Gull 5.365.8362cy (argenteus), April 19 2002, Boulogne-sur-Mer, France (50.42N,1.34E).
A bird with a Dutch ring: Vogeltrekstation Arnhem 5.365.836. Ringed as pullus July 07 2001 at Neeltje Jans - Oosterschelde. Now in 2cy, this individual shows the common moult stage in 2cy argenteus by April: the juvenile wing-coverts and tertials are abraded, the primaries are bleached brown and the bill-base starts to turn paler. The upper lower scapulars have been replaced for second generation feathers, with the last moulted scapulars showing buffish tones on the centres. Arnhem 5.365.836 was seen again in October 2002 (see below).
Argenteus will show a second partial moult of head and body-feathers in spring. This partial moult starts in January and ends by May, bringing birds in so-called "first summer" plumage. The head and under-parts will turn white by June.
The juvenile tertials, lesser, lower lesser and median coverts have a notched pattern, worn by April. The juvenile greater coverts show a 'piano-key' pattern; in argenteus on the outer greater coverts as well. This pattern has gone lost as the white parts of the feathers are worn away. The juvenile tertials are very abraded as well.
The secondaries, primaries and primary coverts are bleached, but the juvenile inner primaries still show an obvious pale window, prominent from below and above. The under-wing is rather uniform grey-brown patterned. The tail has a broad sub-terminal band and isolated dark bars on the basal half.
Research in the 1990's (e.g. H. Vercruijsse) revealed that at least 50% of the Dutch argenteus from SW Netherlands are true migrants, while the other 50% stay within 100 km's of the colony (limited dispersal within the delta of large rivers in the SW of the Netherlands). Migrants follow the coastline south and the majority winter at the coast of Belgium and NW France. Once the wintering grounds are established in the first year, birds are very bound to this particular area, even wintering in the same harbour, using the same resting places and feeding grounds. This habit is found in other species as well (Glaucous Gull, Pontic Gull, etc).
In this bird, all the primaries, tertails, wing-coverts and tail-feathers are still juvenile. In some birds it's very difficult to ascertain whether the inner coverts are still juvenile or whether they are already moulted to second generation, barred feathers, which appear very similar to the juvenile feathers.
By April, some argenteus will show three generation of scapulars: still a few juvenile scapulars in the rear lowest row (first generation), a few very fresh lower upper scapulars with a clear buffish tone and a neat white fringe, still with a similar pattern as the second generation feathers (third generation) and most scapulars are slightly worn upper scapulars and upper lower scapulars (second generation).
In this bird, in the rear lowest lower scapulars, the last juvenile scapulars can be found. All other scapulars are slightly worn second generation feathers. Just visible on the back are the last moulted second generation lower scapulars, which have a greyish base and a brown barred pattern. There is still a neat white fringe at the tip of those fresh scapulars.
image above: 5.365.8362cy, April 19 2002, Boulogne-sur-Mer, France (50.42N,1.34E).
image below: 5.365.8362cy, October 04 2002, Boulogne-sur-Mer, France (50.42N,1.34E).
From late April onwards, argenteus has a complete moult during summer to so-called "second winter" plumage. This complete moult will be finished by October and overlaps with the partial autumn moult which starts in September and often include scapulars, a few inner wing-coverts and upper tertials. The head has been moulted in July-August and shows neat fine streaking by October. The under-parts and rump are pale, mottled with brown. From mid-May, the scapulars and mantle were replaced for third generation feathers, starting with the replacement of the lower upper scapulars. The pattern of the third generation scapulars is more or less similar to the anchor pattern of the second generation feathers. By October, argenteus acquire some new adult-like pale grey scapulars.
From early April, the inner primaries were shed and new second generation primaries grown in. On average, the last juvenile primary (P10) was shed in the first week of August. The majority of argenteus complete the primary moult by the first week of October, when the new second generation P10 will be fully grown. The new primaries are dark with a tiny pale tip on the fresh flight-feathers. The four inner primaries show an extensive pale window, as both inner and outer-web are pale greyish white-brown, contrasting with the outer-wing. A pale inner-web can still be found in P7.
The second generation tail-feathers show a clear-cut blackish tail-band with only isolated blackish markings on the white basal half.
By early October, argenteus finish the complete moult, with the central greater coverts and outer lesser coverts moulted last. Between September and November, the average 2cy argenteus start a simultaneous partial autumn moult in the wing-coverts, including the upper tertials, some median and lower lesser coverts and often the inner greater coverts.
This 2cy argenteus has finished the complete moult, as P10 is fully grown. All rectrices and secondaries are fully grown as well, replaced by second generation flight-feathers. This Dutch 2cy argenteus is illustrative for argenteus replacing wing-coverts in the partial moult: the two upper tertials, the two inner greater coverts, the two inner median coverts plus median coverts #7-9 and #11-12 are all replaced for third generation feathers. The new feathers are easily recognized by the warm buff centres.