Monday, August 17, 2009

Quiz #313 (2009-3-08) Answer

Click the picture for a larger view.

Answer by Tony Leukering

Mark Peterson took this week's quiz photo at Fountain Creek Regional Park, El Paso Co., CO, 27 August 2008 and it was obvious to this week's respondents that it was of a cormorant and a youngster at that. Because the face pattern is not crisp and the bird's plumage is not blackish, our bird is almost certainly a juvenile. The presence of a large and bright orange gular area should bring us to a dichotomy that is often much more difficult than most field guides suggest: Double-crested vs. Neotropic. Even though some Great Cormorants can have an orange gular area, the bird's lack of white on the belly rules out that species.

Typical juveniles of both species are fairly readily identified correctly, but there is an unfortunate number of 'tweeners that demand very careful scrutiny. This fact is not illustrated any better than with the following story.

When I started contemplating writing the answer for this week's quiz, I looked at the picture again and was suddenly not so certain that I had correctly IDed the beast. So, I sent a request to four of North America's best birders (1 west coastal, 2 east coastal, and 1 interior) asking them to let me know what they thought the correct ID was. I don't know why I didn't send that request to an odd number of experts, because I should have expected what I got: a tie score, 2 for Double-crested, 2 for Neotropic. However, after further discussion, they were unanimous in their ID, so I shall continue.

Most juvenile Double-cresteds are more extensively white below than the quiz bird while most Neotropics are less white. Unfortunately, it's difficult to be certain of the extent of white, as the photo is overexposed and the white that is present may be an illusion caused by that overexposure. So, we'll have to check other features. Additionally, Double-crested is more variable in plumage as juveniles than is appreciated by most birders, as is illustrated by two darker-breasted cormorants illustrated on my flickr site, one from


and one from

New Jersey.

We can be certain that both of those cormorants are juveniles, as their remiges (wing flight feathers) are all of the same generation and there is only one time when that is true of cormorants, when they're in juvenal plumage. This is because cormorants exhibit staffelmauser, a molt strategy that limits the number of large, energy-expensive feathers that they replace in any one molt (see the paper on molt, figure 6, by Peter Pyle), causing adults to always have at least two ages of remiges.

So, let's continue.

Tail length is a good indicator of species in this duo, but the angle of the picture makes it a bit tricky to be certain of our bird's tail length. It does not look particularly long to me, but I am not willing to base my ID entirely on that feature in this bird. That leaves us only head pattern on which to base an ID. Even juvenile Double-cresteds typically sport a supraloral strip of bare orange facial skin that our quiz bird lacks. Additionally, there is a distinct white band behind the orange gular skin. These two features point directly at Neotropic Cormorant and, generally, away from Double-crested. Unfortunately, the latter of these two characters is often present on juvenile and immature Double-cresteds, but the lack of supraloral orange is considered by many to be diagnostic: Double-cresteds always have it and Neotropics always lack it.

Of course, this is biology we're talking about, and biology is rarely that neat. Colorado birders have a slight advantage in this in that I wrote a piece for Colorado Birds on this duo, highlighting a picture taken by Michael O'Brien of a Neotropic Cormorant that exhibited an orange supraloral band (Leukering, T. 2008. Neotropic Cormorant (In the scope column). Colorado Birds 42:226-228.). And, if Neotropic can sport such, surely some Double-cresteds can lack that feature. If this is true, what about gular area shape characters?

Though I've been wrong often enough before now, I believe that our most certain road to correct ID of this quiz bird and the two species, in general, is through careful analysis of the shape and size of the bare skin in the gular area and the shape and orientation of the meeting of that area with the feathering posterior to it. In Neotropic, the area of bare skin is small and the meeting with the feathering behind it angles forward from the gape at about 135 degrees. In Double-crested, the orange gular area is large and the meeting with feathering is nearly vertical (or 180 degrees).

So, despite our bird lacking a supraloral patch of orange and its fairly dark neck, this week's quiz bird is a Double-crested Cormorant. The photographer believes that it is the indivdual that was reported from the site as a Neotropic Cormorant, a review species in Colorado, and one can certainly see from Mark's picture how that mistake could be made.

Incorrect species provided as answers:
Neotropic Cormorant - 7
Great Cormorant - 3

The 8 of 18 providing the correct answer:
Nick Komar
Tyler Bell
George Cresswell
Victor Germain
Al Guarente
Margie Joy
Mark Gilsdorf
Barbara Deneen

Answer: Double-crested Cormorant