Herring
Gull - Zilvermeeuw (argentatus & argenteus): sub-adult July
(last update: 24 maart 2004)
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(2 images) Herring Gull 5cy (argentatus), July 09-17 2003, Tampere, Finland (61.31N, 23.43E).
Two different 5cy argentatus with open
wings, showing the pattern in old fourth generation outer primaries and
fifth generation inner primaries.
The complete moult has started in all 5cy birds with wing-coverts and the inner primaries recently
moulted. The outer primaries are old fourth generation. In all cases, there
is an obvious mirror on P10, and a small mirror on P9. The inner primaries are fully
grown fifth generation flight-feathers, showing the pattern of adult birds. The old tail-feathers
are often completely white and the secondaries appear adult-like grey.
Most obvious immature features are more black on the bill in many birds
and black markings on the outer greater primary coverts. Moult in the
wing-coverts is obvious with the median and many lower lesser coverts
replaced and often the central and outer greater coverts missing. By
mid-July, rectrices and secondaries are not included in the complete moult
yet. The colour of legs is
very adult-like, either pink or yellow (common in SW Finland). The
iris may be very dark, but may also be completely clean yellow, both are
commonly found in 4cy and 5cy argentatus.
From June to December, sub-adult argentatus undergo a complete moult resulting in so-called "winter plumage". From June onwards, the new plain grey wing-coverts are moulted in, starting with the outermost median coverts and followed by the inner medians, the inner greaters, and the rest of median and lower lesser coverts. By August, the first neat streaks can be found on the crown and hind-neck. After the complete moult is finished by late autumn, the head will show extensive 'winter streaking' in argenteus, often neat fine streaking in sub-adult argentatus. Also, by the end of the complete moult, the scapulars, mantle and all the wing-coverts will be replaced and near-adult argentatus can hardly be distinguished from full adult plumage (except more black on the bill and often the dark centres in the outer greater primary coverts).
During the summer, the primaries are replaced. In argentatus, the last outer primary P10 will be fully grown by early December, slightly later than in West-European argenteus. However, first data from Tampere, Finland indicate that 3cy argentatus from this region are not delayed in primary moult compared to argenteus.
Adult northern argentatus show a
different pattern in the outer primaries, compared to argenteus. In
argentatus, especially from northern Scandinavia, the black markings
on P5 are very limited and, if black on P5 is present, this black often
has diffuse edges. More often, these black sub-terminal markings are
completely lacking on P5. In Baltic argentatus populations,
normally black on P5 is confined to the outer-web, with sharply defined
edges.
Argenteus normally has a black sub-terminal band on the top of P10,
dividing the tip from the mirror. If the black sub-terminal band on P10 is
broken, the outer-web of P10 still shows black marking. Argentatus
normally completely lacks sub-terminal markings, also on the outer-web of
P10. Some argentatus completely lack black on the edge of the
inner-web, creating a so-called "thayeri pattern".
