Dead Bird Quiz answers: despair over jaegers

17 10 2013

Once again, I am grateful that Wouter found our blog once upon a time and saves me from my ignorance. The two birds Dennis found in Provincetown represent two of the typical challenges we face here at SEANET: the decomposed bird that retains few identifying characteristics, and the unusual bird that we see only very rarely.

Bird A’s bill is intact, but most of the feathering on the face is gone, so it’s hard to see what it’s profile really looked like in life. I vaguely thought maybe grebe at first, based on what looked like a relatively short bill, but I didn’t feel great about that answer. Wouter suggests it may be a murre, and as soon as I saw his comment, I very nearly smacked my forehead for missing it. What’s left of the feathers on the head seems to show the black and white pattern characteristic of murres. Take a look at Bird A’s bill versus the bill of a known Thick-billed Murre:

Bird A

Bird A

Thick-billed Murre, in slightly better shape.

Thick-billed Murre, in slightly better shape.

Great. We’ve got Bird A fairly well pinned down. But Bird B…oh, Bird B. The good news is, Dennis thought it was a Long-tailed Jaeger, which gave me a starting point. I looked at my Sibley guide and felt good about that i.d. Not certain by any means since I’m not all that familiar with jaegers, but it looked promising. Then Wouter weighed in and wrote that it looked like a Long-tailed Skua. You can imagine my despair when I had what I thought were two diverging opinions. Then, my spirits were yanked abruptly back up again when I looked up that species and discovered that’s what the Long-tailed Jaeger is known as outside of the United States. So both Dennis and Wouter thought it was Stercorarius longicaudus. All praise to Linnaeus.

OK, so what’s a jaeger/skua and what makes our Bird B one of them?

On first glance, the bird looks gull-like, though with a strange plumage pattern, which alerts us that something’s fishy. Looking at the bill of Bird B, we see that part of the keratin sheath has sloughed off, as happens with advancing decomposition. What’s clear though, is that the mid-portion of that sheath (the saddle) was an entirely separate plate from the nail, or the bill tip. In a gull, though the overall bill shape can be quite similar, the bill sheath is one continuous structure that sloughs all at once. These separate plates are what make our Bird B clearly a jaeger or skua and not one of their cousins, the gulls. (All three groups are members of the Laridae family.) Skuas have the separate plates, but they are generally bigger and bulkier than the jaegers. It also appears, from my recent reading, that skuas generally show a striking white flash at the bases of the primaries on the upperwing. Though our Bird B is quite beat up, I am fairly convinced that no such patch is present, here so we’ll set skuas aside.

Upper surface of Bird B.

Upper surface of Bird B.

Within the jaegers then, how do we figure out which sort this is? Evidently, it’s not easy. My sources, the Sibley guide and Peter Pyle’s Identification Guide to North American Birds, volume II, both use the word “caution” rather frequently with regard to putting forward a firm i.d. “complex, confusing and subjective” are additional terms utilized. Fabulous. Where to begin.

One thing that jumped out at me in our Bird B is the bold barring on the undertail. That appears more consistent with either a Long-tailed or a Pomarine Jaeger. Less so with Parasitic Jaeger. If we look at the bill, the proportions of that may help us as well. Here’s what Sibley has to say on that:
IMG_4923
Clearly, these rules cannot be hard and fast. Our Bird B’s bill is rather slender, and the nail appears to cover less than half the length of the bill. Those points argue for Parasitic Jaeger. Yet the gonydeal angle in our Bird is basically mid-bill. That’s more consistent with a Long-tailed Jaeger.

Bird B's bill.

Bird B’s bill.

Overall, I am persuaded firsthand of what I had only read about: identifying juvenile jaegers is difficult. I suppose I might come down on the side of Parasitic Jaeger, all things considered, so I am curious what features persuaded other folks that this bird is a Long-tailed. Won’t you enlighten me?

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2 responses

17 10 2013
Wendy Stanton

I am learning so much from your blog!! I thoroughly enjoy reading your step by step thoughts and identification!! Great work!!

17 10 2013
scourc01

Thank you so much, Wendy! Sometimes it feels very discouraging, when I have no idea what I’m looking at. Glad it’s of some help!

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