More on Slaty mantles
Dma3@aol.com
Fri, 12 Mar 1999 06:11:50 EST
Hi Ospreyers:
More info along the same lines from Bird ID Frontiers. Note especially Nick
Lethaby's comments about the first photo in the 1994 Birding article by
Gustafson and Peterjohn. The Conowingo bird is close to this photo in mantle
color, width of trailing edge, tongue tip pattern, and head shape. Nick
Lethaby thinks that it is a a hybrid. It is clear that much more taxonomic
research needs to be done on gulls from this part of the world, and that
identification of the Conowingo birds as a Slaty-back seems very premature.
Dave Czaplak
Reply-to: nick@ELANIX.COM (Nick Lethaby)
To: BIRDWG01@LISTSERV.ARIZONA.EDU
>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.
To begin with, none of these observers to my knowledge has actually spent
any time in Asia looking at numbers of Slaty-backed Gulls. Therefore, their
conclusions are based on limited data.
(Hereafter
>referred to as GP.) Figure 6 shows the complete range of variation in mantle
>color of specimens from the USNM (Smithsonian).
This assumes that the specimens are correctly identified as pure
Slaty-backed Gulls, which seems unlikely given the lack of knowledge even
now on E. Asia gulls
> 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
>
The problem with the cline theory is that birds in Hokkaido are not
particularly pale mantled. Let's look at the origin of the birds with paler
mantles:
- The type specimen from the Commanders. The main gull species here is GW
Gull. Interbreeding is proven here are believed to be regular. I think the
paper I have lists 4 apparent SB Gulls present (although even these had a
long call like GW Gull) but about 30 hybrids. On probablility, the specimen
is likely a hybrid.
- The Guam specimen. Guam is further south than Hong Kong. I checked 5
years of Hong Kong Bird reports and all SB Gull records were first-years.
In contrast, adult Vega Gulls are regular there. Therefore an adult gull
with a Californicus-gray mantle in Guam seems much more likely to be a Vega
Gull.
- The Video footage from Kamchatka. Again there is a proven record of
hybridization from Kamchatka. In addition I recall Angus Wilson finding 2
mixed pairs in the one colony they studied on the coast there. Therefore
any evidence from this area (and quite possibly the Kuriles) needs to
explain why GW Gull hybridization is not the cause.
- The photo from Hokkaido of a paler mantled bird in GP. I have been to
Hokkaido and did see one such bird amoung hundreds if not thousands of
birds that show no such variation. I feel it is easily explained as a hybrid.
- The OH and MD birds. These birds are extralimital and cannot be used as
evidence of pale-mantled Slaty-backed Gulls. The MD birds at least has
numerous other features (head shape, primary projections, leg color,
indistinct white tongues) that suggest it is not a SB Gull.
In Honshu, I find it difficult to distinguish between some Vega and
Slaty-backed adults. However, this seems likely due to variation in Vega
rather than Slaty-backed. Vega is known to hybridize extensively with
taimyrensis and the resulting 'swarm' shows a wide range of mantle
variation. In Hokkaido, where Vega is rare in mid-winter and SB Gull
abundant, I do not see varying shades of gray. I think the concept of SB
Gull having a subspecies with a more blackish mantle (a la
occidentalis/wymani) deserves some investigation. However, I suspect the
Russians would have already found this, given their splits of obscure
warblers etc.in the same area.
I would also point out that if I went to any beach in OR in winter I could
videotape lots of "Western Gulls" with mantles as pale or paler than
California Gull. However, these are generally believed to be hybrids with
GW Gull. Why should SB Gull be any different?
Nick Lethaby