Herring Gull - Zilvermeeuw (argentatus & argenteus)
(last update: 08 december 2003)
(2 images) Herring Gull adult (argenteus), May 13 2002, Dintelhaven, Maasvlakte, the Netherlands (51.59N,04.02E).
By mid-May the inner primaries are shed, nicely illustrated by this individual which has dropped P1 (Primary Moult Score - PMS: 1). This bird died today, hit by a car. It shows the common black pattern on the outer primaries for local Dutch argenteus. A black sub-terminal band can be found on P5. This band is broken at the centre and broadest along the feather edge. A no white spot is visible between the sub-terminal band and the grey centre of P5. Both P9 and P10 have a mirror. The round mirror on P9 is confined to the inner-web. The P10 mirror merges with the white tip, though there is an indication of black sub-terminal marking at the feather's edge, on both the inner and outer-web. The black sub-terminal band limited as triangular spots on the edges of the inner and outer-web. The black patch between the apical spot and the white inner-web (the medial band) is very broad along the edge of the inner-web on P10. Argenteus normally shows such a broad medial band, where it is restricted in argentatus. Note also the diffuse division line between the grey centre and the black outer-web of P9 (and also P10). Such a diffuse division, with much irregularities ("saw-blade motive") can be found commonly in Herring Gull, where e.g. cachinnans often shows a neat clear division between the black outer-web and the whitish centre. From below, the pale centre of P10 runs down gradually to 1/2 of the feather on the inner-web.
Argenteus has a partial moult of body and head in spring. It starts in January (when adults in the Netherlands may already return to the colonies) and is completed by April, leaving birds in so-called "adult summer" plumage. The head is clean white and the white primary tips are slightly worn and the dark patches bleached. Otherwise, this plumage much resembles the "adult winter" plumage. The bare parts are more saturated, as the breeding season has started.
From June (often mid-May in Belgium and
the Netherlands) to October, a complete moult will bring birds in
so-called "adult winter" plumage. A new set of primaries will
grow during the summer months and by mid May, the first, innermost primary
will be dropped in argenteus in the Netherlands.