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Tuesday, 18 January 2011

The Rainham SLATY-BACKED GULL - further expert comment


Some years ago I spent a fair bit of time researching a paper on SBGUs, along with Jon King, that we never got to finish and never will. Hybridization with GWGUs on the Kommander Islands is well-documented (can’t provide the reference unfortunately) and other birders (I think Angus Wilson) have seen odd mixed pairs along the coast of Kamchatka. I think that there are some specimens of hybrids in Russian collections too. Having said all this, I don’t think this hybrid combo is all that common because the two forms don’t meet on the mainland and I saw only one presumed hybrid in Hokkaido where there are thousands of wintering SBGUs and dozens/100s of GWGU. I suspect that there are hundreds of SGBU/GWGU hybrids but not thousands.

There is a paper by a Russian (again don’t have reference anymore) that reported that Vega and SBGU commonly hybridized where there ranges met around the N. end of Kamchatka. It’s likely that many of these hybrids would winter in Honshu where vega is common (it’s rare in winter in Hokkaido). At least on my first trip, I remember experiencing quite a bit of confusion as to where vega stoped and SBGU started when it came to mantle coloration. I don’t remember being so confused on my next winter visit to Honshu when I understood the large gulls a bit better. However, Vega and SBGU are similar enough in many respects (string of pearls, brighter pink legs, etc.) that hybrids might not be too obvious at times.

As to other hybridization of large gulls in the Pacific, GWGU interbreeds extensively with several species and American Herring and Glaucous also interbreed commonly.

Nick Lethaby