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

(last update: 08 december 2003)

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Herring Gull 4cy (argenteus), March 16 2003, Boulogne-sur-Mer, France (50.42N,1.34E).

The scapulars appear completely adult-like grey but in the wing-coverts and on the tertials much vermiculated brown-black markings are visible. Probably, the upper tertials were once replaced in the partial autumn moult and it might be one generation older than the other tertials. The greater coverts were moulted in two waves which met about at #7-#8. The greater coverts were replaced last in line and consequently they appear much plainer grey. So, although these greater coverts most probably belong to the same (third) generation, there are clear and obvious differences between the first moulted coverts (the very abraded inner greater coverts #4-6) and the most recently moulted coverts (#7-#8).The three innermost greater coverts, like the upper tertials, are probably one generation older. 
The outer primary-covert are dark, unlike the grey adult-like primary-coverts of 5cy and older birds. Last autumn, so-called "third winter" birds (4cy by February) have replaced the all-brown immature primaries for more adult-like primaries, which show a white tip, a grey basal half and one or two mirrors (P9 and P10). But 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 broad dark sub-terminal band on P5 and a dark faint spot on the outer-web of P4. The dark patches are not sharply demarcated as in adult, but the dark outer-web gradually dissolves in the grey inner-web. Note the dark vermiculated markings on the tail-feathers in this individual.

From January to April, 4cy argenteus moult to so-called "third summer" plumage. By April, the head and under-parts will appear all-white. The tail-feathers, wing-coverts and primaries are not replaced, therefore most birds strongly resemble "third winter"  birds, although the white primary tips wear away and dark patches start to bleach. The iris is often yellowish, the orbital ring yellowish-orange and the bill predominantly yellow with a red gonydeal spot and limited black markings along the culmen by April.