Lesser Black-backed Gull (graellsii & intermedius)

(last update: 14-11-2017 )

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Barth's research: validity of race intermedius?

From the 40's to the 70's, as one of the first persons, the Norwegian ornithologist Edvard Barth questioned the validity of subspecies intermedius, the main form of Lesser Black-backed Gull occurring in Norway. This text is a summary of the article by E. K. Barth and some extra notes about the current situation. Remember that nominate fuscus was more widespread in these days.

In the last decades, much has changed in the distribution of West-European Lesser Black-backed Gulls. Nowadays, they are breeding in large numbers in Britain, Norway and the Netherlands. The breeding range has expanded south along the Atlantic coast to France and the Iberia peninsular. Year 2000 LBBG's can be found from Iberia all the way along the coast north to the White Sea, in the U.K., the Faeroes and on Iceland. Small populations can be found in the NW of the United States. 
In France the sub-species graellsii breed side by side in mixed colonies of michahellis (P. Yésou), it is sympatric with michahellis, and mixed breeding occurs only in very low numbers (which can be regarded typical in the contact zones of expanding species). 
The same can be said about nominate fuscus, which breeds parapatric with heuglini. (They live in different, but neighbouring geographical conditions, without hybridization along the contact zone). 

The group of Lesser Black-backed Gulls is variable in upper-part coloration, measurements and behaviour, varying from the larger graellsii to the smaller fuscus. Intermedius is, as the name suggests intermediate. Fuscus is small, has blackish upper-parts, migrates to the south-east and many individuals show a step-wise moult (Staffel mauser, after Stresemann & Stresemann). The colour of the upper-parts of the larger western representative graellsii is mid-grey and these birds migrate south, along the Atlantic coast. The variation in intermedius varies clinally. Most of the general idea was constructed by the extensive research by the Norwegian ornithologist E. K. Barth, published as Contribution No. 86, Zoological Museum, University of Oslo, called: The circumpolar systematics of Larus argentatus and Larus fuscus with special reference to the Norwegian populations.

Research in the 40's, 50's, 60's and 70's: Edvard K. Barth

Barth, as a Norwegian ornithologist, was much interested in the variation of Larus argentatus and Larus fuscus in Scandinavia, where both species show large intra-specific (sub-population) variation and he collected material from 1942 to 1966, altogether 690 birds. All birds were adults, collected during the breeding season. Based on this material and skins examined in other parts of NW Europe, Barth examined and analyzed  the material on morphological criteria: mantle colour and chroma notation (in Munsell value, measured mechanically with reflectometer equipment), colour of soft parts (in Munsell value), measurements of body-parts (in mm) and egg-dimensions (in mm). Additional information was collected about geographical distribution and migration pattern.

Barth's classification: the third sub-species intermedius

Map with clustered column charts for Scandinavian location. The aggregate stacked charts, categorized per sub-species can be found by clicking the next three thumbnails.  The last link holds the research data. Although figures have been prepared with most care, I take full responsibility for all errors in any chart and table.

The total material examined can be found in this Table of results from Barth's research. In this table, all data are presented and the complete group of LBBG is classified in three types: the dark form (corresponding with fuscus fuscus), an intermediate form (corresponding with fuscus intermedius) and a British form (corresponding with fuscus graellsii). The division lines between the colour classes are drawn as follows: 
1. dark form: darkness of upper-parts equal to or lower than Munsell value 2.6,
2. intermediate form: darkness of upper-parts between Munsell value 2.6 and 3.2,
3. British form: darkness of upper-parts equal to or higher than Munsell value 3.2.

From the measurements of the upper-parts of all individuals, Barth searched for evidence to classify three sub-species: the well-known graellsii and fuscus and the intermediate intermedius in which he succeeded; from publication of his data onwards, LBBG's have been divided in these three sub-species. The whole of the 'British area' showed only limited overlap with the intermediate samples and the same was true for his measurements of dark versus intermediate samples.

Graellsii has long been considered a sub-species of Larus fuscus, as was already suggested by Brehm in 1867. It's a common breeder in the U.K., the Faeroes and from the 1920's it start colonizing Iceland as well. Barth's measurements in the 60's indicate they all belong to the pale British form and the same could be said about birds from the Netherlands, although the conclusion could only be based on two individuals from this country. 

Fuscus, the very dark mantle form, can be found in the central and northern Baltic area and in northern Norway, forming a marked homogeneous group in the 60's. The picture has changed somewhat since the studies of Barth. Nominate fuscus has had poor breeding successes in e.g. Norway and Finland (see the M. Hario summary) and on the contrary, intermedius has expanded it's range. However, large scale inventories as Barth's research haven't been executed since and his distribution figures still are the most detailed.

Between these two homogeneous "centres" (the countries / islands in the west with graellsii and the Baltic states with fuscus in the east) lays an area inhabited by intermediate coloured birds, both regarding the colour value (darkness) of the upper-parts and regarding the chroma notation (degree of brownish tinge). The data provided by Barth gave rise to acceptance of the intermediate form to sub-specific level: Larus fuscus intermedius. For intermedius as well, a homogeneous population was found by Barth, along the west coast of Norway, near Stavanger. This location was already proposed by Salomonsen in 1963, who situated the centre for intermedius on the island of Rot, just off Stavanger. 
Obvious heterogeneous population were found in the contact zones: southernmost Sweden and the whole of Denmark (especially Anholt). To give an idea of several ringed birds from the area:
An adult ringed in 1989 at Bohuslän (SW Sweden)
An adult ringed in 1992 in Skane (S Sweden)
An adult ringed in 1972 on Anholt (Kattengat)
An adult ringed in 1991 on Anholt (Kattengat)
An adult ringed in 1987 in Jylland (Denmark)
An adult ringed in 1993 in Jylland (Denmark)
A 3cy bird ringed in 1999 in W Denmark 

Several other heterogeneous populations can be found in Kristiansand, on Tarva and in East Finnmark. Areas with heterogeneous population have a marked high coefficient of variation (C.V. value in the table). The general idea of the complete Larus fuscus population is that of a cline: palest birds in the west, darkest birds in the east and intermediate birds in between, with hybridization is small contact zones. Still, this is the general accepted overview of Lesser Black-backed Gulls in Europe (e.g. P Yésou "Systematics of Larus argentatus-cachinnans-fuscus complex revisited", in: Dutch Birding 24-5 2002) and the Dutch Rarity Committee is going one step beyond by splitting the LBBG group in two full species.

Chroma values in Lesser Black-backed Gulls

The research of Barth in the 60's indicates the three forms of Lesser Black-backed Gulls have their own geographical centres, based on the darkness of the upper-parts. Another characteristic measured by Barth pointed in the same direction: the degree of brown tinge on the upper-part feathers (the chroma notation). Observation in the field indicated that the eastern dark form have the upper-parts warmer brown hued, fruity brown, where the western pale form never shows such a warm brown hue, not even on the old abraded feathers. 
The colour value of the chroma notations for the three forms are plotted in the chart left (figure 6 in Barth's paper): The statistical analysis of colour value (ordinate), and the chroma notation (abscissa) of the three forms of L fuscus. Single lines represent the range; rectangles indicate one standard deviation (SD) on either side of the mean; solid bars indicate 99% confidence intervals for the means (99% CID). The number of birds in each sample are given in this chart, the values for SD and 95% CID can be read from the table. 
Again, the distribution of LBBG in NW Europe, based on chroma notation clearly leads to three forms: dark samples (whole Baltic and North Norway to Tarva), Norwegian-Swedish intermediate samples (More to Bohuslän) and the British samples (Iceland, Faeroes, British Isles, the Netherlands). Denmark is plotted between the samples of the dark and intermediate types; representing a mixed population. 

Zones of contact

By the time Barth examined the skins of LBBG (in the 60's), there wasn't much known about the exact contact zone between graellsii and intermedius. Currently, the term "Dutch intergrade" is used for birds breeding in Belgium and the Netherlands north to German coast of the Wadden Sea, showing intermediate characteristics between graellsii and intermedius. The lack of sufficient material is nicely illustrated by the birds from the Netherlands that could be examined by Barth: two individuals. But the contact zones in Scandinavia were quite obvious in his days, although interesting enough two birds collected in northernmost Möre showed Munsell notations close to the British form! 
Barth mentioned two 'geographical centres' for nominate fuscus: the Baltic and northern Norway. Contact zones are located north of Möre along the coast of central western Norway and between Denmark and southern Sweden. These contact zones make it impossible to say something about the origin of a single individual, but on population level, paired clusters can indicate the likelihood of dark birds belonging to intermediate populations and visa versa. Barth uses the Coefficient of Difference (CD value) in line with Mayr (1953). For the area north of Möre, Barth's data give a theoretical joint of non-overlap of 94%. To give a better idea on geographical diverted populations: 

80% of the Tarva birds differ from 100% of the Möre-Bergen birds,
57% of the Möre-Bergen birds differ from 100% of the Tarva birds.

When you take the populations from the geographical centres:
81% of the North Norway birds differ from 100% of the Stavanger birds,
84% of the Stavanger birds differ from 100% of the North Norway birds.

The Danish birds represent a mixed population and doesn't show differences on sub-specific level to either side, nor to the dark form, nor to the intermediate form.

Other measurements

The measurements of bill and wing dimensions shows obvious differences in the three sub-species as well, although not as clear as in mantle coloration. 
Barth also found significant larger eggs of the intermediate form compared to the dark form (within Norway) and the later egg laying date of the dark form. But again, these data are not as convincing as the mantle coloration, as the SD value was rather large.
The soft part coloration is much richer in the northern populations than in populations from southern Norway. Barth measured the swimming webs of Finnmark nominate fuscus to be strong yellow and strong yellowish orange. Birds from Stavanger and the Oslo Fiord showed much less ochre, moderate yellowish orange to very light yellow, although the range was large. The colour of the orbital ring didn't differ much, from reddish orange to brilliant red. 

Migration

Ring recoveries indicate the strong tendency of intermedius to migrate south - south west. Both metal ringed ands color ringed individuals can be found along the coast of the Netherlands and Belgium; and further south along the Atlantic coast of France, Iberia, Morocco and West-Africa. Just check the October and November-December LBBG Sections of this website to find various examples of ringed birds. 
Nominate fuscus predominantly heads south east, as this link of the Finnish Ringing Centres shows: a direct link to the distribution map of Finnish-ringed Larus fuscus fuscus with recoveries of ringed individuals worldwide.

Barth mentions five birds from northern Norway, all migrating in SE direction in line with the direction of migration for Baltic fuscus. In the contact zone of intermedius and fuscus in southern Sweden the picture is clearly illustrated by the findings of Curry-Lindahl (1961) who stated that "no Larus fuscus from Öland or Skane has migrated to the south or south east", they are only found to migrate to the south west. Salomonsen (1963) added that the Danish populations from Bornholm (see Map) and the other islands in eastern Denmark migrate to South Italy and the eastern Mediterranean. Barth presumes these birds of Bornholm, Skane and Öland belong to the sub-species intermedius.

Westkapelle (51.33 N, 3.25 E), situated along the Southwest coast of Holland, is one of the best places to study intermedius LBBG in winter plumage at close range in the Netherlands. As long as it is not freezing, a couple of hundreds remain here during November and December.

All age-classes can be studied at Westkapelle and all pictures (right-hand side) of adult winters were taken in November and December. Based on ring recoveries most present birds originate from Scandinavia (50-80%), particularly Vest-Agder, southern Norway.

Tables

Chart of Munsell value of fuscus Although figures have been prepared with most care, I take full responsibility for all errors in any chart and table.

Chart of Munsell value for intermedius Although figures have been prepared with most care, I take full responsibility for all errors in any chart and table.

Chart of Munsell value for graellsii Although figures have been prepared with most care, I take full responsibility for all errors in any chart and table.

Table of results from Barth's researchAlthough figures have been prepared with most care, I take full responsibility for all errors in any chart and table.

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 November (arrested moult excluded). Sample size for this graph is 12 ringed birds + 367 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 November (arrested moult excluded). Sample size for this graph is 9 ringed birds + 874 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

GRAELLSII FROM ICELAND

GRAELLSII FROM BRITAIN

GRAELLSII FROM FRANCE

DUTCH INTERGRADES FROM BELGIUM

DUTCH INTERGRADES FROM HOLLAND

LBBG FROM PORTUGAL

GRAELLSII FROM GERMANY

INTERMEDIUS FROM DENMARK

INTERMEDIUS FROM NORWAY

INTERMEDIUS FROM SWEDEN

UN-RINGED LESSER BLACK-BACKED GULLS