Dead Bird Quiz answers: geeked out for skeletal anatomy!

18 03 2013

You know you’re a little too into dead seabirds when it’s easier to identify an inside out skin bag of partial seabird bits than it is to identify a completely intact hawk. But so it is.

Bird A had most people thinking alcid of some kind. Murre or Razorbill, particularly. Or, as a colleague of mine suggested in an email: “the rare and elusive pygmy pelican.” As you can see, I am well suited to that job, with such colleagues.

Bird A's leg labeled as I have interpreted it.

Bird A’s leg labeled as I have interpreted it.

I thought alcid was a possibility initially as well, but some things were nagging at me. The wing chord is rather small, approximately 15cm, whereas RAZOs and murres are usually in the 19-22cm range. The dark wing with clean, unmarked white underwing does fit Razorbill particularly well, so that i.d. is tempting. In thinking about this bird, I focused a bit more on the hind end. The body is inside out, but the general shape in very boxy and appears compressed. The legs have been turned inside out and the feet are not evident. At first glance, it looks like the feet have been severed from the legs, but I would argue that the feet are probably tucked inside the skin, and the leg bones pointing up to the top of the frame are actually the femur (thigh bone) and “shin bone” (tibiotarsus). The end of the femur that once formed the hip is now dangling free up near the top of the image. That means the femur itself is very short and curved, and the tibiotarsus is quite long. It also appears to have a projection that extends up above the knee joint. This looks to me like a cnemial process–a jutting lever of bone where the strong thigh muscles attach. All of these are the marks of foot propelled divers, and the cnemial process is especially pronounced in loons. Alcids, on the other hand, propel themselves mainly by flapping their wings underwater and don’t need such specialized knee apparatus. Bird A is too small to be a loon, which made me think it may be a grebe. I know I have grebes on the brain, but I really can’t find another way around that  that leg anatomy, if I am oriented correctly in my interpretation. Based on the size, and the amount of white in the underwing, I would venture to say that the bird is a Horned Grebe.

Image from a 1982 paper comparing tibiotarsi (shin bones) of an extinct diving bird (A), a loon (B) and a grebe (C). The spear-like structures pointing up are cnemial processes of the tibiotarsi.

Image from a 1982 paper comparing tibiotarsi (shin bones) of an extinct diving bird (A), a loon (B) and a grebe (C). The spear-like structures pointing up are cnemial processes of the tibiotarsi.

Bird B generated a strong consensus, thankfully, since I am woefully inept at non-seabird i.d. Though Cooper’s Hawk was also a common response, most of you concur that this bird is a Red-shouldered Hawk. And Lori Benson herself, who found the bird, identified it as the same. As a rank amateur, I cam glad this bird bears some clear identifying signs that I can compare with my Sibley guide. The red “shoulders” are evident on this bird, and the striking wide black bands on the tail interspersed with thinner white bands appear, to me, to mark this bird as an adult.

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