DBQ answers II

14 12 2015

No more shirking or dodging; I must at last face those wings known as Bird B. To jog your memories, here’s what we’re working with:

DMinsky7855-24633DMinsky7854-24633

Dennis, who found the wing, wondered if this might not be something other than a standard issue species. Everyone who has looked at the wing (and ventured to reply) concurred that this is a loon of some kind. The clean white underwing by itself could indicate a few different species groups, including shearwaters and grebes. But looking at the upper wing, we see buff colored chevrons at the terminal ends of the secondary coverts. The wing chord looks to be around 28cm. These features together tell us this is a loon, and a pretty small one (common loons have a wing chord in the 33-40cm range). Red-throated loon is the default i.d. for a small loon on the east coast, but it’s not the only possibility. Dennis granted that it could certainly be a RTLO, but thought something just seemed a bit off about in terms of its overall coloration and the nature of those pale chevrons. Since Dennis has seen many, many a dead bird, I think it worth a look when he notes something atypical about a carcass. Dennis thought perhaps Pacific loon should be on our consideration list. If we consider Pacific, we should also consider Arctic since the two are almost always uttered in the same breath and can be difficult to distinguish themselves. Both Arctic and Pacific loons would have a wing chord in the range of Bird B’s; both average larger than RTLO, but a wing chord of 28cm would fit with any of the three species. Strangely, given what one might assume from the names, the Arctic loon would be much more of a rarity than the Pacific in these parts.

This situation calls for the use of two of my favorite resources: Peter Pyle’s identification guide, and the Slater Museum’s online wing collection. You can check out a whole suite of loon wings from various species and times of year here. Fortunately for me, they have a few Pacific loon specimens there to look over, in breeding and non-breeding coloration. As with many loons, breeding plumage includes bold, clear, pure white spots on otherwise black upper wings. Our Bird B does not have any white spots or dots at all, but that lack is typical of a bird no longer in nuptial raiment. Beyond being a non-breeding bird though, what we have here in Bird B appears to be a young bird. In the case of RTLO and Pacific loon, both have pale edges to the secondary coverts during the first year, and that first year plumage is retained into well into the first winter, so the timing is right for Bird B, which was found at the end of November. ┬áTo parse out the differences between juvenile RTLO and Pacific loon wings, we can look at Peter Pyle.

IMG_7742.JPG

Secondary coverts in Pacific loons. In juveniles, a pale, terminal band gives the impression of light, nearly white crescents over the upper wing.

IMG_7743

Contrast the preceding with these secondary coverts from RTLO. The far left image is a juvenile and the pale coloration here is much narrower and comes to more of a point, giving an impression of chevrons rather than crescents.

Considering this, I am of the opinion that Bird B is decidedly chevroned and now crescented. And so, though I had fervently hoped we might have a first ever Pacific loon in our database, I fear it is not, and is, instead, our old friend the Red-throated loon. As ever though, if I am missing something critical, I know all you super-pros will write it with the correction.

Advertisements




DBQ, part deux

17 03 2015

I have feeling so very remiss about SEANET lately, not least because I cannot blog as frequently as I might like during the high teaching season. To partly make up for this, in this second installment of the most recent DBQ, I have labeled photos with orange arrows. This way, it will appear that I have been doing something more substantial than my usual.

The features I elected to label with orange arrows are not entirely arbitrary. When I looked at Bird B’s photos, my instant thought was “This is a loon.” Of course, that kind of bolt from the blue is insufficient to a DBQ answer, so I then took my usual next step, which is to ponder what it is about the overall Gestalt of this carcass that brought the word loon instantly to my lips. First, the sternum shaped (not labeled with an arrow.) It’s elongate, which, as Edward also pointed out, makes this not a grebe, which is the other pointy-beaked, white-under-winged group of birds one might consider for this i.d. Second, the small bit of patterning visible on the back feathers (shown with orange arrow).

Slide1
Can you make out the faint, light colored chevrons tipping the otherwise dark feathers? That’s a loon thing. What kind of loon though? We have at least some of the head to work from, though the mandibles have parted ways from the upper beak, complicating matters. Still, take a look at this helpfully labeled photo:
Slide2

Here, we can see a v-shaped white patch at the base of the upper bill. This is quite suggestive to me of a Red-throated Loon, as no other loon has such extensive white on the face, especially in the region extending up the forehead from the bill base. Without the usual hallmark of the RTLO though–the jaunty and somewhat smugly upturned look to the bill–it’s not a slam dunk i.d. As Edward pointed out, the upturned appearance of the bill of RTLO is largely due to the shape of the mandible, the upper bill being actually quite straight, as in this specimen. Alarmingly though, Edward suggested Bird B might be NOT a RTLO, but an Arctic Loon! We have never, in the history of SEANET, gotten one of those. Given this dearth, I am tempted to heed that old vet school warning about jumping to rare diagnoses: “When you hear hoofbeats, think horses, not zebras.” But then I am whipsawed by indecision when I consider all my rotations in zoos, and the memorable day I got to blow dart a zebra with its vaccinations. And then, coming out of my reverie, I remember about identifying Bird B again.

I perused the internet for some nice photos to share with you, and I found one by someone else in the world who likes to photograph rotten old carcasses! Here is a mostly skeletal Arctic Loon (known in other parts of the world as a “Black-throated Diver”:

800px-The_remains_of_a_Black-Throated_Diver_I_255618814

Photo by Miika Silfverberg via wikimedia commons

You may note that this species has what looks almost like a droopy, downturned bill: just the opposite of what we see in a RTLO. And here is a cartoon version of the birds by L. Shyamal, which gets across the main aspects of their patterning:
410px-GaviaArctica.svg

I argue that our Bird B has more white on the face than an Arctic Loon would. Add that to the extreme rarity of that latter species in our neck of the woods, and I think I am right. But I love to argue and debate, so if anyone can convince me this is something other than RTLO, bring it on!!!