This web article covers some of the less-discussed and generally more subtle field marks to keep an eye out for when you encounter a Bay-breasted/Blackpoll in the field this fall.
HEAD: Bay-breasted average plainer overall with a less conspicuous eyeline as is seen with Blackpoll. With Blackpoll, the dusky eyeline sets off a light supercillium (eyebrow). The throat on first fall Bay-breasted can be off-white and slightly buffy; with Blackpoll, the throat is usually yellowish. The bill on Bay-breasted averages slighter more heavier-set than Blackpoll. From its base to half way along the lower mandible shines a beautiful bluish-gray in good light. With Blackpoll, the bill is slightly thinner, with the base of the lower mandible averaging slightly more dusky-yellow, fading to dark by the tip. In-the-field GISS note: the forehead on Bay-breasted is slightly steeper than on Blackpoll, and the neck, slightly huskier, imparting a bull-headed look and light “peak” above the eye in some postures. Kenn Kaufman (1990) illustrates this subtlety very nicely in his Advanced Birding guide.
UPPERPARTS: Both Bay-breasted and Blackpoll showcase greenish upperparts overlain by dark streaks. The upperparts are usually deep olive green in Blackpoll, and usually lime-green with Bay-breasted; regarding the upperparts to a Bay-breasted, these tones are especially bright in the nuchal region (upper nape). The intensity of streaking is age-related, with first fall females showcasing the lightest streaking, and adult males showcasing the densest streaking (this is most true re: fall Blackpolls). On some Bay-breasted in particular, note the bluish-gray colouration to the lesser coverts (i.e. the shoulder) and also to the lower rump, and basal portion of the tail feathers. This colouration tends to bleed into the upperside of the tail feathers, especially the outer ones. The aforementioned traits show well on a foraging bird with its tail cocked upward. The wing bars on Bay-breasted average broader and slightly bolder in contrast than on Blackpoll. Both species showcase conspicuous white edging to the tertials but average subtle differences to the edging to their primaries and secondaries. Look for a slightly greenish-blue cast in Bay-breasted, and a slightly greenish-yellow cast in Blackpoll.
UNDERPARTS: Bay-breasted are generally unstreaked below, though some specimens show faint, indistinct olive streaks to the sides of the breast. The underside is buff-white and the sides usually show faint “bay” tones. First fall females can show virtually no “bay” tones to the sides. The buff tones extend into the undertail region on all examples, limiting the contrast between buff underside/white undertail. With Blackpoll, the underside streaking is quite variable and, to a large degree, age-related. First fall females are the most lightly streaked, adult males the heaviest. First fall birds showcase olive streaking to the sides of the breast down to the mid-flank; adult fall birds showcase black streaks concentrated on the sides of the breast, with some examples showing a streaked throat. The bright pale-yellow tones throughout the underside, crisp streaks to the sides of the breast and flank area, and delineated white undertail is look unique to fall Blackpoll Warblers.
LEGS: Bay-breasted of any age class typically show grayish legs and feet, often with a bluish cast throughout. With Blackpoll, first winter birds typically show dark-gray or blackish legs, and adults typically show straw-coloured legs. Blackpolls of all age classes typically showcase yellowish soles to their feet.
Figure 1: Fall Blackpoll (left) and Bay-breasted (right), a comparative study photo to use to follow along with the above analysis. Many thanks to Violet Verhoog, Bill McDonald, and Ronnie Dentremont for allowing me to showcase their photos.
Figure 2: A fall Bay-breasted, likely an adult female, showcasing greatly reduced back streaks and buff tones to the sides. September 2015 – Nova Scotia.
Figure 3: A first fall Blackpoll Warbler showcasing very limited flank streaking, dark legs, and dusky-yellow feet. September 2015 – Newfoundland.
Figure 4: First fall Bay-breasted (left) and Blackpoll (right); the drabbest of the drab re: “Baypolls. Note the very limited “bay” tone to the flanks in this particular Bay-breasted; what is seen is a light dusky-buff blush. With the Blackpoll, note the diffuse, olive streaks to the chest and sides. On this particular Blackpoll, the legs are black and only the soles of the feet are yellow, as is typical of first fall birds. Regarding underpart colouration, a light, buffy base colour to the underparts is seen with this Bay-breasted. On the Blackpoll, a yellowish colour to the throat and chest, followed by a light, dusky-yellow colouration throughout the underside, is seen.
Many thanks to Violet Verhoog, Bill McDonald, Ronnie d’Entremont, Randy Wheeler, and Christine Baird for use of their superb photos. Their photos greatly enhanced the educational value of this web article.
Dunn, J. and K. Garrett. 1997. A Field Guide to the Warblers of North America. Houghton Mifflin Co., Boston, MA.
Hough, J. 1996. Pine and “baypoll” warblers: plumage variation and identification problems. Birding 284-291.
Kaufman, K. 1990. Advanced Birding. Houghton Mifflin Co., Boston, MA.
Whitney, B. 1983. Bay-breasted, Blackpoll, and Pine Warblers in fall plumage. Birding 15: 219-222.