Monday, January 5, 2009

Quiz #281 (2009-1-02) Answer

Click the picture for a larger view.

Answer by Tony Leukering

This week's quiz was a doozy, though two of the species present were correctly IDed by all: Northern Gannet and Great Black-backed Gull. At that point, nearly all respondents went wrong somewhere, with six respondents providing answers with all species correct, but without enough species listed and three respondents getting all species present correct, but providing answers with too many species.

When I took the picture, I didn't really see the Northern Gannet (a nice adult), because I was actually taking pictures of the raptor. That bird was crossing Delaware Bay headed south as I was taking the ferry north from Lewes, DE, to Cape May, NJ, on 18 November 2008. The gulls, being gulls, objected to the raptor's presence and were mobbing it. The Northern Gannet was just minding its business and was simply passing through.

So, the real trick of this quiz was to correctly ID the raptor and each of the gulls. With the provided caveat that there were no local rarities in the picture, we can safely eliminate all species that would be rare on or around Delaware Bay, which helps us tremendously with the gulls.

As the raptor was the focus of the picture, we'll start with it. Though quite a few species were provided as answers, the raptor's strong buffy-orange coloration on the underparts that lack streaking; the narrow and long wings; the long, yellow legs; and the long and strongly-banded tail really leave us with no options other than a juvenile Northern Harrier (and almost certainly a male, due to the narrow wings).

Now, onto the gulls. With the direct size comparisons with the two "Northern" birds, we should be able to determine that all six of the gulls are sizable (as noted by a few participants). Taking the large size and combining it with the list of Delaware Bay gulls, we should be left with a short list of options: Herring, Iceland, Lesser Black-backed, Glaucous, and Great Black-backed. Iceland is probably on the small side for our quiz birds, but I'll retain it for the nonce.

But, not for long, as we can eliminate both white-winged gulls because all of our gulls have at least some distinct black on the tail, a feature not present on either Iceland or Glaucous gulls, no matter the age. With a somewhat tricky triad of gulls remaining, I thought that I'd start with one gull in particular -- the top right gull with the black-tipped white tail. That bird appears to be heavily checkered on the wings and back and the crispness of that plumage suggests that it is a juvenile (in juvenal plumage) or one that has initiated its pre-formative molt, but one that is still well less than a year old. The fact that it has a tail band rules out Herring Gull and the bird's pale greater coverts contrasting with the dark secondaries means that we can rule out Lesser Black-backed Gull. So, this bird is a first-cycle Great Black-backed Gull.

Looking at the rest of the gulls, we should be able to note that there are four others that share our initial gull's pattern of white underparts and a black-tipped white tail. Additionally, those four are also quite large so I feel comfortable calling all of them first-cycle Great Black-backeds, too. That leaves us with the dark-mantled gull in the upper left, the gull directly above the Northern Gannet, and the bottom right gull to ID. The first of these is the one that, I believe,
caused more respondents problems than any other. Ageing the bird should help us. The presence of black in the tail means that it's not a full adult, but the nearly all-pale bill and the nearly uniform coloration of the upperparts means that it's not a very young bird, either. These features tell us that it's a third-cycle gull. By enlarging the picture and looking at the wingtip, we can see that the outermost primary is extensively tipped white, which is all we need to make the ID:
Great Black-backed Gull. Additional features, such as the size of the head and bill, point in that direction and away from Lesser Black-backed Gull. Finally, in my experience, most North American Lesser Black-backed Gulls of this age have much more black on the bill.

For the gull above the gannet, note that the black wingtip contrasts strongly with the paler gray inner wing and that the bill is mostly pale. The lack of any mirrors on the outer primaries and the large amount of black on, at least, the outer part of the right side of the tail should tell us that it is probably younger than our third-cycle Great Black-backed Gull. The bill and tail features also helps us to eliminate Lesser Black-backed from consideration, as a second-cycle LessBack would have more black in the bill and, probably, less black in the tail. That leaves us with an ID of Herring Gull.

Our final quiz bird in the picture is the full-spread bird in the bottom right. We should notice that it has a large mirror on p10 and a small one on p9, so its wingtip pattern is very adult-like. However, its tail still has a fair amount of black, so it, too, is a third-cycle gull. The black on the tail is too extensive for a third-cycle Great Black-backed and probably too extensive for Lesser Black-backed. However, this bird also has pink legs and only slight contrast between the color of the wing linings and that of the underside of the secondaries, the latter meaning that the bird is a paler-mantled gull.

Tallies of incorrect answers for quiz species:
Glaucous Gull - 1
Peregrine Falcon - 1
Ring-billed Gull - 3
Lesser Black-backed Gull - 8
Thayer's Gull - 1
Sharp-shinned Hawk - 1
Laughing Gull - 2
Cooper's Hawk - 1
Merlin - 1
Iceland Gull - 2

The 1 of 23 respondents providing the correct answer:
Robert McNab

Answer: Northern Gannet, Northern Harrier, Herring Gull, Great Black-backed Gull