From: James V. Remsen
LABIRD: as a follow-up to my previous post on this, I photographed a series of fall specimens of Yellow-bellied Flycatcher taken in Louisiana so that you can see what they look like. Beware that extrapolations from specimens to the way a bird looks in the field has limitations (and eyerings very difficult to assess), but with that caveat …
1. Immatures (by skull ossification), ventral view, late September and October: https://www.flickr.com/photos/126193733@N06/29588678090/in/datetaken-public/
You can see that there is a fair amount of individual variation, but that these birds share varying degrees of yellowish in throat area and in belly, separated by a diffuse breast band, typically with vague paler streaking (a characteristic flycatcher plumage theme for those of you familiar with tropical species). The individual variation itself is important to keep in mind,
2. The same set of immatures, lateral view: https://www.flickr.com/photos/126193733@N06/29847285536/in/datetaken-public/
Here you can see how greenish overall they look, with fairly narrow yellowish wing bars sometimes faintly tinged buff. Most importantly, you can see that the sides of the throat below the eye tends to be greenish olive and darker than the rest of the throat, thus constricting slightly the yellowish to the center of the throat (vs. broad pale throats in other eastern empids, e.g. Acadian.
3. Adults (by skull ossification) taken here in August and early Sept., ventral view (Steve Cardiff pointed oout to me that we don’t have any certain adults past mid Sept): https://www.flickr.com/photos/126193733@N06/29799250301/in/datetaken-public/
Here you can see the same features as in the immatures from same angle.
4. Same set of adults, lateral view: https://www.flickr.com/photos/126193733@N06/29768674782/in/datetaken-public/
Same as in immature except that the wingbars are whitish.
What you can’t see without including other species is that Yellow-bellied Flycatcher is a small empid, notably smaller than Acadian (the species most frequently mistaken for YBFL), with a smaller, shorter bill than Acadian, and that it is slighty darker-winged than most empids, increasing the contrast between wingbars and wings. These photos are in my Flickr account, so I think you can download them and magnify them if so inclined. For those who missed my previous post on this, the reason for all this is that I am convinced that Yellow-bellied Flycatcher is the most over-reported species in Louisiana relative to its true abundance (and have zeroed the eBird filters to force people to provide better evidence).
Dr. J. V. Remsen