The best trick (but not the only trick) to separating Tennessee (TEWA) from Orange-crowned Warblers (OCWA) is to get a good look at the undertail coverts. This is actually surprisingly easy as you are often staring at the birds from below with binocs. Both species belong to the same genus, Oreothlypis, and thus look structurally similar. However; OCWA have yellow undertail coverts that are the same color or “yellower” than the rest of the underparts. First Fall TEWA also are very yellow below but even the freshest of those have white (or white slightly tinged with faint yellow) undertail coverts. OCWA also have faint streaking on the breast and flanks, an indistinct dark eye stripe, pale supercilium, a longer tail, slimmer shape and a more slender decurved bill and pale (yellow or whitish yellow) eye crescents that are more prominent than in TEWA. Another helpful clue is that Tennessees often show narrow and inconspicuous pale yellowish wingbars, which are formed by the pale tips of the greater coverts, while OCWA usually does not.
These two species also have quite different timing to their annual migrations. TEWA are one of the earliest Fall migrants and sometimes even begin arriving in WI in late July/early August. They are commonly seen throughout the rest of migration into October. OCWA are a much later migrant and typically don’t arrive in WI until mid to late September. After 13 years of intensive passerine banding, Hawk Ridge Bird Observatory (HRBO) has only banded two OCWA in August (Aug. 30-31). The majority of birds don’t arrive in Duluth until mid to late September.
As you are picking through those warblers in the next few weeks, keep the aforementioned tips in mind when searching for that elusive Orange-crowned. If you should see an OCWA before September 10th, make sure to document your sighting in the comments field of your eBird report and feel free to upload a picture to the eBird Flickr gallery!
For more photos and ID tips for fall warblers, also see:
Grosshuesch, D. (2008). [Orange-crowned Warbler data from Hawk Ridge in Duluth, MN]. Unpublished raw data.
Curson, J. et al. 1994. Warblers of the Americas: An Identification Guide.