Slaty-backed Gull - 27 Dec 2018

I drove out to Grace Lake (Visteon) in Wayne Co. this morning to take yet another crack at finding the reported Slaty-backed Gull (Larus schistisagus) found by Curt Powell. My last two attempts failed, but allowed me to get some comparison images of Lesser Black-backed Gulls. Skies were overcast with rain expected this afternoon. A damp, chilly 40F and a stiff breeze made digiscoping a chore, but possible.

I arrived at 10 am and found Pat Jakel, Tom Gass and others scoping the 1000 or so mostly Herring Gulls on the water. They had the Slaty-backed Gull in view, and I was quickly able to pick it out among the lighter Herring Gulls, smaller-but-slightly darker Lesser Black-backed Gulls, and cleaner-looking, darker Great Black-backed Gulls.  This photo-essay is made possible mostly by frame-grabbing 120 fps HD video capture of the gull swimming and flying. I had to zoom the scope the full 60X in order to get sufficient detail to see diagnostic ID marks (EFL ~2100 to 3000 mm).

The first thing I noticed about the bird is the amount of streaking on the head relative to nearby Great Black-backed Gulls (clean, white heads) and Lesser Black-backed Gulls (lighter-streaking on head). The putative Slaty-backed Gull in the above image (center-left) has a distinctly yellower (orange-yellow) bill relative to the Herring Gulls and Great Black-backed Gull (creamy-yellow). Chest streaking is not as intense as on the head. Size-wise it appears slightly larger than the Herring Gulls and the even smaller Lesser Black-backed Gulls. It is almost as large as the Great Black-backed Gulls swimming nearby. This is consistent with Pyle (2008) and Howell and Dunn (2007).

I managed to get some wing flap images that showed underwing pattern. It is surprisingly similar to the Lesser Black-backed Gulls I photographed a few days ago. The primaries on the Slaty-backed Gull appeared a bit more gray relative to the blacker primary tips on the LBBG. On this bird the P10 Primary tip was completely white, with no black terminal tip. Honestly, I don't know how it would be possible to tell the two birds apart in the field w/o seeing leg color.

Luckily, I managed to see and photograph the fleshy-pink legs in flight when the entire lake erupted with flying birds. I was taking a 120p video of the Slaty-backed Gull at the time, and managed to track the bird as it flew toward me, then banked, and landed back on the water. The following images are all frame-grabs from the video with comments made in the captions. 

takeoff

pink legs visible on dark-winged gull

note pink legs (top bird)

"String of Pearls" on primary tips; All-white P10

Pink legs, P10 vs. smaller window on Herring Gull (top)

slaty-mantle coming into view

Note dark P6-P9 feathers

Herring Gull (top) shows small window on P10

digiscoped!

all-white P10 and white-tongues on P6-P7

white on wrists?

pink legs

pink legs

pink legs and underwing profile

landing

compare primary tips of Herring (top) vs Slaty-back (on water)

I've spent the night trying to determine if a hybrid exists in this bird. Possible hybrids exist with Western Gulls and Glaucous Gulls (?) but I couldn't find any images of a Greater Black-backed x Lesser Black-backed Gull. This bird seems to fit appearance-wise and size-wise of a Slaty-backed Gull, so for now I am having to lean toward a good confirmation.

Jean Iron has some nice pics of adult Slaty-backed Gulls from a recent post. 

Comments are welcomed! I would love to hear from (way more) experience gull experts either confirming or questioning the identification.

References

Pyle, Peter, Identification Guide to North American Birds, Part II, Anatidae to Alcidae, 2008, Slate Creek Press, pp 673-675.

Howell, Steve N.G., Dunn, Jon, Gulls of the Americas, Peterson Referece Guides, 2007, Houghton Mifflin Company, pp 213-220, 438-441.

String of Pearls
http://www.gull-research.org/schistisagus/5cyjan.html

Grace Lake (Visteon Pond), Wayne, Michigan, US
Dec 27, 2018 10:00 AM - 11:15 AM
Protocol: Stationary
Checklist Comments:     Myself, Pat Jakel, Tom Gass, and 3 others; See blog post at https://birdingthroughglass.blogspot.com/2018/12/slaty-backed-gull-27-dec-2018.html for discussion/more photos.
4 species

Herring Gull (Larus argentatus)  1000
Great Black-backed Gull (Larus marinus)  2
Lesser Black-backed Gull (Larus fuscus)  5
Slaty-backed Gull (Larus schistisagus)  1     See blog post with images / discussion at https://birdingthroughglass.blogspot.com/2018/12/slaty-backed-gull-27-dec-2018.html. Larger than Herring Gulls, slightly smaller than Great Black-backed Gulls. Similar in appearance to Lesser Black-backed Gulls but larger and with pink legs (photographed in flight). Also appears to have "string of pearls" on primaries that are visible in flight.

View this checklist online at https://ebird.org/checklist/S51009578

This report was generated automatically by eBird v3 (https://ebird.org/home)