Duck Bone Identification Walk Through

In the process of removing a lot of young coconut palms from under our large coconut palm, some duck bones were unearthed.

Judging by the petite size (and a bit of hopeful bias) I initially thought they could be mallard (Anas platyrhynchos).
However, this was disproved almost immediately, ruled out by the sternum. I new mallards had wider xiphial areas (circled) than a handful of other waterfowl (Muscovy ducks, many geese, and swans, etc.), but I didn't realize just how wide. Not all mallards have the posterior lateral processes (in rectangle) connected to the xiphial area like this reference from the Ohio Virtual Museum, but they're all close. Certainly not a fit for the unknown sternum.
This would hold true for all Anas species, which eliminates them and leaves the two more likely options; a Muscovy Duck (Cairina moschata, specifically Cairina moschata var. domestica) or Egyptian Goose (Alopochen aegyptiaca).

(from https://virtual.imnh.iri.isu.edu/Osteo/View/Mallard/580)

So I got out Muscovy and Egyptian Goose bones for comparative analysis.

First lets start with the sternum.

The Egyptian Goose has a very broad xiphial area (circled) compared to my two Muscovy sternums (larger male, smaller female) and the unknown sternum. The Egyptian Goose sternum in general seems to be wider than the Muscovy and unknown, so it's looking to just be a petite Muscovy duck.

But then I went to the humerus.

(Male Muscovy duck is seen here)
In the square box is an indent known as the capital groove. The unknown humerus and Egyptian Goose humerus both seemed to be more dramatic than the Muscovy. Now I'm considering if this is a goose, not a duck.

On to the pelvis.

Circled on all pelvises is the ischiadic foramen, the opening (foramen) in the ischium of the pelvis. The humerus didn't convince me the unknown was a goose, but this pelvis is getting me there. The foramen of both the male and female ducks is larger, with an almost second foramen extending from the first. I have used such comparisons in my identifications before, so now I'm quite sure this unknown is actually an Egyptian Goose!

And now here comes the tarsometatarsus.

Uh oh. As I went into my box of goose bones to get the tarsometatarsus I immediately realized something was off. Was the unknown bone broken? When I got out the Muscovy tarsometatarsus, however, I realized that no, it was in fine shape. Dirty, yes, but not broken. Egyptian Geese apparently have very long tarsometatarsus, and this unknown bone was not that. And so this becomes a cautionary tale of the inherent randomness in nature, as well as the issues with using features very susceptible to ossifying and changing shape with age.

The unknown is most certainly a Muscovy.

The largeness of the foramen and vague difference in the humerus (which I struggle to find myself now reflecting after a few days) are just not strong enough to trump the wideness of the Egyptian Goose sternum, and, most of all, the size of the tarsometatarsus.
So how do we know what is actually reliable to determine as an identifiable trait?

I am not professional trained by any stretch. I have no formal education in what I do. Not until maybe a year into my self teaching did I buy a proper comparative osteology book, Osteology for the Archaeologist by Stanley J. Olsen.
However, even trained professionals use unreliable identification measures subject to the same issues with what I did with the foramen.
I learned somewhat early on into my studies that professionals often use palatal structures in identification, but that always seemed imprecise to me. I never bothered to study the bottom of bird skulls of IDing, and when people ask if that's an important view I'd say for some people, but I don't find it necessary. And, upon getting my first proper comparative osteology book, I realized I wasn't unjustified in my assumption. From Olsen's Osteology for the Archaeologist:

"It must be pointed out that there is some individual variation in this palatal complex, not only among bird groups, but also regarding the age of individual birds within these groups For instance, in the young of some gulls, crows, and hawks, the anterior margin of the pterygoid becomes detached from the rest and fuses with the palatine. These palatal types must be considered as of taxonomic value only when used as part of the diagnosis which is based on other osteological characters as well."

In short, things that can easily be subject to changes with age can be used for identification as a supplement, but certainly not alone. I personally think I'll retire using the shape of the ischiadic foramen to diagnosis species within a group, though I still intend to use it to differentiate between family groups I never encountered a problem with that thus far, but I suppose time will tell if that's reliable. But there is a much bigger different between a tiny coot ischiadic foramen than the ischiadic foramen of any waterfowl, even if the individuals experience variance.
I've also had issues with using pneumatization patterns for ID, which makes sense. Most bones as far as I'm aware tend to ossify or have changes in ossification with age, and therefore something as delicate as tiny holes in the bone would surely be subject to morphological changes with age. Younger ducks, I have found, tend to have more pneumatization on the bottom of the sternum than older ones. Which makes sense when you think about how many vertebrates ossify more with age.

Ultimately, I think only using one or two traits on one or two bones can always lead to issues, even if they don't experiences the issues discussed above. A well rounded look, exploration and analysis of whatever morphological features you have seems to be a good plan to make the more through ID possible.

Posted on 23 de setembro de 2020, 02:37 AM by lizardking