Dead Bird Quiz answers

26 09 2014

Another set of unanimous decisions on these three specimens. Edward and James both came back with answers of A) cormorant (probably double-crested); B) Juvenile herring gull; C) no idea/?

That’s where I came down on these as well, but I wanted to post them because they each illustrate one of the challenges in identifying dead birds, even when the carcass is lovely and intact, as in Bird B.

For Bird A, we mainly have the skull, and thus the bill, to go on. The bill is thin, but has a substantial curve, and a prominent hook at the tip. Cormorant does immediately come to mind given these features, but I always try to think “What else could this possibly be?” For Bird A, the only other species group that seemed even remotely possible was the shearwaters. Their bills can have a similar curve and hook. “But wait!” I hear you shriek, “Shearwaters are tubenoses, and I don’t see any tubes on top of that bill!” Well I don’t either, but we must always contend with the possibility that decomposition and general falling apart can alter features rather profoundly. Bird A has lost the keratin sheath that overlies the bones of the bill during life. That sheath sloughs off rather readily as the carcass weathers, and if the tubes on the noses of the tubenoses were only a feature of the bill sheath, and not the underlying bone, then we might indeed see a skull that looks like this.

This image of the skulls of both an extinct (top) and extant species of shearwater from a paper by Ramirez et al shows the general features of shearwater skulls when the bill sheath is no longer present.

extinct Lava shearwater skull (top) and Manx shearwater skull. (from Ramirez et al, 2010.)

extinct Lava shearwater skull (top) and Manx shearwater skull. (from Ramirez et al, 2010.)

In these images, you can see that the overall curve of the bill is not as great as in our Bird A, and you can see distinct nares (nostrils) atop the bill, though the distinctive tube structures are indeed, far less evident without the sheath in place. Cormorants, on the other hand, have a rather different looking skull:

cormorant_sideview

Note the lack of evident nares in this cormorant skull. (Photo: Dominique Harre-Rogers, copyright Smithsonian Institution)

In this photo from the Smithsonian Institution, it’s clear that there is no rise in the bone at the level of the nares as there is in the shearwaters. In fact, it’s hard to really see the nares all that much at all here. This is a feature of pouchbills like cormorants and gannets; in fact, they lack external nares entirely. We who handle the birds live must always pay attention to how we restrain the bill since, if we hold the bill closed entirely, the bird has no way to breathe.

All this explanation is really just to tell you what you all apparently already knew: Bird A is a cormorant. It’s difficult to say which species, since an accurate culmen length relies on the bill sheath being intact and seeing where it meets the feathers or facial skin. Not possible here, for obvious reasons. Based on their ubiquity, and the overall shape of the skull, Double-crested is the more likely.

Bird B was beautifully intact, so it may seem strange that I used it here as a challenge. But there were features on this carcass that I think could throw some people off. Everyone concurs that this is a juvenile Herring Gull. mlyons6759-15258-1Indeed, there are many features that would lead us there: the overall gray over the belly and underwings are characteristics of young Herring Gulls that differ from Great Black-backed, Ring-billed, and Laughing Gulls, the three most likely alternatives. So why not just call this a Herring Gull and move on? The color of the legs and the color of the mantle (the back between the wings) caught my eye in this bird. The legs look reddish to me, which makes me think of Laughing Gulls as they have reddish black legs as adults. Our Bird B also has quite a distinct brown pattern to the mantle, where most of the juvenile Herring Gulls we see have more of a grayish wash. The Larusology blog does a good job helping people identify gulls, and this post about Herring vs. Thayer’s gulls points out that this phase is a normal one in young gulls–the feathers over the mantle are large and have a scaled appearance, while the wings have a smaller pattern that looks more checkered, very much like our Bird B.

Overall, the weight of the evidence falls on the side of Herring Gull, especially the conspicuous features like the color of the rump at the tail base. In young Herring Gulls, this is dark, where in Laughing Gulls, Ring-billed and Black-backeds, it’s white. None of those species have this much gray and brown over the entire breast, belly, and under the tail. So we’re left with that strange reddish color to the legs. My only guess is that they usual pinkish color has been altered by some trick of the light in combination with the effects of desiccation. In any case, this bird didn’t fool anyone, apparently, and really only had me wondering what was up with it. I’m always hoping to find some weird gull hybrid too, so maybe I see oddities where they are not actually present.

Finally, for Bird C, I agree with our respondents: no way to tell from what we have, which is a rather shredded wing missing a bunch of its coverts. “Unknown bird” it is. You can’t win ’em all.

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One response

27 09 2014
Edward Soldaat

In addition: another important feature to distinguish cormorants from shearwaters or gulls is the absence of the depressions for the salt glands above the eyes. In this case skull and bill were too big for any shearwater, only a giant Cory’s Shearwater would have come close. But in smaller cormorants the lacking of visible nostrils and salt gland depressions are important characteristics. Interesting is also that cormorants and darters (not in gannets, pelicans or other) have a small dagger shaped bone connected to the back of the cranium, embedded in the strong musculature of the neck: the occipital style.

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