Mantle Color of Slaty-backs
Dma3@aol.com
Sun, 7 Mar 1999 12:07:53 EST
The identification of the bird at Conowingo as Slaty-backed is by no means
certain or complete, but if there are problems with the bird they are of a
more subtle nature than Angus Wilson pointed out.
I thought that everyone was aware that there are two "flavors" of Slaty-
backed Gull. Here is the background for those who were not. Dwight described
the mantle in definitive plumage as "dark neutral gray to slate black." There
is excellent footage of Slaty-backs in the video "The Large Gulls of North
America," by John Vanderpoel and Jon Dunn. (Hereafter referred to as the
video.) Jon Dunn states that Slaty-back has variable mantle color ranging from
dark gray about like that of occidentalis Western Gull, to very dark gray,
darker than wymani Western Gull, and near that of intermedius Lesser Black-
backed Gull.
Mary Gustafson and Bruce Peterjohn wrote an excellent article in the August
1994 issue of Birding, which covers this variation in detail. (Hereafter
referred to as GP.) Figure 6 shows the complete range of variation in mantle
color of specimens from the USNM (Smithsonian). Birds in the video and in
photos from Japan and Alaska typically vary through the range of the left hand
three birds. The right hand bird is near vegae or californicus in color. I
believe that there are only two such specimens in the museum, and that one of
them was an extralimital record from Guam. The right hand bird may represent
an uncommon extreme or a hybrid.
Jim Stasz got some great shots with his big lens of the standing bird at
Conowingo. My overall impression from watching the bird for several hours
under all possible lighting conditions, was that it was just a bit darker than
in the photos, almost imperceptibly paler that graellsii, and obviously darker
that californicus (and hence vegae), by DIRECT comparison. In figure 6 of GP I
would say that it closely matched the 2nd from the right bird, or was a bit
darker than it. In figure 7 of GP I would say that the Conowingo bird matched
the right hand bird, and in figure 8 the 3rd bird from the left. (They are all
the same specimen, but note how the shades vary even under these controlled
conditions.)
GP report "mantle color in this limited sample exhibits a geographic trend.
The darkest Slaty-backeds were collected in Alaska during the summer. Birds
with paler mantles were collected in the summer on the Kurile Islands and the
Kamchatka Peninsula and the Commander Islands (type specimen)."
The video has extensive footage from Lake Kuril'skoye, at the tip of the
Kamchatka Peninsula. Some examples of birds with obviously paler mantles,
about the same shade as that of the Conowingo bird, are at 0.36 2.21 5.04 5.12
and 5.16 into the Slaty-back footage. Note especially the two birds with
different mantle shades at 2.21.
Grant does not describe variation in mantle color for Slaty-back, but note
that his figure 61 and photo 508 illustrate obviously paler birds, contra his
statement that they are darker than wymani.
So why does Slaty-back have two flavors? To quote GP: "Whether this
geographic trend indicates a cline, with darker birds in the north blending
into paler birds nesting farther south, a hybrid swarm, or an undescribed
subspecies, remains to be determined." Slaty-backed is reported to hybridize
with vegae Herring Gull around the Gulf of Anadyr (Handbook of Birds of the
World, vol3), and its range also overlaps that of Glaucous-winged Gull. It is
possible that this group of gulls forms a chain of intergradation analogous to
that of the Herring--Glaucous-winged--occidentalis Western--wymani Western
Gulls of the Pacific coast of North America. It has been several years since I
have looked at Stejneger's type specimen (in the USNM) from the Commander
Islands, but I believe it was a medium dark, about like the Conowingo bird. If
Slaty-backed is ever formally treated as comprising two subspecies, like
Western Gull, then it appears that the darkest bird lack a name.
Rick Blom, other observers, and myself have been discussing the possibility
of hybrids from day one. We are more concerned about the relative narrowness
of the tongue tips, obviously wider than those of marinus, but apparently at
the narrow extreme for Slaty-back, (although very close to some drawings in
Dwight). We think that an F1 marinus X smithsonianus is ruled out, but would
like to see photos of the adult wingtip of such a bird. Given that F1 Kelp X
Herring hybrids look more like Kelps that like Herrings, it is not impossible
that this is a graellsii X smithsonianus, a known hybrid. Are there any photos
out there of that beast? Can anybody help?
Will post more comments later.
Dave Czaplak