Herring Gull - L. argentatus

This website deals with the Herring Gulls of the taxa argentatus and argenteus. Argenteus is an abundant breeding species on Iceland, Britain, NW France, Belgium, the Netherlands and N Germany. The taxon argentatus breeds in NE Europe, from Denmark, the coast of the Baltic Sea, Scandinavia and coast of the White Sea. In the centre of the breeding range (Denmark and the NW German coast), birds show intermediate characteristics, indicating there is a cline in argenteus-argentatus. Argenteus in Western Europe is pale-backed, slightly smaller and with relatively much black in the wing-tip; argentatus in the NE Europe is larger, slightly darker on upper-parts and show less black in the wing-tip.
Due to the large breeding range, there is much intra-specific variation in argentatus, with most large, powerful birds in the northern populations, showing the most darkest upper-parts, longest wing and most white in the outer-wing. In this region, some populations may hold substantial number of yellow-legged argentatus. (referred to as "Finnmark yellow-legged argentatus"). In adult plumage, the black sub-terminal spot on P5 is often lacking or strongly reduced to a diffuse streak, broken at the centre. Sometimes P6 lacks black as well.
In the Baltic region, other yellow-legged argentatus populations occur ("Baltic yellow-legged argentatus"), with adults showing even brighter yellow legs in summer and the upper-parts only slightly darker than West European argenteus, still much paler than Yellow-legged Gull (michahellis). Those Baltic argentatus often show a black sub-terminal spot on P5 in adult plumage.
On these web-pages, we largely follow P.J. Grant: "Gulls, a guide to identification" and E.K Barth's publication as Contribution No. 86, Zoological Museum, University of Oslo: "The circumpolar systematics of Larus argentatus and Larus fuscus with special reference to the Norwegian populations".

The differences between typical western argenteus from the U.K. and northern Scandinavian argentatus is rather straightforward on primary pattern P5-P10. However, there is a gradient and overlap in features of birds from populations in northern Germany, Denmark and southern Norway.
Identification is even more challenging in birds from Baltic populations, which show both characteristics of argenteus and argentatus, but sometimes also of the southern taxa michahellis and particularly cachinnans.
Adult birds from the western populations argenteus show more black in the outer primaries, compared to northern argentatus. In argenteus, P5 normally shows small black sub-terminal markings, concentrated on the outer-web as a clear-cut black angular spot and in some birds extending on the inner-web as a diffuse streak. There may be some black on P4 as well, although this might be a clue for near-adult birds. In argentatus, especially from far northern Scandinavia, the black marking on P5 is completely lacking or is very limited and diffuse.
Argenteus often has a black sub-terminal band on the top of P10, dividing the tip from the mirror. If the black sub-terminal band on P10 is broken, there is often still black on the inner and outer-web's edge of P10. Normally argentatus lacks sub-terminal markings on the outer-web of P10.

Baltic argentatus often have the black marking on P5 confined to the outer-web as a single rectangular small black square, with well-defined borders (not diffuse as in most Finnmark birds). In most Baltic birds, the white tip of P10 fully merges with the mirror, without a single sign of a sub-terminal band. The greyish-white 'tongue' on the inner-web is often broad and continues far down P10, almost creating a thayeri pattern on P10. The shape of the division line between greyish tongue and black outer-web can be an identification clue: In cachinnans this division line often makes a strong angular curve and has a smooth straight line, unlike the pattern of Herring Gulls, which often show a "saw-blade" motive or otherwise much irregularities along the division line. Sometimes wear of the outer primaries may be useful as a first indication of origin: An early moulting species as cachinnans on average show more abrasion in the feathers than later moulting argentatus.

Especially birds from north Scandinavian Finnmark may show the characteristic 'thayeri' pattern in the outer primaries P9 and P10 (column 5 of tip pattern P9 in the table). This pattern is named after the Thayer's Gull, which often shows a particular pattern in the outer primaries: the mirror and the white tip merge and are connected to the pale inner-web (see this image). In other words: the black medial band of the outer-web doesn't fully reach the edge of the inner-web and there is no sub-terminal band. In Thayer's Gull this pattern may be very obvious and, together with the 'string of pearls' may be indicative for identification.

By October the majority of adult Lesser Black-backed Gulls moulting the outer primaries. There is a clear difference between presumed birds lingering around and birds which will probably migrate further south quickly. The difference can be found in the primary moult strategy.

Some adult have a large moulting gap in the central primaries (for example P8-P10 old and only P4 fully grown and P5 still slightly shorter than P4) by the first days of September. It is hard to believe these birds will travel large distances non-stop as such moult gaps are highly inefficient for this migration pattern. Probably these birds, which can be found moulting in a period roughly from late August to late November stay in the area throughout their primary moult. They migrate further south either forced by cold winter or when the primaries moult arrives at the far outer primaries.

The other group is represented by birds with a very small moult gap in the primaries or with a very low primary moult score, indicating that primary moult will probably suspended until arrival at the wintering grounds.

remaining SPRING
starting date:	30 Apr	07 May	14 May	21 May	04 Jun	11 Jun	30 Jul
week no:	18	19	20	21	23	24	31
0	0	0	0	0			0
1	0	0	0	0			0
2	0	0	0	0			1
3	0	0	0	0			1
4	0	0	0	0			37
5	0	0	0	0			22
6	0	0	0	0			3
7	3	2	1	2			0
8	16	22	26	18	1	1	0
9	30	13	16	10	1		0
10	66	33	37	10			0

n=	115	70	80	40	2	1	64
average REM P:	9.4	9.1	9.1	8.7	8.5	8.0	4.4
95% CID =	0.15	0.23	0.20	0.29	6.35		0.17
inverse =	1.98	1.99	1.99	2.02	12.71		2.00
Variantie (s²) =	0.68	0.90	0.84	0.83	0.50		0.46
SD (s) =	0.82	0.95	0.91	0.91	0.71		0.68
SE (s_x) =	0.08	0.11	0.10	0.14	0.50		0.09

% class:
0	0	0	0	0	0	0	0
1	0	0	0	0	0	0	0
2	0	0	0	0	0	0	2
3	0	0	0	0	0	0	2
4	0	0	0	0	0	0	58
5	0	0	0	0	0	0	34
6	0	0	0	0	0	0	5
7	3	3	1	5	0	0	0
8	14	31	33	45	50	100	0
9	26	19	20	25	50	0	0
10	57	47	46	25	0	0	0

% control:	100	100	100	100	100	100	100

Table above, data for the Netherlands (2000-2010), results for spring & summer (aggregated week): number of remaining old primaries in adults.
0 = 0 old primaries left (P10 dropped); 10 = all 10 primaries still old and present.

Above: Scatter Plot for PMS in adult Herring Gulls. Number of remaining old primaries throughout the year. Sample size is 1.329 birds.

new: SPRING
starting date:	30 Apr	07 May	14 May	21 May
week no:	18	19	20	21
0	104	60	66	30
1	7	9	12	7
2	4	1	2	3
3	0	0	0	0
4	0	0	0	0
5	0	0	0	0
6	0	0	0	0
7	0	0	0	0
8	0	0	0	0
9	0	0	0	0
10	0	0	0	0

n=	115	70	80	40
average NEW P:	0.1	0.2	0.2	0.3
95% CID =	0.08	0.10	0.10	0.20
inverse =	1.98	1.99	1.99	2.02
Variantie (s²) =	0.18	0.16	0.21	0.38
SD (s) =	0.43	0.40	0.46	0.62
SE (s_x) =	0.04	0.05	0.05	0.10

% class:
0	90	86	83	75
1	6	13	15	18
2	3	1	3	8
3	0	0	0	0
4	0	0	0	0
5	0	0	0	0
6	0	0	0	0
7	0	0	0	0
8	0	0	0	0
9	0	0	0	0
10	0	0	0	0

% control:	100	100	100	100

Table above, data for the Netherlands (2000-2010), results for spring & summer (aggregated week): number of new, fully grown primaries in adults.
0 = no new primaries visible. So either P1 still old, or P1 dropped, but the very tip still not visible.
1 = P1 longest new primary. Range between "at least the tip of P1 is visible" until "tip of P2 still shorter than P1".
2 = P2 longest new primary. 3 = etc.

Above: Scatter Plot for PMS in adult Herring Gulls throughout the year. Longest new primary is classified "fully grown". Sample size is 3.271 birds. Birds scored on NEW primaries.

The first date in our analysis on adult Herring Gulls is 02 May 2006 (week 18 - starting 30 April), on which we already found birds with missing primaries (some birds had already shed 3 primaries). This group of actively moulting birds, 43% of the sample, must have started the complete moult somewhere around mid-April and probably several birds may have started earlier.

The last scoring day has been 03 December, when still 20% of the sampled birds had to grow P10. The final stage of the complete moult for all birds has to be somewhere after mid-December. On 03 December we scored 59 birds with P10 fully grown, 13 birds with P9 fully grown and 2 birds with P8 fully grown. They all had no old primaries left.

The first time a new primary (P1) was visible, was also on 02 May 2006 (week 18 - starting 30 April). In this week, 90% of the sample did not show new primaries, but 9% did. Four birds in the sample of 115 birds alreday had P2 as longest primary (fully grown). As soon as the tip of P1 was visible, this was automatically scored "fully grown", and this was the case in 7 birds. For the other primaries to be scored fully grown, they had to exceed the previous primary in length (so P2 fully grown, only from when it is longer than P1).

Onset of primary moult can best be illustrated by a sinusoid for the first few weeks and last weeks, but there appears to be linear dependency over much of the central moult period.