I have contemplated writing this for some time. Now, with the recent discovery
of what is sure to be Ontario's 5th record of Slaty-backed Gull, and so many
birders in the province actively searching for this species, I thought it wise
to point out a significant identification feature that is seemingly being
ignored, or at least overlooked, by those wanting to clinch the ID of a
suspected adult individual. I don't suppose that many reading this post have
actual experience with this mega-rarity, so skill level and expertise in gull
identification may not matter much if one is limited in his/her knowledge to
what the field guides indicate. The species is very distinctive, and even
novice birders should be able to distinguish it from possible confusion
species, given adequate views. However, many birders may be unaware (and
understandably so) that Slaty-backed Gull occasionally hybridizes with the
"Vega" Herring Gull (if you follow the AOU taxonomy) as well
as Glaucous-winged Gull. Consequently, in the case of adult birds, there are
notable plumage differences between pure adult Slaty-backeds and those of mixed
parentage which should be looked for.
Jon R. King and Geoff J. Carey addressed this identification concern in an
excellent article likely unfamiliar to many subscribers to ONTBIRDS:
"Slaty-backed Gull hybridization and variation in adult upperparts colour"
[Birders Journal, Vol. 8 No. 2 (April and May 1999) 88-93]. Gull watchers in
possession of this issue would do well to re-read it, since discovery of
Slaty-backed Gulls in Ontario is clearly not as pie-in-the-sky as it may once
have seemed to be. For birders not so lucky, I will summarize the main points
which should be borne in mind when trying to confirm the identification of a
possible adult Slaty-backed.
(1) Adult Slaty-backed Gulls are very dark and uniform in upperparts
coloration, i.e., the dorsal surfaces of the mantle, scapulars, etc.
(2) Pure adult Slaty-backeds should be very dark bluish gray with black (not
dark gray or gray) outer primaries.
(3) The upperparts are clearly and consistently darker than our graellsii race
of Lesser Black-backed Gull, as well as "Vega" Gull and, of course,
Glaucous-winged Gull.
(4) For those who are familiar with Black-tailed Gull, the upperparts are
comparable in shade to the darkness of this species (and may not be appreciably
different).
The authors note that while hybridization is by no means extensive, it does
occur regularly, particularly in the northern and northeastern portion of the
species' range. (I never saw an adult Slaty-backed Gull in Korea that I thought
a potential hybrid.) Since vagrants to North America are most likely to have
originated in regions where hybridization is known to occur, it suggests the
need on our part to go one step further in our systematic identification. What
the field guides -- and even Jon Dunn in The Large Gulls of North America video
-- tell us is not enough. Seeing the "diagnostic," so-called "string of pearls"
is not enough. Unless we can at least state with certainty that the outer
primaries of a probable adult Slaty-backed Gull are indeed jet black, there may
be legitimate doubt as to whether the bird is of pure Slaty-backed descent.
Therefore, until the possibility of a hybrid gull is satisfactorily ruled out,
perhaps we should qualify our
tentative identifications with the p-word: "putative."
(Note: For immature Slaty-backeds, ruling out potential hybridization is
probably impossible at this time.)
Cheers,
Randy Horvath
Windsor, Ontario and Busan, ROK
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