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Excerpts from Jim Conrad's
Naturalist Newsletter
from the July 5, 2009 Newsletter, issued from the Siskiyou Mountains west of Grants Pass, Oregon:
WESTERN TIGER SWALLOWTAIL
The Audubon guide says that the Western Tiger Swallowtail, PAPILIO RUTULUS, may be the most conspicuous of all butterflies in the West. You can see one near my place above.
That picture is about the same size and the butterfly is posed the same as in the picture taken this spring in Mississippi of an Eastern Tiger Swallowtail, so it's interesting to compare them. That picture is at http://www.backyardnature.net/n/a/swalltail.htm.
Placing one picture on one side of my computer screen and the other species on the other side, really I can't see any difference between them that I'd trust if trying to separate them in the field.
I know that what we have here is a Western Swallowtail only because that species occupies approximately the western third of the US while the Eastern species occurs in the rest. Some slight hybridizing is reported along the dividing line.