Two of the most challenging species to separate from each other during fall migration are the Blackpoll and Bay-breasted Warblers – so much so that observed individuals are often referred to by birders as “Baypolls”. In this article, Wisconsin warbler guru Tom Schultz (one of two illustrators of the Peterson Field Guide to Warblers of North America) breaks down key identification features for these confusing fall warblers.
The plumages of Blackpoll and Bay-breasted Warblers are indeed very similar, with both species looking substantially different than they did during the month of May. Complicating matters is the fact that the two species are also structurally very similar – with both having long primary projections, short tails, and long undertail coverts. Both species have bold whitish wingbars, white tipping on the primary flight feathers, and prominent white tail spots.
Fortunately, many individuals have markings that are quite distinctive to species – for example, once narrowed down to being a “baypoll” warbler, if a bird is observed to have any strong blush of “bay” color on the flanks, it can be safely identified as a Bay-breasted. Alternatively, if the “baypoll” can be seen to have obvious blurry dusky streaking on the upper breast and flanks, it can be safely named a Blackpoll. Also, if one can see any yellow at all on the legs, feet, or toe pads, the bird can be identified as a Blackpoll. A Bay-breasted at any age will typically have all-blackish or grayish legs and feet – with no hint of yellow (although some may show dull pinkish on the toe pads).
The challenge, however, is with intermediate birds that may not show the distinctive markings mentioned above. In these cases, one must look carefully at a combination of several different characteristics – and hopefully a consensus can be attained that will point to one or the other species.
Here are some of the secondary characteristics that will help you to separate these two similar species during fall migration. The head and upperparts of both birds are greenish, but on a Blackpoll the green is more olive in tone, compared to the brighter, more yellowish-toned green on a Bay-breasted. On average, the Blackpoll tends to have a stronger dark eye-stripe than does a Bay-breasted, but this character is somewhat variable.
On the underparts, a Blackpoll typically has a more lemony-yellow tint to the whitish color of the breast, while a Bay-breasted will show a more buffy-yellow tint. As mentioned above, a Blackpoll will usually show some blurry dull-greenish streaking on the breast, while a Bay-breasted will usually lack any hint of streaking. If you can see the long undertail coverts, a Blackpoll will typically show white feathers here, while those of a Bay-breasted are usually tinted with buff.
Mentioned above are the distinctive differences in the color of the feet, but there is also a strong tendency for a difference in bill color. The upper mandible of both species is mostly dark, but the base of the lower mandible is pale. If the pale area appears to have a hint of yellowish, the bird is likely a Blackpoll, but if the pale bill base is pinkish the bird is a Bay-breasted.
Of course, the field marks described above will only be diagnostic if you have eliminated other similar warbler species – so that would be the first step!
INTERESTING NOTE ON DISTRIBUTION
The Wisconsin eBird team, with the help of an astute Chippewa Falls birder, recently discovered an interesting trend in relative abundance of these two species across the state during fall migration. In most parts of the state, but particularly the north and east, Blackpolls are reported more often and in greater numbers and sometimes significantly so. However, in the west, the balance shifts as Bay-breasteds are slightly more common. So your perception of which species is more likely to be seen during autumn is probably heavily swayed by where you live!
The explanation? Both species breed north of Wisconsin across a broad swath of Canadian forest but they appear to take distinctly different paths in reaching their wintering grounds. Blackpolls move easterly toward the Atlantic Coast before flying over the ocean en route to South America, while Bay-breasteds more often take a more direct route via the Mississippi Flyway. The Blackpoll’s west to east push across southern Canada clips northern and eastern Wisconsin and their numbers are likely increased there by being concentrated along the Great Lakes shorelines.
WANT MORE CONFUSING FALL WARBLER PHOTOS?
Try this excellent website: http://www.giffbeaton.com/warblers.htm
Text by Tom Schultz, with contributions from Ryan Brady
Photos by Nick Anich, Ryan Brady, Brian Hansen (pbase.com/bhansen), and Bruce Steger