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

(last update: 14 september 2004)

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Herring Gull sub-adult (argenteus), April 12 2001, IJmuiden, the Netherlands (52.27N-04.33E). 

The scapulars appear completely adult-like grey but in the wing-coverts and on the tertials many vermiculated brown-black markings are visible. In many sub-adult argenteus the wing-coverts are barred, with the dark bars thinner than the pale bars, except in the median coverts and inner greater coverts, which appear rather plain adult-like grey. The same often applies to the tertials, with the upper tertial less patterned that the other tertials, although the white tip of this upper tertial has been slightly worn. Of course, these are the feathers often included in the partial autumn moult in 2cy argenteus and the grey feathers may be at least one generation older than the other wing-coverts and tertials. 

The primaries show a clear mirror on both P9 (normally a very small mirror) and 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. Note the dark markings on the tail-feathers in this individual.

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.