Have you made the trek to a Vermont mountaintop to hear and see our signature montane bird, the Bicknell’s Thrush? Many birders have, and upon hearing it’s distinctive song, submit their record to Vermont eBird. Seen a Gray-cheeked Thrush in Vermont? Likely not, as it does not breed here and it is such a rare, passing-through migrant that a Rare Species Documentation form (RSD) needs to be submitted to the Vermont Birds Records Committee if suspected.
Chances are that most of us birders are little prepared to see and document a bird from this complex outside of the breeding season and montane habitat, much less to distinguish between the two. Long-time researchers of the Bicknell’s Thrush at VCE had this to say about documenting them during migration:
Having studied Bicknell’s Thrush for over 20 years and handled many hundreds, as well as Gray-cheeked Thrushes on migration, it is not possible to distinguish between the two without some VERY good documentation, such as: (a) songs or call field descriptions, if you can tell them apart, or recordings; (c) excellent photos with back, side and head, they are color morphs that are separable; (d) banding with detailed measurements if they are outside of the overlap; and (e) any other features that possibly can help segregate the two.
We get photos all the time from banders and sometimes they are good enough combined with measurements to call the ID. But, most of the time, we struggle with those. There is overlap (a) in sizes, (b) in color morphs,(c) in similar songs and calls, and (d) even migration timing somewhat overlaps — this is a very tough group to sort out to species, not to mention hybrids. Yes, hybrids. I have one well documented with DNA to be a Veery-Bicknell’s cross. It used the song and call of both species!
So, during migration if there is not very good documentation for a bird, it should be listed as Gray-cheeked/Bicknell’s Thursh. On the Bicknell’s montane breeding grounds, identification of the Bicknell’s should be okay, as the breeding habitat is now quite well documented. But, even here I have a bit of caution. One time we captured a late migrant Gray-cheeked in early June in the middle of a Bicknell’s study plot high in the mountains! This is of course extremely rare, but something to be aware of.
So, if you are in the montane fir forests, hear the distinctive Bicknell’s song or call or get a good look at a Bicknell’s Thrush, submit your observation to eBird as a Bicknell’s Thrush. However, during migration (April-May; September-October) outside of known Bicknell’s thrush habitat, unless you are able to obtain the extraordinary details listed above, either submit your observation as Gray-cheeked/Bicknell’s Thrush [use Add Species on the submission page], or prepare the detailed Rare Species Documentation Form and submit it for possible approval by the Vermont Bird Records Committee. Together, we’ll be able to better document the migration of these amazing songsters in the Green Mountain State and beyond.