I think the top pic here shows most clearly the very distinct contrast between the very white neck and the dark bluish gray head on this bird, which I have read is a very important field mark in differentiating Blue-headed from the less contrasted Cassin's Vireo.
Birds of North America online also notes that the crown on Cassin's Vireo is "brownish gray, with little contrast to brownish olive gray back;" This vireo does not show any brown in it's plumage and the bottom pic shows what I think is a clear contrast between the dark bluish gray crown and the greenish back on this bird. SeEtta
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