Thanks Jan, it more or less echoes a writing we prepared after seeing similar kind of birds in Zagreb. And it also brings me back to the start: 'what is the definition for features of adult heuglini'? (asked myself this before, like in adult smithsonianus or adult cachinnans. Character set and classification of score options. Now does this also occur in local birds?).
This reflects my feelings about accepting out-of-range heuglini. I have no problem calling these birds heuglini, fair enough, but can we be specific on the set of features (maybe we can even define a diagnostic set?). On the German forum Martin Gottschling made a start, late moult (P9 growing, P10 old), black on P2-P10, and Martin Becker added black on primary coverts and broken black band on P3.
It remains to be seen if this combination can be found in one single western bird.... So far, it seems single features are possible in western birds, but simultaneous occurrence of all 4 features? I have not seen such a bird yet.
Some traits may be not very useful to add to the set, like black on primary coverts. From Moerdijk research we know:
The majority (76%) of breeding Lesser Black-backed Gulls showed black on the primary coverts (n = 962) and we found no marked difference between the sexes (males 78% (n = 434); females 74% (n = 496); Pearson χ2 = 2,622; P = 0,105; df = 1). Our data of known-age Lesser Black-backed Gull (n = 76) suggest that both the number of primaries showing black (b=-0.071, r2=0.072, P=0.020) and the presence of black on the primary coverts (b=-0.189, r2=0.046, P=0.068) are age-related (table 2). We scored 54 adult birds of known age in 6cy-14cy. 41 birds (75%), in all nine age-classes, were found showing neatly demarcated black pigmentation. 62 breeding LBBG in our sample are considered to be ‘sub-adult birds’ showing primary coverts with extensive black markings, pale brown tips and diffuse borders. 666 birds show limited black markings which are neatly demarcated; we consider such birds to be ‘mature adults’ (see plate 3).
Source: http://gull-research.org/moerdijk0506/m ... mcovs.html
Number of primaries with black is good to include, as only 1% (n = 931) of Dutch birds showed 9 primaries with black:
The average number of primaries with black pigmentation in Lesser Black-backed Gull was 6.9 (SD 0.61, n = 931), and the sex difference was just significant (Mann-Whitney U = 100938, Z = -1.992, P = 0.046, male n = 433, female n = 498). In Lesser Black-backed Gull too, the number of primaries with black is age-related, when sub-adults are compared to known-age adults (sub-adults: mean 7.2, SD 0.68; adults: mean 6.7, SD 0.64, Mann-Whitney U = 1000, Z = -4.071, P < 0.001, sub-adult n = 62, adult n = 53. See table 4). However, we found some exceptions, as older birds like 10cy LBBG o67, 12cy LBBG yET, 12cy LBBG o65 showed 8 and even 9 primaries with black. When sub-adults (62 birds, of which 70% are males) are excluded from the analysis, the difference between the sexes is no longer significant (Mann-Whitney U = 89591, Z = -1.294, P = 0.196, male n = 390, female n = 480).
Source: http://gull-research.org/moerdijk0506/m ... black.html
Maybe it's good to check other features in a similar way, for both populations, and then compare the two populations?