Albert de Jong
Larus (cachinnans) mongolicus AL59 adult, May 26-28 2013, Telmen Nuur, Mongolia (48°49'N, 97°10'E). Picture: Andreas Buchheim.
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Adult bird, trapped in NE Mongolia. Mongolicus adult AL59.
Orbital ring (orange-)red.
Bill colour - from: Phenotypic variation and systematics of Mongolian Gull, by: Pierre Yésou, in: Dutch Birding 23: 82, 2001.
The bill is yellow, varying from pale yellow to bright orange-yellow, commonly with a paler tip. The red gonydeal spot usually does not reach the upper edge of the lower mandible, falling short by 2-3 mm. Dark markings (spots or broken lines in front of the red gonydeal spot) occur in one out of three birds. Of 107 adult-plumaged birds trapped at nests and examined in the hand, 17% showed dark markings on the upper mandible only and 15% on both mandibles (in one extreme case, the markings formed an almost complete dark bill band of c 3 mm width).
Eye colour - from: Phenotypic variation and systematics of Mongolian Gull, by: Pierre Yésou, in: Dutch Birding 23: 82, 2001.
The orbital ring was vermilion-red in all adult-plumaged birds I examined in the hand. Pyzhianov & Tupitchyn (1992) described the iris as ranging from pale ashy-grey to olive-grey to dark smoky-grey. 54 and 68% of their samples of Lake Baikal and Lake Khubsugul, respectively, were pale eyed. They found no correlation be tween iris and leg colours (a point I did not check).
FIGURE 2 Variability of iris pigmentation Mongolian Gull Larus (cachinnans) mongolicus at Lake Baikal, Siberia, Russia (redrawn from field sketches of birds in hand) (Pierre Yésou). Source Surfbirds: IDENTIFICATION OF ADULT VEGA GULL: FIELD OBSERVATIONS FROM JAPAN, FEBRUARY 2003 by Chris Gibbins.
In the birds l examined in the hand, the iris was dull yellow (very pale, almost whitish, in some), usually peppered with grey (pale bluish-grey to dark grey-brown) minute spots in variable density over a much variable extent of the iris surface. In some birds, the grey was peppered quite uniformly over the iris, often with a few small aggregations forming dark marks on the overall pale eye. In other cases, dense grey spots formed one or more large dark areas over the eye while sparse spotting left the yellow iris colour showing over the rest of it (figure 2). Eye darkness is not sex related. Pale eyes and dark eyes are found in both males and females and pairing occurs irrespective of the iris colour (of eight pairs, the male had darker eyes than the female in four cases and the female had darker eyes than the male in three cases; the partners of the remaining pair showed similarly coloured eyes).
Such a high variability of iris pigmentation makes it difficult to accurately comment on the eye colour of mongolicus. In a first analysis of 43 birds in the hand, l classified 28% of them as pale eyed, 42% as intermediate and 30% as dark eyed. A more detailed analysis, including two more samples (table 3), showed that nearly one-third was pale eyed while grey spots covered more than half of the iris surface in another third. Really dark-eyed birds, however, accounted for less than 10%.
In the field, colour assessment is less easy. Of 236 adult birds studied through binoculars or telescope, 89% were classified as yellow or yellowish eyed. It thus seems that eyes classified as pale grey in the hand are perceived as yellowish in the field while dark-eyed birds accounted for only 4% in the field sample (not statistically different from the 8% found during the in-hand examination).