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

(last update: 22 september 2004)

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Herring Gull sub-adult, September 21 2004, Scheveningen, the Netherlands.

Probably a 4cy argenteus. This bird resembles an adult argenteus in many respect, but the black markings on the bill are too extensive for full adult birds. Note the pink-based bill and dark orbital ring. The primary moult score: P7 fully grown, the tip just over P6. Note the greater primary coverts on the old primaries. 

From June (often mid-May in France, Belgium and the Netherlands) to October, a complete moult will bring birds in so-called "sub-adult winter" plumage. A new set of primaries will grow during the summer months and by October, the old primaries are all shed in argenteus in the Netherlands and northern France. In most adults (!), the new primary P6 is just visible beneath the tertials and shows a clear sub-terminal band. By late-September, P6 and P7 are fully-grown as well. At the same time, almost all wing-coverts have been renewed. In summer, the head is still largely white, but by September, most birds develop extensive 'winter streaking', in sub-adults strongly mottled brown contrasting with the white breast as in an executioner's hood. The tertials show obvious white tips. The iris is white-yellow with extensive speckling. The bill has a red gonydeal spot confined to the lower mandible. The orbital ring is yellow-orange in most argenteus, but often dark in autumn.

September primary moult scores in sub-adult & adult Herring Gulls at Scheveningen.

. 30.08.2000 01.09.2000 01.09.2000 05.09.2001 07.09.2000 18.09.2000 21.09.2000 21.09.2001 26.09.2001 30.09.2000
0 . . . . . . . . . .
1 . 1 . 3 3 . . . . .
2 4 4 9 19 15 . . . . .
3 7 5 41 32 30 . . . . .
4 . . 10 4 . 7 . . . .
5 . . 1 . . 35 42 7 24 2
6 . . . . . 36 60 24 62 38
7 . . . . . 11 20 9 12 43
8 . . . . . 2 3 . . 1
n: 11 10 61 58 48 91 125 40 98 84
average 2,64 2,40 3,05 2,64 2,56 5,63 5,87 6,05 5,88 6,51
Variance (s2) 0,25 0,49 0,38 0,48 0,38 0,77 0,58 0,41 0,36 0,33
SD (s) 0,50 0,70 0,62 0,69 0,62 0,88 0,76 0,64 0,60 0,57
SE (sx) 0,15 0,22 0,08 0,09 0,09 0,09 0,07 0,10 0,06 0,06
Notes:
30.08 - 07.09: number of remaining old primaries sub-ad and adults (thus 1 = P10 still old).
18.09 - 30.09: number of new primaries in adults (hence 6 = P6 full grown).
*) By September, local argenteus get company from argentatus from Norway, which have on average lower moult scores and from argentatus from the Baltic region, which have equal moult scores probably.