Here are his comments from his email (I cut out non-relevant portions).......
"Not all Acadians are yellow to the vent in the fall, but many are, with a greater tendency in immatures than adults.
Yellow-bellied tend to have yellower throats, especially in the center, than do Acadians, which are usually whitish (but see the photos above for some faintly yellow throats). Add to this that the reflections from nearby foliage always seem to me to make empids look yellower than they really are when in the shade, and you can see the problem.
Acadians are COMMON migrants in our woods (in Louisiana) in fall. They are often noisy -long series of "wheep" calls are frequent and differ from the "bu-zweeet?" song (?) that we hear in summer. In contrast, YBFL is rare to uncommon in fall, especially away from Cameron, and I have yet to hear one make a sound here (although others have).
Part of the problem is NOT your fault. The very popular Sibley guide illustrates a "Juvenile/1st winter" bird that doesn't match any of our specimens in being whitish bellied and not nearly green enough; an individual that looked like this illustration cannot be a fall YBFL. This might be the most inaccurate illustration in the book. Likewise, Sibley's "1st winter (Jun-Feb)" illustration of Acadian is whiter below than almost any specimen in our collection and is thus highly misleading (almost as bad as the YBFL). Check the real deal specimens and judge for yourselves."