Early July and from
July 26-30 2001 we did small surveys on Yellow-legged Gulls at Le
(near Boulogne-sur-Mer) and Etaples (near Le Tourquet). In these surveys, we
scored moult stages to determine timing of moult in primaries (and wing-
coverts and body-feathers) in sub-adult michahellis.
By June, sub-adult michahellis
occurs only in small numbers at Le Portel and the near surrounding.
But by July, more Yellow-legged Gulls disperse to northern quarters
to rich feeding grounds and to moult the primaries. At the dumping site of Dannes,
near Le Portel, mixed flocks of mainly Lesser Black-backed
Gulls (100's) from NW and N Europe can be found including tens of
adult and sub-adult Yellow-legged Gulls.
Regarding identification: the difference between 3cy birds and 4cy
birds can best be established by the outer primary, which is still
unshed by July: 4cy birds show a small but obvious white mirror on
P10, while 3cy birds have the (second generation) outer primary all
brown. In birds with very worn outer primary tips it may be
difficult to ascertain whether a mirror has been present or not. Other
clues can be found in the bare part coloration, with 4cy strongly
moult in July:
As can be seen in most of the images,
sub-adult Yellow-legged Gulls look very untidy and mottled in their
July plumage. Simultaneous moult is in progress in the primaries,
wing-coverts, tertials, scapulars and the
tail-feathers. Fresh moulted wing-coverts can be found in
the median row and outer lower lesser coverts in most cases and these fresh grey median coverts
normally strongly contrast with the brown-hued lesser coverts and
old greater coverts. The old coverts have very abraded fringes and
often a brown hue. By early July, the
upper tertial has been shed in many birds, as is the case with the
stage and pace of remiges and rectrices:
The table below shows the average
score of immature Yellow-legged Gulls, range between 3cy birds to
5cy birds. Individuals in 3cy show no mirror on P10, 4cy birds have
various wing-coverts, tertials or tail-feathers with black markings
and 5cy birds are very adult-like. All birds showing black on the
bill combined with dark outer primary coverts were included in this score.
It's hard to predict the score of new primaries; too many resting
and sleeping birds could not be scored on the new fresh grown inner
primaries (over 35% of the immature birds present were not scored in
detail and are clustered in the table under "P?"). The
average score of old primaries is 8.1, hence on average less than
two old primaries are still present in the last week of July.
In 3cy birds, the early-July score
averages 7.0 (n=33 on July 10 2002), so three old primaries left. By the end of July, the average primary moult score is
8.5 (n=39 on July 29 2001; n=53 on July 30 2001), only slightly
higher than 4cy michahellis.
In adult birds, the early-July
averages 6.3 (n=66 on July 11 2001), so more than three old
primaries left in most birds. By the end of July, the average primary moult score is
7.6 (n=50 on July 28 2001) and 7.7 (n=56 on July 29 2001) in adults,
so slightly behind compared to 4cy michahellis. Adult birds
were selected on plain grey primary coverts.
By the time P4 is fully grown, 2cy
birds normally start the complete moult in secondaries and rectrices. 3cy birds
start slightly later, when P5 is fully grown, although individual
variation may exist. Sub-adult michahellis probably follow a
moult timing similar to 3cy birds. Old tail-feathers may still show
much black, typically found on third generation rectrices, but old
feathers are not necessarily third generation in michahellis.
As can be seen in previous sections, tail-feathers may be included
in the partial autumn moult in both 1cy and 2cy, so old
tail-feathers in July sub-adults may be completely white as well.
However, the new white tail-feathers can be recognized by the neat
fringes and tips.
Secondary moult hasn't started yet in late-July, but
is initiated in early-August.
strategy of body & covert moult in July
Wing-covert moult in 3cy and sub-adult michahellis vary
between individuals. In general, conclusions based on average sub-adult michahellis
- Moult of head feathers is in
progress, creating a "moth-eaten" appearance.
- sub-adult birds show plain adult-like
grey scapulars. It's
difficult to score missing feathers in the complete grey
upper-parts, hence we have no scapular moult scores. In general,
the upper lower scapulars look very fresh, the lower upper
scapulars look older and worn and some of the scapulars in the
lowest row seem to be missing in sub-adult michahellis by
the end of July.
The complete wing-covert moult is in progress throughout July. The
ordinary moult strategy and sequence start in the outer median
coverts, the innermost medians and innermost greater coverts, the upper tertial
and the outer lower lesser coverts. Early July, the new median
coverts are visible (first the outer median coverts), more so as
the outer lower lesser coverts are dropped. By the end of July,
the complete median covert bar is renewed, the lower lesser
coverts are growing, the outer greater coverts are missing
(exposing the secondaries underneath) and the innermost greater
coverts have been renewed. In the tertials, the new upper two
feathers are visible and the central tertials are dropped.
Section for explanation of feather tracts.
(3cy-5cy) remaining old and fully grown new primaries, July 28
m-old (average): 1.9 remaining old primaries; so moult score at
SD = 0.77
P?: many birds sleeping, fully grown primaries hidden by
tertials, so P4 or less.
Aggregated survey done at resting and preening sub-adults
in 3cy, 4cy and 5cy (with at least black central markings on
the outer primary coverts).
*: including one bird with P2 fully grown.
01: michahellis sub-adult, July 10 2002, Etaples, NW France.
A sub-adult with active moult in the
tail-feathers and some black on the cutting edge of the lower mandible.