Herring Gull - Zilvermeeuw (argentatus & argenteus): sub-adult July

(last update: 24 maart 2004)

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(3 images) Herring Gull 4cy (argentatus), July 14 2003, Tampere, Finland (61.31N, 23.43E).

4cy argentatus with open wings, showing the pattern in old third generation outer primaries and fourth generation inner primaries.
The complete moult has started with many wing-coverts and the inner primaries recently moulted. The outer primaries P6-P10 are old and still third generation with a diffuse mirror on P10. The inner four primaries are fully grown fourth generation flight-feathers. The old tail-feathers and old secondaries show many dark markings the tail somewhat resembles 3cy birds (see also this ringed bird). 
Fully grown P4 and old P6 is a very small moult gap for Herring Gull, and this is mirrored by the moult in the wing-coverts. There are hardly any moult gaps and only a few feathers have been replaced: the outer median coverts and the upper tertial. Rectrices and secondaries are not included either. The colour of bare parts is much adult-like.

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".