adult: September

Last update of this page: 07-12-03 17:33

By September, the large majority of adult LBBG have left the Netherlands and must have move south towards Portugal. Many of the Dutch adults, which were seen feeding youngs and those suspending primary moult start migrating and must have joint these migrating groups. Nevertheless, many adults can still be found in the Netherlands, but as September progresses, more adults from Scandinavian intermedius colonies move in. This may be a reason for the "observed mere stand-still in primary moult progress" in adults: less advanced individuals from Scandinavia congregate with local Dutch intergrades. 

In this September Section we present primary moult score surveys 2001 & 2002 from the Netherlands (Maasvlakte) and NW France (Boulogne-sur-Mer). No definite conclusions on geographical variations can be drawn from these data. Therefore we started to focus on ringed adults only from year 2003 onwards;  metal ringed and Darvic ringed birds of known origin. 
Anyone checking local Lesser Black-backed Gulls in September will have noticed the large individual variation in primary moult scores. The commencement and pace of primary moult is an individual "choice", influenced by other prime goals like feeding pulli and migration. The birds observed in NW Europe apparently "chose" to stay and their September primary moult scores are not necessarily representative for "average NW European LBBG", as we simply do not know the primary moult strategy of early migrating adults.
So two question can be raised: how to exclude influences from northern populations and what PMS can be expected in early migrating birds? 
Best option to reduce the second problem could be found in simultaneous counts along the entire West European coastline, or counts which only involve ringed individuals (thus from certain origin) at stop-overs and winter quarters. Particularly those adults which migrate south as soon and as quick as possible, may be found with low PMS or suspended moult.
In the Netherlands we concentrate on ringed individuals to eliminate the first problem (intermedius joining local Dutch intergrades), but we still have insufficient ring observations to make reliable analyses possible. 
The problem of (presumed) later moulting intermedius, moving into early moulting graellsii groups is not a problem restricted to adults, but applies to other age-classes as well.

Although the input of later moulting intermedius may be expected, the average primary moult score is hardly influenced as it seems. From the table below, the average "remaining old primary score" falls by almost two primaries over a period of 4 weeks. This matches nicely with the August estimates. 

Subspecific PMS: graellsii

In the Netherlands, the nominate type graellsii can be found in low numbers in September. Normally, they can be recognized by their slightly paler grey upper-parts, fiercer facial expression, stockier jizz and more advanced PMS. By September, adult graellsii may arrive from Britain, but the nominate form also can be found breeding in Belgian and Dutch colonies. 
We do not collect sufficient field data on ringed graellsii in the Netherlands to draw any conclusions; notwithstanding the fact that September adults disperse throughout the entire southern part of the North Sea, following vessels on rich fishing grounds and taking rest at the closest beaches, either in Britain, Belgium or the Netherlands. 
There are hardly articles or papers dealing with September adult LBBG. By far the most extensive surveys were done by the Severn Estuary Gull Group in Gloucestershire, with LBBG-expert Peter Stewart. He wrote two papers on PMS of British graellsii, one still in preparation.

During the period February 1986 to October 2001 no less than 12,842 LBBG were caught using cannon-nets, and from this total 6,539 PMS were recorded. For the month September, 407 adults have been recorded. The number of old primaries present in adults are presented in the table below. Peter uses half monthly intervals.

Peter also recorded the PMS of adult birds, using the method described by Ginn & Melville (1983). Each primary receives a score: score 0 (old feathers), score 1 (old feather missing, new feather completely in pin), score 2 (new feather visible up to 1/3), score 3 (growing feather 1/3 to 2/3),  score 4 (growing feather more than 2/3, still with basal waxy sheath), score 5 (new feather fully grown). The total PMS for all 10 primaries varies from 0 to 50 for any single wing.

Subspecific PMS: Dutch intergrade

The form "Dutch intergrade" refers to the LBBG populations in Belgium and the Netherlands. These colonies are situated between graellsii (British Isles) and intermedius (Norway, Sweden, Demark) and breeding birds in the Netherlands and Belgium show characters of both taxa. We have extensive field surveys of primary moult scores for adult LBBG from the large colony at Maasvlakte (near Rotterdam) but the September scores may be effected by migrating intermedius, which are abundant, especially in the second half of the month. From 2003 onwards, we focus on ringed birds only, making sure birds originate from the Netherlands or Belgium. Although the figures in the table give some indication for the average PMS of Dutch LBBG, a few comments must be made:
1. The PMS recorded here refer to LBBG which are still in the Netherlands. We don't have sufficient data to draw conclusions on the complete Dutch population.
2. Different colonies are involved, with birds from Maasvlakte, IJmuiden and Moerdijk (inland location).
3. Although it seems easy to construct a PMS progression line, this is not wise. Especially the counts for September involve many different birds, only seen on one or a few occasions. Individual birds follow different moult strategies, and this may vary between Dutch colonies. 

Subspecific PMS: intermedius

The form intermedius breeds in Norway, Sweden and Demark. Compared to either graellsii or Dutch intergrades, intermedius is on average a late moulting bird. Most birds are a shade of grey darker than Dutch intergrades, but some intermedius appear just as grey and robust as the average local Dutch intergrade. However, strong western September winds may bring the typical small intermedius to Dutch shores, very dark grey (almost blackish) with typical jizz: short necked, short legged, attenuated rear end and dove-like appearance. And often a remarkable low PMS. The few comments made under the Dutch intergrade caption apply to intermedius as well.

Peter Stewart has caught intermedius in Britain. He only recorded 10 birds for September on a total of 68 birds. The average PMS for intermedius "falls mainly between the mean and lowest moult scores obtained for adult graellsii", according to Peter. However, intermedius may show a very advanced PMS as well, already by mid-August.

conclusions

As could be expected, LBBG which spend the month of September along the Dutch coast may origin from many different colonies. The PMS of either graellsii, Dutch intergrades or intermedius do not necessarily reflect the average of a complete LBBG population, as many birds quickly migrate south, early in the season. Particularly these birds may follow a different (delayed) moult strategy.

Tables & Graph

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 September (arrested moult excluded). Sample size for this graph is 278 ringed birds + 1294 unringed birds.

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 September (arrested moult excluded). Sample size for this graph is 112 ringed birds + 152 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).

LINEAR REGRESSION

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.

LINEAR REGRESSION FOR OLD REMAINING PRIMARIES

Complete sample: Linear regression (n = 6171, R2= 0.820, sign: P<0,000)
PMS = -0,381* WEEK + 17,967

Ringed adults: Linear regression (n = 2069, R2= 0.800, sign: P<0,000)
PMS = -0,325* WEEK + 16,270

Unringed adult: Linear regression (n = 4102, R2= 0.779, sign: P<0,000)
PMS = -0,513* WEEK + 23,031

LINEAR REGRESSION FOR NEW FULLY GROWN PRIMARIES

Complete sample: Linear regression (n = 2969, R2= 0.930, sign: P<0,000)
PMS = 0,302* WEEK - 5,933

Ringed adults: Linear regression (n = 1209, R2= 0.888, sign: P<0,000)
PMS = 0,252* WEEK - 4,747

Unringed adult: Linear regression (n = 1760, R2= 0.833, sign: P<0,000)
PMS = 0,442* WEEK - 11,906