Sorry to be late to the party, but one other thing to consider on the
Mourning/MacGillivray's challenge is call note. It's fairly distinctive
in the two species. Mourning issues an odd, flat, almost scratchy "chit"
or "chet" note; MacGillivray's offers a short, sharp "tik" note. Not as
easy as it may seem in writing, but it's something to consider if the
bird resurfaces. Also note that hatch-year Mourning Warblers and even
some female after-hatch-year birds can show narrow eye-arcs. But this
bird, with its whitish throat and thicker eye arcs, does look pretty
good for MacGillivray's. I don't like the tail proportions at all for
MacGillivray's, but I'm less familiar with that as a field mark.