Tuesday, October 14, 2014

Blue-headed vs. Cassin’s Vireos

This October I banded several Blue-headed Vireos. I have previously posted on this species. In 1997, genetic studies split the Solitary Vireo into Blue-headed, Cassin’s and Plumbeous vireos. Looking at this rather dully plumaged female, I wondered if a Cassin’s Vireo would be recognized if it wondered into Minnesota from the Pacific Northwest or from somewhere else in the far West.
Plumbeous Vireos, which nest in the southwestern Rocky Mountains, northeast to the Black Hills, are actually found closer to Minnesota than are Cassin’s. Plumbeous Vireos are much darker birds, with slate-gray sides. But Cassin’s Vireos could stray into Minnesota. I believe the bottom photo, which I took in southern Arizona in 1970, is a Cassin’s Vireo. Key field marks of a Blue-headed Vireo include: 1) the gray of the head sharply contrasts with the white sides of the throat; 2) the back of the is much brighter than most Cassin’s Vireos; and 3) Blue-headed Vireos are bright white below, unlike the dingier Cassin’s Vireo. My October birds, as expected, were all Blue-headed Vireos—but I remain vigilant.

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