Herring
Gull - Zilvermeeuw (argentatus & argenteus): sub-adult April
(last update: 14 september 2004)
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Herring Gull 4cy (argentatus), April 19 2002, Boulogne-sur-Mer, France (50.42N,1.34E).
The scapulars appear completely adult-like grey (and create a "saddle effect") but in the wing-coverts and on the tertials many brown-black markings are visible. Especially in argentatus the immature pattern on most wing-coverts is prominent: The wing-coverts are barred, with the dark delicate lines thinner than the pale bars, strongly contrasting on these pearly white basis. In the central greater and median some feathers appear rather plain adult-like grey. These are probably the last moulted feathers in the wing-panel. This is supported by a few feathers in the carpal edge, which are similarly grey.
The tail is all white and the primaries show a clear mirror on P10. In the complete summer moult, 3cy argenteus replace the all-brown immature second generation primaries for more adult-like primaries, which show a white tip, a grey basal half and one or two mirrors (P9 and P10). Still, the size of the dark patches and the amount of black in the outer primaries is more extensive when compared to adults: there is often a more black in the sub-terminal band on P5. The dark patches are not sharply demarcated as in adult, but the dark outer-web gradually dissolves in the grey inner-web. Also, in most near-adults the primary coverts are not completely grey but still show a black centre in the outermost greater primary coverts.
From January to April, sub-adult and adult argenteus moult to so-called "summer plumage". By April, the head and under-parts appear all-white in argenteus, but argentatus has this partial moult slightly delayed and and as result may appear strong patterned on the head by April. The tail-feathers, wing-coverts and primaries are not replaced in this partial spring moult, therefore most birds strongly resemble "third winter birds", although the white primary tips wear away and dark patches start to bleach. Compared to sub-adult argenteus, the change in colour of the bare parts is delayed in argentatus as well: the iris is still not bright yellow in many sub-adult argentatus, the orbital ring is dark and the bill still has a pinkish base and obvious bill-band, completely obscuring the red gonydeal spot by April.
