Sometimes in the fall we encounter a bird that seems to lack many useful marks. Fortunately, that fact can be a mark in itself! A few of these birds are drab Pine Warbler, drab Yellow-rumped, Orange-crowned and Tennessee, female Black-throated Blue, drab Cape May, and Blackpoll or Bay-breasted. We can see these “markless” birds if we just do a scan of the finders.
So now let’s look a little closer, and I think we’ll find that there are actually some very useful characteristics that we can find that help us separate these similar-looking species. Our bird seems to have a thin, long supercilium...that’s a good start. Below the eye there’s a white undereye arc...that’s going to be helpful, too. There’s also a yellowish wash in the underparts, contrasting with the darker, drab blue-green back. And note that white mark on the wing at the base of the primaries...that could be good.
Scanning these drab birds again, I see something that stands out...only one bird has a white mark on the wing like our bird...let’s take a closer look at Black-throated Blue female.
Sure enough, I can see that the white “handkerchief” on the wing is in fact diagnostic! That makes it easy, but what if there were no mark on the wing, as with some first-year females of this species. We still have a couple of good things to work with...for one, the plain darker blue green back and the unstreaked buffy underside is a good and unique pattern for Black-throated Blue. Coupled with that thin eyeline/undereye arc you have a pretty solid ID.
That makes it easy, but what if there were no mark on the wing, as with some first-year females of this species? We still have a couple of good features to work with...for one, the plain darker blue-green back and the unstreaked buffy underside are patterns unique to Black-throated Blue. Coupling those points with that thin eyeline/undereye arc and you have a solid ID: This bird is a female Black-throated Blue Warbler.