Sunday, August 19, 2007

The Brown-eyed Vireo Problem

I was just treated to the best impression of a Bell's Vireo that I have ever seen a White-eyed Vireo pull off. Not only did this bird (presumably a juvenile male) have nice dark brown eyes and faintish whitish spectacles with no trace of yellow, but he was singing a song that, with only a little effort and selective listening, was easily interpreted as a mumbly version of the classic Bell's "wheedlewheeedlewhee wheedlewheedlewoo." So why wasn't this a Bell's Vireo, you might ponder? Well, because other than looking and sounding like a Bell's, it didn't look and sound like a Bell's. By this I mean its size, proportions, actions, "flickiness" or lack thereof, were all typical White-eyed Vireo. Its wingbars were a bit too crisp, its head, bill, and tail shape didn't look un-WEVIish in any way. Plus, if you listened more objectively, you had to admit that his mumbly song had a few chips and some "chickaparea" like phrases thrown in, and honestly, well, it was just the juvenile babblings of a baby boy who hadn't learned to form proper words yet.

I see these Brown-eyed Vireos every year in late summer and early fall, of course; anyone who lives amidst an abundance of nesting White-eyed Vireos does. Every year there are one or two like this bird that really tug on the temptations to make it into a BEVI not a WEVI. But I considered this bird blog-worthy for one particular reason: I decided out of curiosity to see what the major North American field guides had to say about this particular ID problem. I was astounded and profoundly disapointed to discover that they do not address it at all! I grabbed the four guides I had handy, and here's a case-by-case breakdown:

Eastern Petersons: Illustrates the head of a young WEVI with a bold glowingly yellow set of spectacles. No mention of jizz or behavioral differences. Dark eye and white eye-ring listed as marks for BEVI.

Robbins: Illustrates young WEVI looking exactly like adult except for dark eyes. Mentions that yellow spectacles will distinguish it from Bell's. No mention of jizz or behavior; indeed it references the two species as "counterparts" which could be taken to suggest there's little difference.

NGS (an older copy): Juvenile WEVI not illustrated. Text says only that is is duller with "gray or brown" eyes. BEVI not even compared with WEVI in text; but (finally!) the distinctive shape, jizz, and behavior of the BEVI are mentioned.

And finally, the current state of the art, Eastern Sibley: "First winter" WEVI is illustrated with dark eye but flaming yellow spectacles and flanks. "Bright yelow spectacles" are listed as a distinguishing mark "for all" WEVIs. No mention is even suggested of the white-spectacled juvies of late summer and fall. Entry for BEVI is extensive including behavior, jizz, shape, and several distinguishing marks that might eventually let a novice or overeager intermediate birder realize that their dark-eyed, white-spectacled WEVI is NOT actually a BEVI, but not very quickly.

So the gist is this: The popular American field guides would leave most any birder who relied on them thinking that any dark-eyed, whitish-spectacled, wingbarred, smallish vireo is a Bell's Vireo, unless they are very dilligent in investigating the subtler aspects of bird ID. Why no guide even bothers to mention that some Juvenile WEVIs can be very drab and appear to have whitish spectacles, hence other field marks must be examined, is a mystery to me. Just another argument for learning to bird primarly from other birders and then only secondarily from field guides, but not everyone has that luxury.


At 3:24 AM, Blogger cyberthrush said...

Field guides are only volumes of massive generalizations. Generalization works well in the physical sciences (i.e., if a "law" of gravity works in one part of the universe it seems to work well in other parts), but in the biological sciences generalizations can be straightjacketing and misleading as regards individual cases. The variables in biology are so many and complex, and the methods involved so imprecise, most generalizations have exceptions. This is so even in molecular biology. In short, field guides are just "guides," and not gospel.


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