I photographed this Vireo about 10 years ago during spring migration at my farm Se of Portland OR.
I've always suspected it to be a hybrid Warbling x Red-eyed.  On the other hand, perhaps it's a first
year Red-eye.  In western Oregon, Red-eyed is a rare but regular summer visitant, which sings and
(attempts to) breed.  Seems logical it might settle for a Warbling Vireo partner.

Note that this bird bears some resemblance to the vireo recently discussed in California.
However, it shows a dark line over the white supercilium, and some
contrast between the gray back and head.

Bob O'Brien, Portland OR    green@agora.rdrop.com
Comments appreciated, or reply to the list.

At the time I was interested enough to examine speciments at Oregon State University.
These are larger images you can view if you wish.

Warbling upper view      Warbling side view     Red-eyed