Lorraine
There was a terrific article by Matt T. Heindel on the Solitary Vireo
complex in Birding, Dec, 1996, pp 458-471.
Basically, Cassin's Vireo breeding range includes BC, WA, the northern tip
of ID, most of Oregon except the SE corner, and most of CA. The breeding
range of the Plumbeous Vireo includes S MT, SE ID, WY, UT, NM, and most of
NV, AZ,CO and just a bit of W. TX. The Blue-headed Vireo's breeding range
includes all the other (eastern) areas that used to apply to the whole
complex.
Hope this helps and I hope I got all the state abbreviations right.
Cheers
Gail Spitler
Johnsons Landing, BC
spitler at wkpowerlink.com