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".

Continental European Herring Gull L. a. argenteus are known to show phenotypic characteristics which are intermediate between their British (L. a. argenteus) and Scandinavian relatives (L. a. argentatus). However, nothing has been published to date on the moult pattern and detailed phenotype of birds breeding in the Netherlands.

The industrial complex Moerdijk in the Netherlands holds a mixed colony of Herring and Lesser Black-backed Gulls. Colonisation of Moerdijk started in the early 1980s. By 2006, an estimated 650 pairs of Herring Gulls and 2900 pairs of Lesser Black-backed Gulls bred in the colony. In 2005, 157 breeding Herring Gulls and 434 Lesser Black-backed Gulls were colour ringed at Moerdijk, with 158 and 604, respectively, ringed in 2006.

We used these data collected at Moerdijk to describe the phenotypic characteristics and commencement of primary moult. Our data illustrate the large intra-population variability that exists for these taxa, even within a single colony. We discuss the implications of this variability for field identification of the various sub-species of Herring and Lesser Black-backed Gulls on this website.

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.