Island bird quiz answers

19 12 2014

This is one of those quizzes that’s made me feel rather good about my abilities. While I encounter a lot of wings in my work verifying SEANET reports, we don’t get many skulls, so I am always a bit leery of trying those. My basic strategy is not particularly systematic; I look at the skull and see what jumps to mind, and then I check skullsite.com to either confirm or deny my suspicions. When I looked at Skull 1, I thought oystercatcher immediately. I was therefore gratified to find that American Oystercatchers (AMOY) do breed on the island where this skull was found, and also that our readers concurred; Wouter, Edward and James all asserted AMOY as well. When I considered a bit more what had made me jump to this i.d., I came up with the bill shape, which is long and has a long nasal aperture (the opening on the upper bill). The shape of the cranium is very rounded, with a open, round orbit (eye socket). It reminded me somewhat of a woodcock, which makes sense since the two species are shorebirds by classification. Though the woodcock stalks the woodlands eating worms and such, their skull anatomy reveals their affiliation. The finder of this skull was also curious whether we could tell the age of the AMOY based on the skull. I don’t know, but if anyone would, it’s our esteemed readership, so if they have thoughts on that, I hope they will weigh in. But in any case, check out these dome-headed cousins!

Eurasian woodcock (photo: Ronald Slabke)

Eurasian woodcock (photo: Ronald Slabke)

American Oystercatcher (Photo: Peter Wallack)

American Oystercatcher (Photo: Peter Wallack)

The second skull is a more familiar friend. The bill is clearly a gull’s, though it can be challenging to tell a Great Black-backed from a Herring Gull, especially with only an oblique view and no measurements. I tend to be conservative on these i.d.s, at least for the purposes of our database, so I would not likely go further than to say it’s a large gull–either GBBG or HERG.

The set of wings at first looked like a tough call. The overall roundish shape and dark color got me thinking of a duck, but a nondescript dark wing could be a lot of kinds of ducks. On close inspection though, I was gratified to see a little hint of bluish purple on a couple of the feathers that I suspect are the remnants of a speculum. What birds have a bluish speculum? Mallards and American Black Ducks do. How can we tell the difference? In this case, it’s challenging. These wings are pretty far gone. The two best ways to differentiate the wings are by the presence (Mallard) or absence (ABDU) of white bars bordering the speculum, and, on the underside of the wing, a characteristic brown streaking at the wrist in ABDU, and plain white in Mallards. That region of the underwing is thoroughly degraded in our mystery bird, and I am leery of judging too much based on what’s left of the upper wing, but I don’t see much evidence of any white borders anywhere here. I would, therefore, lean toward ABDU in this case. Though I am open to discussion and correction, as ever, dear readers.

This American Black Duck is trying to show you both the upper and underwing characteristics of his species. (Photo: Dick Daniels)

This American Black Duck is trying to show you both the upper and underwing characteristics of his species. (Photo: Dick Daniels)

And look at this obliging Mallard's wing. (Photo: Malene Thyssen)

And look at this obliging Mallard’s wing. (Photo: Malene Thyssen)

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2 responses

19 12 2014
Edward

I can go along with you on the duck wings. Since there is no trace of white bordering of the speculum it might be Black Duck indeed. I’m not familiar with that species, so I guess you are the expert in this case.
Ageing an Oystercatcher skull is hardly possible. This is definitely an adult/ full grown bird since the ossification is complete. In very old birds the skull may show tiny protrusions and more pronounced ridges, but I’m not sure that it tells much about the actual age. It may be possible to distinguish extremely old birds from not so old adults and juveniles, but not more than that.
The gull looks a Herring Gull and not a Great Black-backed to me. The size could be established by measuring a lid or diameter of a similar plastic bottle. That would provide a reference for calculating the length of the skull. But by judging both items I stick to Herring Gull.

31 12 2014
scourc01

Thanks as ever!

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