I have had two inquiries about Tropical Parula in the last 24 hours, prompted by a bird currently in Austin, Texas. You can check out that bird in photos by Andy Balinsky and photos and some background by Martin Reid. It has the throat pattern and extensively yellow underparts of Tropical Parula, but the pale eye-crescents and the extensively green crown of Northern.
In The Sibley Guide to Birds I illustrated a drab Tropical Parula with faint pale eye crescents, which is now the focus of questions. That illustration is based entirely on one specimen which was collected in southern Texas (and is currently at the Academy of Natural Sciences in Philadelphia). I strongly suspected that it was a hybrid, and discussed it with Jon Dunn, who also looked at the specimen and included a special mention of it in his and Kimball Garrett's Peterson Guide to Warblers. We both concluded that it could be a hybrid but could also be just a pale and drab variant of Tropical, since the only thing that looked odd for Tropical was the drabness and the eye crescents.
So when it came time to paint those illustrations and reduce the text to fit, I simply called it a variant of Tropical, didn't mention the hybrid possibility, and left out all the background info.
Since 2000 the evidence has piled up suggesting that Northern and Tropical Parulas do hybridize (see especially Tony Gallucci's blog post from 2005). Martin Reid also has photos of a male Tropical-like Parula with faint white eye crescents from southern Texas in 2005. I have not seen any similar specimens or photos from outside of the geographically limited area in southern Texas where the two species meet, but Tony Gallucci's map suggests tantalizingly that he has found intergrades far to the north and east of Texas. I hope we will learn more details of that soon.
From Louisiana this week, Erik Johnson has posted photos of a pure-looking Tropical Parula, but he includes the comment "many, if not most, Tropical Parula sightings outside the normal range (i.e. in Louisiana) have some evidence of Northern Parula genes or characteristics."
So I would now say that the bird I illustrated in the Guide is a hybrid, and that the Austin bird (which matches my illustration closely) is also most likely a hybrid.
And of course this just brings up more questions:
How widespread is the introgression? or How many US Tropical Parulas are pure?
Is the northeast Mexican population of Tropical Parula just part of a cline between Northerns and true Tropicals to the south? (Tony Gallucci has some interesting comments on this)
Should they be considered all one species?