What we can glean from severed legs

6 05 2015

Now, for the final installment of this latest DBQ. Bird C was nothing more than a leg, mostly stripped of its flesh. Remember it?

Bird C: Severed,  skeletal leg found by Dennis Minsky on Cape Cod.

Bird C: Severed, skeletal leg found by Dennis Minsky on Cape Cod.

What we can see here is that the skin that is left on the lower leg is black, and there is one small hind toe with a fairly small nail visible. The bones we see here, from the top, are the femur, which meets the tibiotarsus at the knee joint, the tibiotarsus, bearing the tag and the tie, then the hock (ankle) joint, and finally, the tarsometatarsus, which has the black skin on it.

For our crack team of dead bird experts, this specimen was not particularly difficult, it’s clear. But for lesser mortals, let’s work through some possibilities as an exercise.

For me, that color leg brings to my mind two main possibilities since this leg is fairly large: loon and goose. What other traits would we expect were this a loon? First, the skin-bearing tarsometatarsus is extensively flattened side to side, giving it an almost blade like appearance head-on. Bird C looks to have a rounder leg that that. The toes are no help since they aren’t here, but we can use the proportions of the leg bones as well as a bit of knee anatomy.

Leg anatomy of loons. Note the proportionately short femur and the pointed projection at the knee joint.

Leg anatomy of loons. Note the proportionately short femur (f) and the pointed projection at the knee joint.

If you have ever seen a loon attempting to ambulate on land, you have witnessed an animal quite literally out of its element. With its legs set so far back on its body, the front heavy animals can manage only a couple labored steps at a time. In addition to having the legs set so far back, the birds’ short femurs offer limited ability to swing the leg with enough clearance above the ground. Why this absurd arrangement? Because of this:


Click the image for more pics and a bit of video!

When you watch a loon in the water, it makes perfect sense. Legs set as far back as possible on the body works in the water the same way an outboard motor works on a boat: you want that propulsion system at the back of the craft. Unless you are like me and my hapless fisherman father when we tried (once) to use a trolling motor. We mounted it on the side of the boat and proceeded to describe large circles on the lake for an hour or so before giving up.
In the images of loons underwater, one cannot even see where the short little femur is on the body because it is snugged up under the thick feathers. The reason that femur is so short comes to down to a clever bit of physics. Though the femur is short, it is overlain by thick bands of quadriceps muscles. These are so beefy, they need extra attachment points where they insert at the knee. That reinforced structure is the spear-like projection, called the cnemial process, that juts up from the knee in the image of the hind limb bones. Muscle can attach along that whole length. When those strong muscles contract, though they are short, they pull on that cnemial process lever arm, and for a relatively small contraction distance, the tibiotarsus makes a very wide arc. That’s the kick you see in the video of the swimming loon.

You notice that the loon keeps its wings tucked in tight while swimming. This is in stark comparison with a wing-propelled diver like the Thick-billed Murre. These birds are a bit awkward in the air, seeming to be just on the brink of crashing. Their flight is labored, with their heavy bodies and proportionately small wings. But, as with the loons, all forms of locomotion are evolutionary trade-offs with others. In this video clip of murres underwater, you can see their short, pointed, stiff wings are just the thing for swimming. Note that their legs, on the other hand, just trail behind them, as they would during flight in the air. Their hind limb anatomy resembles a land bird much more than a loon since they did not evolve to use their legs and feet for swimming.

All this has not yet answered much about Bird C. Except that now, looking at the leg, we see the bones look similar in length, and proportional. This argues against a leg-propelled diver like a loon. So we could be looking at a wing-propelled diver, or, more likely, not a diver at all, but a bird that feeds like this:

Swan dabbling for food. (Photo by Etan Tal)

Swan dabbling for food. (Photo by Etan Tal)

In dabblers like this swan, as well as geese and the non-diving ducks, the birds are limited to the aquatic food they can reach from this absurd upturned position. The feet and legs do little more than help a bit with balance and orientation. A long neck helps with this, but it also helps to feed on immobile prey like plants. To catch more active foods, the ability to pursue is a major benefit.

Bird C–large sized, black legged, likely not a diver–seems too big to be a Brant, but too small to be a swan. The consensus on this one is Canada Goose, which is fitting given how many of them we’ve had reported this spring as the ice melts.

Now, my friends, I am off to learn about forest ecology and land management for several days, blissfully without internet. I’ll see you here when I get back!