NGS isn't terribly specific, but roughly speaking:

eastern = blue-headed (contrast between blue-gray head, greenish back)
west coast = Cassin's (smaller, slightly paler overall)
Rockies/Great Basin = plumbeous (almost no yellow)



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From: Lorraine I. Potter[SMTP:105570.1263 at compuserve.com]
Reply To: 105570.1263 at compuserve.com
Sent: Saturday, October 17, 1998 7:18 PM
To: Tweeters-owner
Subject: Question on Solitary Vireo Split

Tweeters:
Would someone out there be kind enough to help me out? The Solitary Vireo
was split into three: Blue-headed, Cassins and Plumbeous. I would like to
know the approximate geographical range of each of these three birds. If
there is a publication that answers my question, I would appreciate knowing
of it. I have checked everything that I have and can find nothing. I
guess my material is just not recent enough. Thanks in advance.
Irene Potter, Tacoma
105570.1263 at Compuserve.com