Dead Bird Quiz, er…answers?

20 11 2012

‘Tis the season for American thankfulness, and I, for one, am thankful for Wouter van Gestel who always helps with the Dead Bird Quiz. Wouter and I are in full agreement on Bird B: an immature Black Skimmer. The upperwing is marbled, with a prominent white band on the secondaries. And the clincher is the all white underwing, which most immature gulls don’t show. Which brings us to the problem presented by Bird A.

Black Skimmers show white underwing even when young, and have a longer, thinner wing than their gull cousins.

Wouter picked up on the same cues I did on Bird A: gull-like wing shape, intermediate wing chord (39cm), dark gray/brown upperwing (suggesting subadult gull), but whitish underwing (somewhat confounding). The little bit of body feathering still evident here suggests that this bird’s belly/breast would have been mostly white. So this is not, evidently, a first year bird. Gulls in the years between hatching and breeding go through an oft maddening series of molts. Field guides tend to depict a sort of idealized average of the annual plumage changes, and it may be that we are dealing with an atypical bird here. The upperwing coloration said immature Herring Gull to me when I first saw it, and the wing chord supports that. But this light underwing gives me pause. Most immature gulls with a mottled gray-brown upper wing also have a darkish underwing. Wouter raised a possibility that had not occurred to me: immature Lesser Black-Backed Gull (LBBG). If you are not familiar with the challenge and the torment that is identifying gulls, check out this post on Larusology. LBBG youngsters can show more white on the underwing, and seem to acquire a white body a bit earlier than young Herring Gulls.

Dennis re-found Bird A on a subsequent walk. Here’s what it looks like in different light.

So, where do I come down on this bird? Sigh. I wish I could continue to hedge indefinitely. I am hearing the voices of my veterinary school professors in making diagnoses: “When you hear hoofbeats, think horses, not zebras.” The Herring Gull is certainly the horse of the seabird world–common, ubiquitous, possibly the number one bird encountered by Seanetters. But LBBG are no longer the rare and exotic zebras they might once have been; sightings of them are on the rise in North America. So, since this bird does not quite match Herring Gull, and fits a bit better with Lesser Black-Backed Gull, that’s the identification I will go with, shoring up my confidence with Wouter’s input. But you can bet I won’t be checking off “very confident” in the database on this one.

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2 responses

20 11 2012
Wouter van Gestel

Thanks for posting that third picture of the gull, it does show some extra clues:
The small white tips on the primaries prove that the bird is nearly adult, probably in its third year. This is also supported by the white body feathers. If the bird is a herring gull, the 6th primary which we can see well should be light-grey with only a dark tip. This is not the case, but the mostly dark-grey look of this feather suits 3rd year LBBG rather well. So personally I’m a bit more confident to name this bird a LBBG.
PS: I agree that large immature gulls are a big pain in the rear end to ID ;)
PPS: I’m glad I got the Skimmer right, as most of my books are European, and we don’t have any Skimmers here.

Wouter

26 11 2012
scourche01

Agreed–those pale tipped primaries made me feel a little better too!

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