularly those found late in the year (Nov-Dec) or early in the year (Jan-Mar).
Bailey & Niedrach (Birds of Colorado; 1965) notes five subspecies of Hermit Thrush (account begins on pg. 617) as occurring in Colorado: auduboni -- This is the form that breeds in the southern Rockies and is, far and away, the most common migrant through low elevations of the state, particularly in the east. It is large, pale, with any rufous bits not strongly rufous, such that it can readily be mistaken for Gray-cheeked Thrush (as I noted in one of the first In The Scope columns in Colorado Birds); the flanks are "medium-pale grayish brown." Sibley depicts this as one of the 'Interior West' forms, and Pyle (1997) notes it among the group of subspecies that he calls "Western Mountain," a treatment followed by eBird. oromelus -- B&N called this "probably a regular spring and fall migrant over the eastern prairies," noting 13 specimen records for the state, concentrated in the east, but with records from Grand and Montezuma counties. Given the much lower level of effort that early collectors and recent birders have spent on the West Slope, I'd guess that the state of this form there would be, if anything, more common. That is because this form is one of the forms listed by Pyle as Western Lowland (which treatment was followed by eBird). This is a medium-small form, browner above than auduboni, but much less bright than eastern forms (more on that, below); the flanks are "pale grayish." This form breeds in the interior from the northern Rockies south to the northern Sierras and winters south and west of there. Recorded occurrence in CO (13 specimens): 22 Apr-16 May (8 specimens) and 26 Sep-29 Oct (5 specimens). guttatus -- B&N list the occurrence of this form in the state as "regular migrant in small numbers, probably on both eastern and western slopes." This form is quite similar to oromelus, but is warmer-colored and with flanks "brownish gray." This form breeds along the Pac NW coast in southern Alaska and in British Columbia; it winters in the SW. Recorded occurrence in CO (11 specimens): 26 Apr-16 May (5 specimens) and 28 Sep-13 Oct (6 specimens). euborius -- B&N list five specimens for Colorado, all from the Denver Metro area (not surprising, since the major CO bird collection is housed in Denver). This is one of the Taiga/Eastern forms of Sibley and Northern forms of Pyle (followed by eBird, as such being medium-sized, warm brown above, very orange-rufous tail, and with flanks "grayish brown, sometimes tinged tawny." This form breeds in interior central Alaska and northern British Columbia, wintering from Oklahoma and Texas east to the East Coast. Recorded occurrence in CO 5 (specimens): 22 Apr-20 May (3 specimens) and 26 Sep-5 Oct (2 specimens). faxoni -- B&N note a single record (as crymophila, which is considered a synonym of faxoni by Pyle), in Denver 8 Feb 1939. This form (Taiga/Eastern, Northern) is medium-sized and the brightest, with a strong rufous aspect to the upperparts and with flanks "tawny-brown." The form breeds across a huge part of North America, from Alberta to Newfoundland and south in the eastern US to Maryland, and winters in the SE. Note that the single specimen is from winter! Now, on to my request. Like most southern Rockies-breeding forms, our local Hermit Thrushes bail out of the mountains and the state fairly early in fall, as do Plumbeous Vireos, Slate-colored Fox Sparrows, and Mountain White-crowned Sparrows. For whatever reason, these southern Rockies forms seem to be able to avoid vagrancy more so than other migrant forms and they seem to be able to completely depart the state for the winter, with few confirmed winter records for any of them. As good records of auduboni in Colorado outside the typical migration bounds (mid-April into mid-late October) are few, the ones found outside that time period deserve extra attention. With the recent advent of wintering by the species in the state, we have more chances to study and photograph such out-of-season birds in hopes of determining to what subspecies or, at least, what subspecies group individual birds may be referred. As with juncos, determining the subspecies enables us to more completely understand the importance of Colorado as a wintering area, because it allows us to determine the catchment area or source area of these birds. With juncos, because we can determine subspecies (or subspecies group), we know that Colorado receives wintering individuals from a wide swath of western North America. Though there are many fewer wintering Hermit Thrushes, determining that catchment area is no less important. Above, I have highlighted flank color of the various forms, as it is a good first indicator of source for the species: Northern birds tend toward warm-colored flanks, with the more eastern ones having fairly rufescent flanks, while gray is more dominant in flanks on western birds (both Western Mountain and Western Lowland). So, I would request that any notes to Cobirds or entries in eBird make some mention of suspected subspecies group, so that we can continue down the road of determining what forms are traveling through or wintering in Colorado. We can certainly do that for birds during the primary migratory seasons, too, but I understand that one can be a bit swamped by things to ogle at those times! Enjoy, Tony Leukering Villas, NJ -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. 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