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
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
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
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.