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This page describes in detail the underwing of First-basic (First-winter) Larus cachinnans, using a photo of an individual from Germany in September by Ronald Klein. I feel there is a complex but shared pattern that can be seen on most published photos of this age class. Before stating my impression of the underwing, I'll list the photos I know about that show the underwing of 1B birds:
ALULA 3/98 vol 4; page 87, fig 17 (a very pale bird)
LIMICOLA Vol 11 Number 2; page57 abb 8; page 59 abb 10; page 64 abb23 (all pale birds)
BRITISH BIRDS Vol 90 Number 9; page 372 plate 103; page 379 plate 128 ( palish birds)
BIRDING WORLD Vol 13 Number 2; page 64 plate 7 ( a darkish bird)


NOTE: I use the expression "medium" and its variations to denote a tone of shading, i.e. it is in-between dark and pale in terms of shade:-
The marginal coverts (A) are usually striped dark/light, continuing onto the first row of lesser coverts and forming a "medium" narrow leading edge.
The next row of lesser coverts (B-Q) are often whitish, forming a pale narrow band; sometimes this band is lightly marked and hardly any paler than the leading-edge band.
Immediately next to the pale band there is a row of lessers (from B through R) that are again striped dark/light, forming a very narrow "medium" band; sometimes this band merges with the outer axillaries (O) if these are well-marked, but often this band forms a diffuse "medium" leading boundary to the pale outer axillaries.
On the inner-third/half of the inner wing (out to point N), the axillaries cover the remaining lesser coverts and part of the bases of the median coverts.
On darker birds the inner axillaries (P-O) are usually more-heavily marked than the outer axillaries, with transverse dark barring and the bars getting thicker distally such that the trailing diagonal edge of the inner axillaries looks darker than the leading edge.
On darker birds the outer axillaries (O-N) are usually paler than the inner axillaries, with fewer dark bars and quite often with no visible bars at all - except that the trailing diagonal edge to this tract is often tipped with a broad transverse bar, thus extending the darker trailing edge of the inner axillaries onto the outer part of the axillaries.
The remaining lesser coverts beyond the axillaries (bounded by B, C, and N) often have a striking pattern of dark/light/dark, formed by a whitish band sandwiched between two bands of very darkly-pigmented feathers - often the darkest area of the inner underwing. Sometimes the dark outer bands pinch together (at N) creating a pale oval in-between.
The median coverts (C-M) are usually mostly pale basally, with a "medium/light" subterminal band; sometimes there are a few "medium" flecks in the largely white basal area, but this band of feathers is normally the palest zone of the inner wing.
The greater coverts (M-E) are usually darker than the medians; sometimes just slightly, but often they are at least one shade darker - but still in the "medium/light" range; there is usually a discernable contrast with the whiter medians. Keep in mind that at certain angles and wing positions the greater coverts can be largely hidden from view.
The secondaries (E-L) are darker than the greater coverts, with at least the distal half of these feathers falling in the "medium/dark" range and thus forming the third "rung" in the white/medium/darkish banding on the trailing half of the inner wing.
Importantly, the darkish secondaries contrast with the pale underside of the primaries - especially the very pale inner Ps and primary coverts.
P1 is normally just slightly darker than P2 - P4 (K), which are very pale; P5-7 have pale bases (G) and "medium" tips (J); P8 and P9 are similar but often have the "medium" tone of the tip extending up the outer half of each feather for about three-quarters of the way to the coverts (H) while P10 is "medium" on both webs all the way to the base ( or almost so).
The greater primary coverts (F) are very pale, while the median/lesser primary coverts (D) are irregularly-mottled light-dark.
Put all this together, and you get a distinct pattern that, despite variation in intensity in these zones, appears fairly constant on the photos I have been able to find. Keep in mind that on typical, very pale birds, the above features are much less evident. I feel that heuglini has the most similarity to this underwing pattern (and some may not be distinguishable; I need to see more photos), while most other forms seen in NW Europe differ in a number of ways from this pattern, including michahellis. Of course, this is just a theory and my sample is small, thus I'd appreciate comments and especially access to photos that confirm or refute my suggestions - thanks.