Umm... at the risk of sounding all snipish, my 1st edition
(14th printing) copy of Peterson's Western Guide (1941, check
out the illustration for Western Grebe) says:

"SOLITARY VIREO. Vireo solilarius. Subsp. (Illus. p. 190.)
Descr. 5-6. A Vireo possessing both conspicuous white
wingbars and conspicuous white 'spectacles.' The Bell's,
or Least, Vireo has these marks indistinct; in the Warbling
Vireo they are wanting. The Gray Vireo has only a
conspicuous eye-ring. The present species can be told from
the Hutton's Vireo by the complete eye-ring and snowy-white
throat.
Voice:?A series of short whistled phrases, with a rising or
falling inflection, rendered with a short wait between phrases.
Subsp. PLUMBEOUS VIREO. V. S. plumbeus.
Breeds in Rocky Mt. region from n. Nev., n. Ut., s. Mont.,
and se. Wyo. s. into Mex. Lacks the contrast of gray head
and olive back of ncxt race.
CASSIN'S VIREO. V. S. cassini.
Breeds in Pacific States. When at close range, in good
light, easily distinguished from the Plumbeous or any
other Vireo possessing wing-bars by the gray head
contrasting with an olive back, almost identically like
the Blue-headed Vireo of the East, which is also a race
of this species. The Plumbeous race is uniformly gray
above."

The revised 2nd edition Peterson's (1961) given to me for Christmas
in 1971, back when Marsh Hawks were still Marsh Hawks, is not as
celebratory in its presentation, but still specifically mentions
all three forms.

--
Mike Patterson
Astoria, OR
celata at pacifier.com

Half-a-bee, philosophically must ipso-facto half not-be.
But half the bee, has got to bee Vis-a-vis its entity...
d'you see?
But can a bee be said to be or not to be an entire bee
When half the bee is not a bee due to some ancient injury?
-Monty Python

http://www.pacifier.com/~mpatters/bird/bird.html