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The following response was recieved on 17 March 2000

Having read the description of the gull in Lincolnshire and seen the field sketch, I agree with Jim Barton that the bird may be a very odd Common Gull, but think that the hybrid possibility is quite likely - some of our Ring-billed Gulls must be getting their end away at European Common Gull colonies surely? I want to make a comment on the tertial pattern. The commonly quoted distinction between the tertial patterns of Common & Ring-billed (wide = Common, narrow = Ring-billed), is (as Duncan Poyser implies), a bit of an oversimplification.

My experience is that on adult Common Gull, the mid-grey base colour continues down to about 2/3 of the way along the exposed tertial and then it gives way fairly abruptly to white, usually without any 'grading'. On adult Ring-billed Gull, the (usually paler) grey continues to about the point at which the darker grey does on Common Gull, and then gradually starts to fade out over a greater distance, to white (i.e. the white tertial fringe on Ring-billed is narrow, but not clear-cut, but on Common Gull it is broad and usually clearly demarcated).

I could imagine a situation where an observer in Britain looks at a Ring-billed Gull whose tertials appear to start getting considerably paler starting at the point 2/3 of the way down the tertial, and thinks "that bird has a wide white band on its tertials", and therefore overlooks it as a Common Gull, or ....

conversely, where someone looks at a picture of a Ring-billed Gull like the one in Grant (plate 143, presumably) and says "that is a picture of a Ring-billed Gull and it has a wide white terminal band on its tertials, so the presence of this feature on a bird I've just found in Britain doesn't necessarily rule out Ring-billed Gull"

I think that Plate 143 in Grant is a bit of a Red Herring - the tertials appear to be lying a little out of place on that bird, and to my eye, the "white" clearly looks pale grey. The black of the primaries on the left wing is showing through, so could be the cause of the apparent greyness, but we can't be sure. I remain to be convinced that Ring-billed Gull can show a tertial pattern like a typical Common Gull.

More detail on the appearance of the tertials of the Lincolnshire bird would be of use (although even if they are "right" for Ring-billed, I still think there is too much else that is "wrong")

From: Steve Preddy Steve.Preddy@cableinet.co.uk