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My Oh Myiarchus: A post for Jochen

March 6, 2008

Last week, Jochen of Bell Tower Birding asked me to help him out on a little ID conundrum he had. Considering Jochen’s help was essential to a trip I made to Michigan last fall that resulted in my life Bay-breasted Warbler, it was the least I could do to throw him a bone and use my access to the North Carolina Museum of Natural Sciences bird collection to advance the power of good. But there’s a cost to you dear readers, which comes in a postponement of an awesome post on Pomarine Jaegers I had ready to go for today, one that will change the way you look at them, and threatens to cast into doubt the usefulness of the major North American field guides when it comes to their identification. You may even throw out your Sibley, it’s that groundbreaking. Did I oversell it enough? That will come this time next week, so set your clocks. The question now is, what will we learn today (aside from the fact that I can be bribed into writing a bird ID post of your choosing for information on excellent birding locations and the promise of beer).

Today, friends, I offer you flycatchers. And not just any flycatchers, Myiarchus flycatchers, as you no doubt picked up from the ever-so-clever title of the post. In particular, two species within the aforementioned genus that could pose some identification problems, as they have to our friend Jochen.

Those of us in the eastern United States are no doubt familiar with the Great Crested Flycatcher. For most of us it’s the only Myiarchus that we see on a regular basis. But the genus is more widespread in the southwest, and those species have historically been regular vagrants to parts far east. These vagrant birds are mostly Ash-throated Flycatchers, but Dusky-capped and Brown-crested are not unheard of, and certainly, any Myiarchus seen in the eastern US in winter is far more likely to be one of these desert wanderers than our regular Great Crested. I’ll be focusing today on the two largest of the North American Myiarchus, Brown-crested (BC) and Great Crested (GC).

Both BC and GC fit the typical Myiarchus mold, both are medium-sized flycatchers characterized by brown upperparts and vibrant yellow underparts with a large bill and a jaunty crest. While similar lengths, BC averages bulkier than GC, with a bill that is wider at the base and heavier overall. BC, as per its drier habitat preference tends to be much drabber, in fact, looking quite a bit like a washed out GC, hence the identification difficulties. You can see this below. From left, birds 1 and 2 are GC, 3 and 4 are BC.

Jochen wanted, in particular, a discussion of the differences in the tail, particularly the interface between the rufous and the brown. I hadn’t given this particular field mark much thought. In fact, Sibley notes for the two birds, that GC has “extensive rufous”, whereas BC has “fairly extensive rufous”. A fine line to be sure, and in my personal experience with the species in South Texas, BC was differentiated by habitat and voice rather then any plumage characteristic. I do, however, remember a couple birds that I wished this field mark had been more available to me. Oh well, what’s past is past, perhaps what I figured out here will help someone down the line.

Anyway, the question now is what causes this difference in the extent of rufous on a feather by feather basis. Turns out the retrices are rather diagnostic. For example, the spread tail of a GC below. Let’s consider the shaft of the feather to be the axis. The entire medial (inside) side of the feather is rufous, the entire lateral (outside) of the feather is brown. 50% rufous, 50% brown, thus the impression is of “extensive” rufous. Central retrices are largely brown. Apologies for the sight of my hideously maginified thumb.

Moving on to a similar posed BC Flycatcher. You’ll clearly note the difference. The lateral side of the feather is half brown/half rufous. Thus 75% brown, 25% rufous. I guess this is “fairly extensive”, I don’t think Sibley would be any more clear if he called it limited rufous, or restricted rufous. There’s less there in any case than the GC, and most importantly, clear bleeding of brown into the medial side of the retrices.

I didn’t look at the other Myiarchus flycatchers this way, perhaps I will now. Tail pattern appears to be a excellent determining factor for this difficult genera given a good look, especially in the winter, when a flycatcher may not be vocalizing in the manner we’d prefer. In any case, any odd or out of season Myiarchus should be looked at very closely.

Good enough for you, Jochen?

One Comment
  1. Jochen permalink
    March 7, 2008 5:34 am

    Cheers, mate, you’ve earned yourself some beer! I’ll respond with a post on my mystery (not anymore) flycatcher as soon as … sigh … I find the time!

    Now on to those Jaegers.

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