During the first few weeks of October, here in Pennsylvania, the diversity of warbler species is declining rapidly. By the third and fourth week of this month pretty much only Yellow-rumped Warblers and Palm Warblers will be around in decent numbers, while most other species have made their way to the southern states, and some already are down to Central and South America. However, this time of the year there are still a few Tennessee Warblers around, as they make their long trek south from Canada’s boreal forests. In addition, there is also the chance to see the Orange-crowned Warbler, a species that is much harder to come by in Pennsylvania. When Orange-crowns are seen here, it is usually later in the season, since this species is a late migrant and also somewhat tolerant of colder conditions. Both of these species are typically seen at or below eye-level compared to the other warblers. They both like to climb around in shrubs, searching and probing leaves for insects. Furthermore, both species are a dull yellowish-green color during fall migration. The fact that these two species are very closely related and are similar in appearance and habits, can lead to quite a bit of confusion in the field.
This collection of photos show just how similar the birds can look and act. All of the photos below show the same individual Tennessee Warbler (taken in Pennsylvania) and the same individual Orange-crowned Warbler (taken in Maryland), in various postures and angles. Both individuals are first fall birds. The birds are climbing around in brush and appear more chunky and heavy-set than other warblers. Both birds have sharp and straight gray bills and plain wings, lacking any wing bars or other obvious features. This overall drab appearance is what can make them look especially similar, particularly at a quick glance. Typically, the Tennessee Warbler is whitish throughout its underside, however sometimes their undersides can be washed green and their undertail coverts can occasionally be yellowish. Orange-crowned Warblers always show a green wash to their undersides and always show yellowish-green undertail coverts that are either as bright or brighter than the rest of the body.
Probably the single best field mark to go off of, when trying to separate these two species is by looking at the markings on the face. The Tennessee Warbler always shows a whitish supercilium that extends from the lores, straight back to the end of its facial disk. The coloration on the head of the Tennessee Warbler does not contrast with the coloration on the bird’s back. On the other hand, the Orange-crowned Warbler lacks the pale supercilium, and instead shows very obvious white eye-arcs above and below its eye (broken eye-ring), as well as a grayish head coloration that contrasts with its greenish body. Hopefully this collection of photos will help you to pick out the two species more accurately and with more confidence in the field!
Broad-side, Undertail View:
Note: The information in this post concerning Orange-crowned Warblers only applies to the celata subspecies, which is the only subspecies occurring in eastern North America. The various subspecies found in western North America are typically quite different in coloration.