Monday, February 1, 2010

Quiz #334 (2010-1-05) Answer

Click the picture for a larger view.

Answer by Tony Leukering

A big, all-white, long-necked bird with a black bill and black legs -- gotta be a swan. As noted by a couple of respondents, swan ID is rarely easy, a caution with which I wholeheartedly agree! However, we're greatly assisted this week by the fact that our quiz bird is an adult; I really dislike juvenile swans -- they're tough!

Though ID of adult swans is fairly straightforward, it can be a bit tricky with some individuals, particularly if the views we get don't include good profiles. Our quiz bird is in profile, but the bill is partially hidden and the posture does not provide an optimal assessment of structural features. Just to be on the safe side, I ran the picture by Steve Mlodinow (currently of WA), who probably studies more swans than >99.99% of ABA-area birders. Fortunately for me, he agreed with my ID.

I am restricting discussion of this bird to the possibilities that are Tundra and Trumpeter swans, as Mute Swan and Whooper Swan are fairly readily eliminated by, if nothing else, bill color.

On ID of adult swans, most of the important features to assess reside on the head, but include:

1) size (preferably with a good yardstick species in proximity),
2) shape of culmen (top edge of bill),
3) shape of the meeting of forehead feathering and bill,
4) relation of forward extent of feathering above the eye to that below the eye,
5) shape of meeting of cere and facial feathering around eye,
6) presence/absence of yellow on the bill,
7) shape of feathering edge at gape, and
8) shape of neck in alert posture.

While there are other features that are useful here and there, the above get at the most useful ones. However, on our quiz bird, we can throw out #s 1 and 8 outright (the bird is by itself and not in alert posture), #2 is not perfectly useful as part of the bill is hidden, and there are enough Tundra Swans lacking yellow on the bill such that #6 is not useful here. Granted, presence of yellow would have sealed the ID but absence is not helpful. Also, #3 is difficult to assess accurately without a head-on (or nearly so) view of the forehead.

Let's look at the remaining features individually.

4) The feathering edge at the distal edge of the forehead is not as far forward as is the feathering edge at the chin -- point to Trumpeter Swan.

5) The bird's eye is directly adjacent to the bare skin of the cere, with no feathering pinching in to nearly isolate the eye from the black of the cere and bill -- point to Trumpeter Swan.

7) The edge of feathering extending from in front of and below the eye to the chin is straight or very nearly so, without any distinct curve downward at the gape -- point to Trumpeter Swan.

Of these three useful features, #5 is probably the one most readily assessed correctly at distance, because it imparts a very different look to the whole face on the two species. This really is a card-carrying Trumpeter Swan, as close attention to the swan-head illustrations in The Sibley Guide will attest.

I took this picture of an adult Trumpeter Swan in February 2006 in the Skagit area, Washington, where Steve M. tells me Trumpeter Swans regularly root around in muddy ag fields, while Tundras don't often do so. Of course, I've seen Tundras rooting around in fields in Maryland this winter, but.... Finally, to answer an indirect query by one respondent and to try to put to rest the misinformation of a previous age, both native swan species have salmon edges to the mandible; it is not an ID feature.

I suppose that the respondent providing a non-swan species in the answer (that person did get the swan correct) must have interpreted some of the stubble incorrectly.

Incorrect species provided as answers:
Tundra Swan - 8
Stilt Sandpiper - 1

Congratulations to the 21 of 30 getting the quiz correct:
William Velmala
John Bissell
Tyler Bell
Al Guarente
Tony Ferdensi
Chuck Carlson
Tom Wilberding
George Cresswell
Kevin Kerr
Mark Dettling
Andy Dettling
Aaron Brees
Peter Burke
Peter Wilkinson
Marcel Such
Nick Komar
Barbara Deneen
Joel Such
Chris Warren
Andrea Smith-Guarente
Bryan Guarente

Answer: Trumpeter Swan