Lesser Black-backed Gull - graellsii & intermedius

By October, all LBBG have left the colonies and surrounding feeding grounds near the Maasvlakte (the Netherlands) and migrate south towards France, Portugal and Morocco. In November and to a lesser extend December, a couple of hundreds remain on continental NW Europe, as north as the south of the Netherlands along the coast at Westkapelle, 65 km south of the Maasvlakte (see map). As it seems, every year a larger group of LBBGs stay "at the most northern latitude as possible", i.e. migrate south only when winter conditions force them to.

By late November 2000 (a rather mild winter) ringed individuals from Scandinavia, Denmark, the Netherlands, Belgium and the U.K. were present at Westkapelle. When winter really started in December 2000, this group moved south as well to unknown wintering grounds, probably the Atlantic coast of West and South-West Europe.

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.

starting date:	24 Sep	01 Oct	08 Oct	15 Oct	22 Oct	29 Oct	05 Nov
week no:	39	40	41	42	43	44	45
REM 0	0	2	97	111	113	122	2
REM 1	19	1	183	165	56	3	0
REM 2	85	2	217	175	55	9	0
REM 3	123	2	106	38	7	1	0
REM 4	44	0	39	11	2	0	0
REM 5	24	0	28	7	1	0	0
REM 6	9	0	29	5	0	0	0
REM 7	5	0	8	3	0	0	0
REM 8	1	0	9	1	0	0	0
REM 9	0	0	3	1	0	0	0
REM 10	0	0	3	1	0	0	0
	0	0	0	0	0	0	0
n =	310	7	722	518	234	135	2
average P =	3.1	1.6	2.2	1.5	0.9	0.2	0.0
95% CID =	0.14	1.18	0.13	0.12	0.13	0.10	0.00
inverse =	1.97	2.45	1.96	1.96	1.97	1.98	12.71
Variantie (s²) =	1.57	1.62	3.30	1.81	0.97	0.33	0.00
SD (s) =	1.25	1.27	1.82	1.35	0.98	0.57	0.00
SE (s_x) =	0.07	0.48	0.07	0.06	0.06	0.05	0.00

% class:
0	0	29	13	21	48	90	100
1	6	14	25	32	24	2	0
2	27	29	30	34	24	7	0
3	40	29	15	7	3	1	0
4	14	0	5	2	1	0	0
5	8	0	4	1	0	0	0
6	3	0	4	1	0	0	0
7	2	0	1	1	0	0	0
8	0	0	1	0	0	0	0
9	0	0	0	0	0	0	0
10	0	0	0	0	0	0	0

% control:	100	100	100	100	100	100	100

Table above, data for the Netherlands (2000-2010), results for October (aggregated week): number of remaining old primaries in adults.
REM0 = 0 old primaries left (P10 dropped).

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries throughout the year (arrested moult excluded). Sample size is 6.171 birds.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries for October (arrested moult excluded). Sample size for this graph is 40 ringed birds + 1888 unringed birds.

starting date:	24 Sep	08 Oct	15 Oct	22 Oct	29 Oct	05 Nov
week no:	39	41	42	43	44	45
NEW 0	0	0	0	0	0	0
NEW P1	0	0	0	0	0	0
NEW P2	0	0	0	0	0	0
NEW P3	0	0	0	0	1	1
NEW P4	4	0	0	2	1	1
NEW P5	10	1	2	17	5	6
NEW P6	4	12	94	62	10	13
NEW P7	0	5	127	162	96	115
NEW P8	0	0	24	136	164	145
NEW P9	0	0	0	0	71	18
NEW P10					2	13

n =	18	18	247	379	350	312
average NEW P =	5.0	6.2	6.7	7.1	7.8	7.6
95% CID =	0.34	0.27	0.08	0.09	0.10	0.11
inverse =	2.11	2.11	1.97	1.97	1.97	1.97
Variantie (s²) =	0.47	0.30	0.42	0.74	0.83	0.91
SD (s) =	0.69	0.55	0.65	0.86	0.91	0.95
SE (s_x) =	0.16	0.13	0.04	0.04	0.05	0.05

% class
0	0	0	0	0	0	0
1	0	0	0	0	0	0
2	0	0	0	0	0	0
3	0	0	0	0	0	0
4	22	0	0	1	0	0
5	56	6	1	4	1	2
6	22	67	38	16	3	4
7	0	28	51	43	27	37
8	0	0	10	36	47	46
9	0	0	0	0	20	6
10	0	0	0	0	1	4

% control:	100	100	100	100	100	100

Table above, data for the Netherlands (2000-2010), results for October (aggregated week): number of new, fully grown primaries in adults.
NEW0 = no new primaries visible (arrested moult excluded in this analysis). So either P1 still old, or P1 dropped, but the very tip still not visible.
NEW1 = P1 longest new primary. Range between "at least the tip of P1 is visible" until "tip of P2 still shorter than P1".
NEW2 = P2 longest new primary. NEW3 = etc.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls throughout the year. Longest new primary is classified "fully grown". Sample size is 2.969 birds. Birds scored on NEW primaries (arrested moult excluded).

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Scoring on longest new primary, which is classified "fully grown" for October (arrested moult excluded). Sample size for this graph is 58 ringed birds + 1266 unringed birds.

The earliest date for birds with missing primaries is 30 April 2006, which is not the earliest day in our complete sample. We scored 98 Lesser Black-backed Gulls prior to that date, some already in February. As can be expected, none of these was in primary moult, and this group was excluded in our further analysis as it will only result in a misfit for the onset of moult, using linear regression statistics.

The last scoring days were in November, with a larger sample (133 adults) on 04 November 2010. From that date we have the last birds with retaining old primaries (13 birds with 1, 2 or even 3 old primaries; 120 birds with no old primaries left). From other days in November, only 10 birds were scored (all ringed adults). They all had no old primaries left.

The first tine a new primary (P1) was visible, was on 08 May 2006. Prior to this date, birds were scored missing inner primaries, but the very tip was still not visible (and therefore not scored). As soon as the tip of P1 was visible, this was automatically scored "fully grown" 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).

Peter Stewart presents onset of primary moult by scores for caught adult Lesser Black-backed Gulls in the Severn Estuary, U.K., repeated in the table below. To read his results, see this PDF file.

Table: the number of old remaining primaries present. Compare PMS of adult graellsii at half monthly intervals. Data from the Severn Estuary Gull Group by Peter Stewart: The primary moult of the Lesser Black-backed Gull (special publication No 1). Scores in the Netherlands can be influenced by unringed nominate fuscus, in autumn in particular.
	Netherlands			Severn Estuary U.K.
	PMS range	N:	Average PMS	PMS range	N:	Average PMS
1-15 apr	10	49	10	8-10	216	9,9
16-30 apr	10	47	10	10	197	10,0
1-15 may	8-10	849	9,8	8-10	167	9,9
16-31 may	7-10	380	9,7	7-10	76	9,5
1-15 jun	8-10	104	9,6	7-10	33	8,6
16-30 jun	7-10	253	9,7	6-10	65	8,2
1-15 jul	6-10	605	9,2	5-10	176	7,4
16-31 jul	-	-	-	4-8	44	6,4
1-15 aug	3-10	143	6,6	3-10	165	5,2
16-31 aug	2-10	1284	5,4	3-8	127	4,6
1-15 sep	1-10	300	3,6	0-7	234	3,3
16-30 sep	0-7	645	2,9	0-7	173	2,1
1-15 oct	0-9	733	2,2	0-4	328	0,9
16-31 oct	0-10	750	1,3	0-3	244	0,3
1-15 nov	0-3	138	0,2	0-3	604	0,1
16-30 nov	0	5	0	0-1	341	0,0

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.