Discusfus: Discussion page on L. fuscus
(last update: 21 juli 2005)
Herring Gull argentatus
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Immature LBBGs resembling fuscus in Western Europe (so-called discusfus LBBG's)
Every spring and early summer, LBBG showing arrested and suspended primary moult can be observed in the Netherlands, Belgium and northern France. Itís believed these LBBG belong to the race intermedius or otherwise resemble the race graellsii, except perhaps 2cy LBBG with second generation primaries in spring. In articles dealing with the eastern nominate race fuscus [Jonsson, Birding World 1998, Rauste, Limicola 1999] this feature (among others) was mentioned as common in fuscus and very rare or non-existing in intermedius and graellsii. Therefore it seemed to be an easy clue in the field to find fuscus in Western Europe.
However, annually about 50 of such birds (LBBG with arrested or suspended moult) have been observed and probably arrested moult occurs in about 3-10% of 3cy LBBG. In all cases these immature LBBG resemble intermedius and graellsii, not fuscus. Several colour and metal ringed 3cy confirm that these birds indeed are intermedius and graellsii, from Scandinavia, the Netherlands and Britain.
For decades it's known that darkness of upper-parts or general appearance of fuscus and intermedius overlap. Now it seems that arrested moult is only uncommon (but not extremely rare) in intermedius, and one may wonder what make the difference between intermedius and fuscus in 3cy.
The question remains (or is back): ďHow to recognize LBBG of the race fuscus in Western Europe?Ē. Fuscus 2cy in spring probably hold best chances. But identification of juvenile, 3cy, 4cy and adult fuscus seem to be extremely tricky and may be impossible when birds are not ringed.
On this page we like to enlarge upon characteristics which (when combined) point towards fuscus, but may be seen in intermedius and graellsii as well (as stand-alone feature or accumulated characteristics in a single bird).
What features can be used best to establish age accurately? This question not only applies to intermedius and fuscus but to all gull species. Is it bare part coloration or generation of primaries and secondaries? As photographs of ringed birds show, 2cy fuscus may return from the wintering grounds with very advanced upper-parts and with complete 2nd generation primaries, almost resembling older (sub)adult fuscus in spring (Limicola 13, p159 plate 35 and this link for examples of very advanced moult). In these cases, bare part coloration points to at least 3cy (with much yellow on the bill, sometimes even red on the gonydeal angle and pale yellow legs). When all the primaries are moulted in spring before departing north and are therefore of a successive generation, 2cy fuscus may very much look like 3cy birds.
The same problem may arise for advanced 3cy, which resemble 4cy, as can be seen in this LBBG intermedius, a probable 3cy from Dannes, France, May 05 2001. It looks like 4cy, but the primaries suggest 3cy (no white tips and no mirror on P10). But in many respects the very advanced upper-parts, white tail and bare part coloration resemble 4cy. Compare this bird with the 4cy bird in front (click thumbnail), which shows white spots on the primaries and a white mirror on P10. Bare parts clearly advocate at least 4cy. Such (advanced 2nd summer?) LBBGs can probably be found in every large group of intermedius in spring in Western Europe.
This is another 3cy LBBG Dutch intergrade in the Maasvlakte colony, May 27 2003, Maasvlakte, the Netherlands. Fortunately, primary moult was arrested at P8, so the outer primaries can be judged second generation. But imagine 3cy birds which do succeed in complete replacemnet of the primaries. Such birds would be very difficult to age accurately.
To summarize: fuscus may return in summer in very advanced plumage and they may clearly differ from same aged intermedius/graellsii. But they do not differ much from one year older graellsii/intermedius and to distinguish between 2cy fuscus and 3cy intermedius may be very difficult, when fuscus returns with complete replaced remiges. Furthermore, 3cy intermedius and 3cy Dutch intergrades may appear very adult-like, just like 3cy fuscus.
On average, fuscus is a small gull, smallest of the LBBG-complex with short legs and long, elongated wings (the hand of fuscus may be as long as nominate argentatus (!): 450 mm.). Of course, females are smaller than males, females have a more friendly facial expression and the overlap between male fuscus and female intermedius has been mentioned in several articles. The fact that size is far from diagnostic is shown by the left individual, which has exactly the size of present Common (Mew) Gulls, Larus canus.
individual has the primaries jet-black, shows no white tips and just an
extremely small faint mirror on P10 is visible (see the problem of ageing above). Grey-tone
is too pale to fit fuscus and is in line with intermedius. Again
moult in the upper-parts is advanced. The bare parts are very adult-like: the legs
are lemon yellow, the red spot is confined to the lower mandible and there is only a small black spot
on the upper mandible. This is one of the smallest birds seen in spring 2001.
The on average short legs (especially the tibia) are not a very reliable feature to distinguish between fuscus and intermedius, on individual level.
The bill in fuscus is said to be different from intermedius, but probably it's the difference between fuscus and graellsii what is meant. The difference between the bills of average fuscus and graellsii from the U.K. is quite obvious. But like fuscus, intermedius and probably to a lesser extend also some Dutch intergrade LBBGs may show a parallel slender bill, sometimes a bit droopy, as the image shows. This is a female, ringed at the Maasvlakte, the Netherlands July 10 1995, now in her 6th summer. Note, besides the bill, the brown (but not mahogany) hue on the upper-parts. It seems bill-height and relative bill length may overlap in individuals and differ only on average on population level.
Normally, 2cy fuscus returns on the breeding grounds with plain, dark, fresh or slightly abraded second and third generation scapulars and second generation wing coverts, while some fuscus show fresh third generation all black coverts in between (BW 11-8, p304 fig 1 and fig 2; 2cy fuscus section). In 3cy fuscus, some individuals already show complete adult-like upper-parts (Limicola 13, p163 plate 42) but in quite some birds, the outer-half of the greater coverts, the outermost median and lesser coverts and some of the tertials show immature brown feathers. However, 2cy fuscus may return with barred or anchor patterned second generation feathers as well, resembling graellsii (Limicola 13, p159 plates 37 & 38; BW 11-9, p 305 fig 3).
Intermedius is variable in second generation scapular pattern. Some may show very dark second generation feathers, black-centred and slightly paler at the edge and tip. In some spring 2cy birds, these plain feathers are very worn and bleached dirty brown, but clearly must have been plain, lacking obvious barring and anchor pattern (see also next paragraph). Left is a picture of a 2cy LBBG, July 06 2001, IJmuiden, the Netherlands. It shows very plain patterned new coverts and scapulars. The centres are black and towards the fringes, the feathers become more brown-grey. Third generation feathers (often found in the median coverts, inner greater coverts and the scapulars) can be present in graellsii/intermedius as early as May and will show abrasion by mid-summer (as in fuscus).
Very dark intermedius may frequent LBBG-colonies as south as the Maasvlakte, the Netherlands (see map). Here, they establish mixed couples with paler Dutch intergrades. This sub(?)-adult summer was photographed June 17 2001. Note the brand new primaries in spring; it's hard to believe these are half a year of age already. Do some intermedius moult the primaries in late winter and early spring, prior to northbound migration? Intermedius from Scandinavia may be as dark as fuscus, as stated by Jonsson "... It is not possible to rely on this feature alone for sub specific identification". [BW 1998, p297 plate 4]. He found about 15% overlap in blackness (click also on the illustration). Of course he refers to adult LBBG, but it seems reasonable to believe an immature offspring of such intermedius might get as dark as an offspring of similar dark fuscus, although Jonsson (in BW 1998, p 307 figure 8) mentions the presence of "pitch black scapulars" in fuscus, as a feature to distinguish fuscus from intermedius. Warm brown hue on old coverts (mahogany hue) is mentioned to be common in fuscus. However, intermedius with old coverts can show strong brown hue on old coverts as well, as is demonstrated by this ringed Norwegian intermedius, September 22 2002, Le Portel, France.
Fuscus has the wintering grounds very far south (sub-Sahara Africa, see ring recoveries section) and this might give reason to moult upper-parts in a much higher pace than West European LBBG, which often winter north of Mauritania, Morocco and recently in (S)W Europe. For long, the main strand of thought was that graellsii and intermedius do not moult mantle and scapulars before departure to the wintering grounds. In fact they do; they sometimes even moult some coverts as early as August in the breeding colony (see the juvenile sections). On the wintering grounds, many intermedius replace the upper-parts and wing-coverts, just as many fuscus do. Subsequently, intermedius may return in spring with slightly worn second generation scapulars and coverts. They may even grow third generation dark plain feathers by May, as this LBBG from Le Portel (France), May 04 2001 shows. It's a 2cy LBBG with already a few third generation dark grey scapulars and moulting the median coverts.
average West European LBBG has most wing-coverts juvenile in spring. But quite some graellsii
and intermedius show advanced moult with all visible coverts included in
the moult on the wintering grounds. Here is an
example of a (presumed) graellsii, Etaples (France), May 05 (!!!) 2001: the
tertials are all second generation and the upper three show much abrasion.
Probably, these tertials were moulted already some time ago. The entire tail
has been replaced by second generation rectrices. Many wing coverts are second generation.
intermedius, this is probably the most advanced moult stage possibly in spring (by early May) in
2cy. Early May, P4 may be shed.
Right is an example of 2cy LBBG (May 11 2001, the Netherlands) with complete second generation body plumage. The tail was recently moulted; the white fringes are obvious. In the scapulars, the last moulted feathers are plain grey with a dark, almost black triangular centre. These examples from early May indicate what intermedius and graellsii may look like in June and July, when feathers start to get older, bleached and worn. Two examples are shown below.
2cy has abraded second generation upper-parts, which have to be
moulted in the winter quarters and shows a new tail. Such 1st summer fall in
3, see 2cy May,
with a simple pattern on all the 2nd generation upper-parts: plain brown-grey
with a pronounced shaft-streak. (Photographed June 17 2001, Naaktstrand
Maasvlakte, the Netherlands.)
Apparently, immature intermedius can show much variation regarding moult, abrasion and pattern on second and third generation scapulars and wing-coverts. In this, they may resemble fuscus to a high degree. The presence of third generation scapulars as early as May is rare, but nevertheless occurs in intermedius. And, what makes things problematic, they may be plain dark.
One of the most striking and easy clues to find immature fuscus in spring is arrested or suspended moult in LBBG in spring [see e.g. Jonsson in Birding World 1998 and Rauste in Limicola 1999: p 156 plate 31, p 157 table 32, p 159 plate 36, p 162 plate 40 & 41]. In suspended moult, there is a break in primary moult. Moult may be suspended during migration, so there is no gap in the wing, which might otherwise hamper efficient flight and maneuverability. After migration, suspended moult is picked up at the point where it stopped before the break. In arrested moult, a new moult-wave starts at the first primary P1. In some cases two waves can be found: halfway the wing moult continues after suspension and simultaneous moult starts at P1. In Western Europe, all these moult strategies can be found in intermedius. Approximately 3-10% of 3cy intermedius and graellsii LBBG show arrested or suspended moult in spring and early summer. Arrested or suspended moult in West European LBBG may correlate with migration distance. It seems reasonable to presume these LBBG belong to the race intermedius. Intermedius is, more than graellsii, a long-distance migrant and needs more time and energy to fulfill the journey back to Scandinavia. Energy can be saved by suspending the moult.
Here are two 3cy LBBG with arrested moult: a 3cy LBBG (right), seen on April 19 2003, Maasvlakte and a 3cy LBBG (left) was seen July 07 2001, IJmuiden, the Netherlands. They show arrested moult in the primaries at P7 (right) and P6 (left). The grey tone is in line with immature Dutch intergrades and intermedius and too pale for fuscus. In the tail, the central feathers have been replaced by fresh white feathers. Some fresh third generation rectrices still have black central markings. Note the pale yellow legs. Spring 2001, over 40 birds showing arrested moult were encountered in Belgium and the Netherlands, by only four observers.
Here are two other LBBG (intermedius?): one 3cy (left) from Le Portel, NW France, May 17 2002. It is actively growing P8. And a 3cy (right), photographed June 30 2001, IJmuiden, the Netherlands. It shows very advanced moult: almost all upper-parts were moulted and there is arrested moult in the primaries. In the section 'accurate ageing', another LBBG is mentioned: a 3cy LBBG Dutch intergrade in the Maasvlakte colony, May 27 2003. Arrested moult can still be recognized by late August, as the next individual clearly shows: a probable 3cy LBBG intermedius on August 29 2003, IJmuiden, the Netherlands, with arrested moult at P8.
Sometimes the distinction between the tips of P5 and P6 is very obvious, although they are the same generation and moult has not been suspended. P5 is the last primary hidden by the tertials and therefore less exposed to sun-light and severe weather conditions. In the picture right, July 7 2001, IJmuiden, the Netherlands, the tip of P5 looks much fresher than the tip of the next primary P6. The difference may become even more obvious by late summer. In real arrested moult, it's often the complete feather that looks more bleached, with a brown hue. To determine arrested moult between P5 and P6 may be difficult, while division between two generation primaries is easier to establish between any other two primaries.
How do you recognize fresh feathers? By the tips, which do not show any wear. Fuscus returns with pretty fresh primaries in northern Europe, especially when moult is suspended or arrested during migration. But intermedius may show very fresh primaries as well in spring, although the primaries must have been half a year of age. See e.g. this intermedius type sub-adult (5cy?) from June 17 2001, Maasvlakte, the Netherlands. Another intermedius showing fresh primaries in spring could be view in a previous paragraph.
Those 3cy showing arrested moult may otherwise look like the average LBBG in their age-class. Here is a 3cy, May 11 2001, Maasvlakte, the Netherlands. P1-P7 show white tips on fresh primaries. P8-P10 are still second generation and P10 has no white mirror. Arrested moult is a rare phenomenon, but when intermedius show arrested moult in the primaries, combined by advanced moult in the upper-parts, these birds can easily be mistaken for fuscus.
At Le Portel, France, three such kind of LBBG were present early May 2001, including a very dark, gentle (female) intermedius. The bird's characteristics strongly approached those said to be diagnostic for fuscus. Another intermedius at the beach is depicted here.
In Belgium, a colour-ringed intermedius (JUH6) with arrested moult at P9 was seen on May 27 2001. The bird was seen before in Morocco (58.03 N - 06.40 E) in February 2001. It was ringed at the south coast of Norway (Vest Agder county). The tail contained a few all-white feathers and the primary tips on P1-P9 were obvious.
So far, this paragraph deals with 3cy and sub-adult intermedius and graellsii. Most 2cy fuscus return to Scandinavia with second generation primaries and by August, these 2cy birds may replace inner primaries to third generation. In NW Europe, we haven't seen such advanced birds. The most advanced 2cy intermedius show second generation wing-coverts, tail and even second generation secondaries. Arrested primary moult in 2cy birds is very rare and so far not recorded in ringed birds.
Having said this about birds in spring, arrested moult may also occur in autumn of course, as this bird left shows, a sub-adult photographed October 22 2001 at Westkapelle, the Netherlands with P8 moulted but P9-P10 still old. Another example can be found in the right thumbnail: a bird with arrested moult at P1 in autumn, thought to be intermedius.
Autumn 2002, many adults and near-adults were checked on their primary moult pattern to get a more detailed overview of the possible moult strategies than may occur in graellsii and in intermedius. Typically, nominate fuscus show a step-wise moult sequence, resulting in several visible sets of primary generations, sometimes up to three sets. These sets are marked by a strong divide, visible in the field. We presumed this feature to be possible for intermedius as well, but we couldn't support the idea until autumn 2003. However, a few LBBG, most certainly from NW European origin and probably in sub-adult plumage were photographed autumn 2002 in NW Europe. While checking groups of 100's of intermedius and graellsii LBBG in October, about 5% of the adult / near-adult birds shows a primary moult strategy indicating suspension or 'near-suspension', often in the outer primaries. In the October sub-adult section, illustrative examples are shown. Most interesting is a presumed intermedius showing three moult waves in the wing. The odd 2cy may suspend moult as well, as this 2cy bird, photographed on October 05 2002 at Boulogne-sur-Mer (NW France) shows. Moult will be suspended at P8.
3cy nominate fuscus always seems to return with a complete replaced tail in the summer (Jonsson BW 1998, p. 309). These new rectrices may nevertheless show black markings. Most important point to establish in the field is wear in the tips. Tail-feathers that are a year of age show worn tips. Rectrices (as in this 2cy example) which have black markings, but without wear in the tips and which show a broad white fringe are almost certainly fresh feathers.
Normally, 3cy graellsii and intermedius LBBG in the colonies show only small variation in general appearance. But rectrices can show distinct differences: a clear tail-band, spaghetti pattern, black-and-white variegate patterns to snow white. Snow-white third generation tails in (presumed 3cy) intermedius occur. Often these 3cy are extremely hard to separate from 4cy. An example of a 2cy (!) October intermedius with an almost white tail can be found here.
2cy intermedius and graellsii normally return with a juvenile tail and start moulting the rectrices after arrival. However, some LBBG return with a complete second generation tail.
feathers are replaced because the old ones are damaged (gulls fight
pretty much and primaries may get lost and have to be replaced). When a single
outer primary is replaced in only one wing (so obvious no parallel wing moult)
it's clear such individuals do not follow an aberrant moult strategy. Sometimes aberrant
moult is hard to establish. An individual may look very advanced in
moult, but when the bird turns around, the view is completely different. here is
a graellsii from Dannes (France), May 06 2001. An example of how important it is to
view both sides of a gull, since the tertials left and right are at a different moult stage.
At a dump near Groningen, the Netherlands a very strange 2cy was present mid June 2001. At one side, all primaries were completely juvenile, the other wing showed completely second generation primaries. The size, feather-pattern and jizz were pretty fuscus-like.
A 3cy(?) LBBG, July 06 2001, IJmuiden, the Netherlands which is in many respects fuscus-like. The problem starts with correct ageing: is it 3cy or still 2cy? Bare parts advocate 3cy, but are these pointed outer primaries still juvenile? There is arrested moult in the primaries (P1-P6 are new, P1 is present and P7 is present). The new scapulars are almost black, it's a small bird, slender, long-winged, short-legged and the pale yellow bill appears almost parallel. The tail has a few all white feathers, while some fresh feathers have black markings. In some respect, it resembles a figure in Jonsson's article (BW 1998, p. 305 figure 4). Are those recently moulted scapulars as pitch black as necessary for fuscus? And what exactly is the lightest extreme in fuscus?
The right image shows another 3cy with arrested moult, at P7. It was photographed on September 02 2003, at Westkapelle, the Netherlands. Fortunately, it was ringed in Norway. Another example, of a probable 3cy fuscus was mentioned in a previous paragraph.
Fuscus most probably occur in Western Europe. If only 1% of
the Scandinavian fuscus follow a western migration route, dozens can be
expected annually in NW France, Belgium and the Netherlands. Unless
ringed, it's hard to get a clear view of the exact figures, since most features
in fuscus seem to apply to at least some intermedius
as well. This is not necessarily a problem when you see these gulls as clinal races of
one species: Lesser Black-backed Gull; and subsequently threat them as such: a
cline in colour and jizz from big and pale graellsii in the U.K.
to darker intermedius, ending in a small all-black fuscus in eastern
Scandinavia. And all types, forms or whatever you call them in between.
page deals with problems separating immature fuscus from intermedius
and adults in autumn. A low primary moult score (un-moulted old primaries or arrested moult at
P1 or P2)
may indicate fuscus, but this phenomenon occurs in graellsii
Some 40% of adult graellsii
may arrest moult at the end of the breeding season. Without doubt
including individuals just moulting up to P1 and P2...
All comments are much appreciated.
Many thanks to: Norman van Swelm, Rik Winters, Peter Adriaens, Peter Stewart, Visa Rauste and many others.
This is an ORG-publication: The Organisation for Research on Gulls supports cooperative field research and gull studies.
Mars Muusse, Theo Muusse & Bert-Jan Luijendijk