Herring
Gull - Zilvermeeuw (argentatus & argenteus): sub-adult April
(last update: 14 september 2004)
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Herring Gull sub-adult (argenteus), April 20 2002, Boulogne-sur-Mer, France (50.42N,1.34E).
This bird strongly resembles an adult, but note the tail and the brown primary coverts, just visible beneath the central greater coverts. Its a male, showing courtship and mating with a full adult plumage female argenteus, so probably a local breeder? This could be supported by the orange-red orbital ring, which is typically yellow-orange in argenteus from Belgium and the Netherlands. First preliminary data on argenteus breeding in the vicinity of Boulogne-sur-Mer seem to differ in this respect.
The scapulars and wing-coverts appear completely adult-like grey. The primaries show a clear mirror on P10. The outer primary-covert are dark, unlike the grey adult-like primary-coverts of adult birds. 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). One year later, 4cy argenteus moult the primaries to fourth generation flight-feathers. 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. The tail-feathers, wing-coverts and primaries are not replaced in this partial spring moult, therefore most birds strongly resemble "winter birds", although the white primary tips wear away and dark patches start to bleach. The iris turns bright yellow in many sub-adults, the orbital ring turns yellowish-orange and the bill will become predominantly yellow with a red gonydeal spot and limited black markings along the culmen by April.
