The Western Tiger Swallowtail and the Eastern Tiger Swallowtail are slightly different in the blue area in the tail end.
Eastern: http://en.wikipedia.org/wiki/Eastern_tiger_swallowtail
Western: http://en.wikipedia.org/wiki/Western_Tiger_Swallowtail
The Western has a range much smaller than the Eastern in the US, but the Western goes up into Canada.
Nice Spotting!
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