"It seems if you study a photo too hard you lose perspective. Your "big" Kumlien's Gull is certainly not that species - not even close. It is without doubt a Herring x Glaucous Gull. You must be aware that Glaucous Gull bill size has considerable variation among races, as well as sex. Your gull's bill is well within normal variation for Glaucous."
Response to Jake:
Glaucous Gull is indeed made up of various races or subspecies. The nominate subspecies Larus h. hyperboreous is the subspecies native to Canada with the nearest other subspecies being Larus h. barrovianus native to northern Alaska. I am aware of no other subspecies of Glaucous Gull other than the native nominate subspecies being recorded in Ontario. So I think "race" is not an issue. I fully agree that the bill size may well be in the range of Glaucous Gull. If you review the measurements in Olsen & Larrson their is overlap between male Kumlien's and female Glaucous in all but one of the bill measurements. The outlier was Bill Depth at Base with a less than 1mm difference. This is really my point that size and structure are very broad and overlapping even between Kumlien's and Glaucous Gulls.
Adult Kumlien's Gull- supersized
Adult Kumlien's Gull- supersized with open wings
My final determination of this bird as a Kumlien's Gull and not a Nelson's Gull is based on the primary pattern not size or structure. The primary pattern is near identical to Type e pattern of Kumlien's Gull Adults as described by Howell and Mactavish in 2003 and illustrated subsequently by Martin Elliot. This great aid to Kumlien's Gull identification is illustrated on page 252 of Howell and Dunn's - Gulls of the Americas. It strikes me as most unlikely that a Nelson's Gull would end up with a near perfect Kumlien's Gull primary pattern.
Wing close up showing Type e Adult Kumlien's Gull primary pattern
Although you state "your "big" Kumlien's Gull" is not that species- not even close"you fail to provide any rationale or explanation for your obvious certainty in this statement. If you are basing this on the size and structure of the bird I have clearly failed to make my point to you that this can be very misleading and is not definitive.
I would be most interested in the rationale for your claim "that this is without a doubt a Herring x Glaucous Gull". I would be pleased to add the comments to this posting. If anyone else feels this is a Nelson's Gull I would be keen on hearing why and will add the comments to the Blog.
Continue on for Part 2 of Does size Matter-Thayer's Gull