Verification of Unusual
Rec. # 2014-003
|Scientific name:||Larus schistisagus|
|Date:||14 December 2013|
|Length of time observed:||7 minutes, then 5 minutes|
|Location:||Logan Fisheries Experiment Station and Logan Landfill|
|Distance to bird:||~30 feet|
|Optical equipment:||Nikon Monarch 10x42 binoculars and Nikon DSLR with 80-400 mm lens|
|Weather:||Cold and hazy|
|Light Conditions:||Soft overhead light through hazy air|
|Description: Size of bird:|
|(Description:) Basic Shape:|
|(Description:) Overall Pattern:|
|(Description:) Bill Type:|
Field Marks and
1. All-black, relatively straight-sided bill with a subtle gonys. There is just a bit of pink blending in to black at the base of the lower mandible, which is expected in Slaty-backed Gull in December.
2. Head paler than the body, but with a lot of dark around the eye like Western and Slaty-backed both show. Smudgy streaking of brown on cream forms a vague collar, a good sign for Slaty-backed Gull. Some Slaty-backed Gulls are identified by a much paler head than this, but first-winter Slaty-backed Gulls are variable in overall color.
3. Solidly black eyes. On very close examination, a very dark brown. Most Larus species share this trait in their first winter.
4. Overall dark coloration, but not as dark as a Western Gull. Darker than most Glaucous-winged x Herring hybrids.
5. Lesser and median coverts overall dark and not as checkered and contrasty as would be on a Herring Gull or Herring hybrids.
6. Distinctively dark-centered and plain (not patterned) greater coverts on the wing. This is not seen on Herring Gull or Thayer's Gull, and can be found on both Western and Slaty-backed Gull. The solid-centered greater coverts also help eliminate the most similar-looking hybrid combination, Glaucous-winged x Herring, which can tend towards this appearance but are not as dark and solid as this bird.
7. Tertials very dark brown, not quite as dark as the primaries, and completely solid-centered. Whitish tips to mostly dark brown tertials.
8. Dark primaries, blackish but not quite black, more of a very dark brown, and with pale tips to each primary visible in the folded wing. The dark but not black
primaries help eliminate Glaucous-winged Gull and are darker than expected in Glaucous-winged hybrids, including Glaucous-winged x Herring. Primaries very skinny and pointed, a subtle character but also a good indication of first-winter Slaty-backed Gull.
9. Underwing shows distinct dark wedge at the outer trailing edge formed by darker tips of outer primaries. This band of darker primary tips when seen from below is indicative of Slaty-backed or Thayer's Gulls. Spread upper wing not seen well or photographed.
10. Legs relatively dark pink, darker than most Herring Gulls for example.
11. Upper and undertail coverts heavily barred with blackish brown on white or cream. Tail appears solidly dark in photos, but was not seen well in life and photos of the tail are from an awkward angle where white bases of retrices might not be visible if they were present.
For comparison, here are two first-winter Slaty-backed Gulls
photographed in Japan that I think strongly resemble this bird:
|Song or call & method of delivery:|
|Habitat:||Shallow wetland and then landfill. Initially associating with one Glaucous-winged Gull, one Herring Gull, and one Ring-billed Gull for direct comparison, but soon flew to the nearby Logan Landfill by itself, where it was relocated.|
were they eliminated:
Western Gull: This bird lacks the obvious gonydeal expansion and slightly
drooping bulky bill of a Western Gull. The head is also too pale for that
species this time of year. There is no visible drooping tertial skirt. Head is
less blocky than Western Gull.
American Herring Gull: The primary and most obvious feature ruling out Herring Gull is the solidly dark-centered greater coverts.
Vega Gull (Vega Herring Gull): As in American Herring Gull, eliminated by the plain, solidly dark greater coverts.
Thayer's Gull: Some of the wing patterning is similar to Thayer's Gull, but that species is excluded by the relatively large and angular head and relatively large bill of this bird. This bird was closer in size and structure to a Herring Gull, which was present for direct comparison.
Glaucous-winged Gull: Easily eliminated by the dark, nearly black, primaries. A first-winter Glaucous-winged Gull was also present for direct comparison.
Glaucous-winged x Herring Gull hybrid: This is the most difficult candidate to eliminate. The overall dark color of this bird argues against this hybrid combination, as GWGU x HERG tend to be lighter than this. Most examples of this hybrid combination have more patterned (less solidly-centered) greater coverts, but extreme examples can approach the appearance of this bird in this trait. When they do, the wingtips are paler, not as close to black as on this bird. These hybrids, like pure Glaucous-winged, tend to more gray and less brown than Slaty-backed Gulls. The legs and feet of this bird were deeper pink than most GWGU x HERG hybrids. This bird had relatively narrow wings, as evidenced by the lack of a drooping tertial skirt that is often visible on GWGU x HERG hybrids.
Other Glaucous-winged Gull hybrids: Primaries too dark for almost all Glaucous-winged Gull hybrids. Bill also too narrow and straight-sided.
this & similar species:
|I have seen this species in various ages and plumages while working as a birding guide on St. Paul Island, Alaska. I have seen many Western Gulls, Glaucous-winged Gulls, and hybrids between them while living in Washington and Oregon. I have seen many Herring Gulls of various plumages around North America, including a few observations of Vega Herring Gull on St. Paul. I have seen the most similar hybrid, Glaucous-winged x Herring, on at least five occasions in Washington, California, and Utah.|
Sibley Guide to Birds, Howell and Dunn's Gulls of the Americas, and several
authoritative gull websites, including the following:
|Observer's address:||1098 Crescent Dr.|
|Observer's e-mail address:||**|
|Other observers who independently identified this bird:||This bird was first spotted by Bryan Dixon, Mike Taylor, and Jean Lown, who identified it as a Herring Gull. Stephanie Cobbold and I saw the bird and thought it was an odd Herring Gull until I reviewed the photos later. The bird was first seen at the Logan Fisheries Experiment Station and was seen flying from there to the nearby Logan Landfill, where we relocated and photographed it. Subsequent attempts by myself and others to relocate this bird in the following week were unsuccessful.|
|Date prepared:||17 December 2013 through 16 January 2014|
|Additional material:||Four photographs|
I have solicited comments from other experts through Facebook and private emails, including these:
"The underwing seems to have the 'shaggy' look I have come to associate with juvenile Slaty-backed. The bill and head shape, short tibia, and bulging chest look great, and the plumage fits well. Many Slaty-backeds have white interior markings in the innermost greater coverts, making them less distinctive, but this one has a classically solid SBGU panel that may well be outside the range of variation of Glaucous-winged x Herring hybrids. . . . There are two Slaty-backed Gulls on this page of the Japanese Gull-Site (2nd page of first cycles) that have darkish, non-contrasting rumps/uppertail coverts, just like your bird. This is obviously an extremely variable character. With that concern out of the way, I'm much more comfortable with it being a Slaty-backed. The resemblance of some Glaucous-winged x Herring hybrids to young Slaty-backed has discouraged most people from identifying SBGUs of this age in the lower forty-eight. See Steve Hampton's page for good discussion and photos of this problem. Many birds are probably best left alone. However, the Utah bird just happens to have several features--exhibited by some but not all Slaty-backeds--that do not make sense for any such hybrid. The structure and upperwing coverts are quite unlike anything I've seen in GW x Herring hybrids (or either parent species). Hampton has photos of one apparent GWxH hybrid with plain greater coverts, but it does not have a distinct Slaty-backed structure as in your bird."
Tristan McKee, California Bird Records Committee emeritus
"Wow-- I think the scaps and coverts look really good [for Slaty-backed Gull], like the birds at http://harrisbirds.com/Slaty-backed%20Gull.htm. I really like the pale line of dots at the bottom of the median coverts. Body, head, and bill shape are good too."
-Steve Hampton, author of the famous gull identification website at http://www.tertial.us/gulls/gulls.htm