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Some Confusing Lesser Black-backed Gulls (Larus fuscus)

by Peter Adriaens

In recent years, some very detailed articles on the identification of Baltic Gull Larus (fuscus) fuscus have been written, notably by Jonsson (1998) and Rauste (1999), which have generated an increased interest in the identification of Lesser Black-backed Gulls, not least in Western Europe. The identification of fuscus, as described by the above authors, is to be based on a number of key features, one of the most important being primary moult, both in its extent and in its timing. The application of these criteria has resulted in several out-of-range Baltic Gulls being identified, and accepted by local rarities committees, e.g. in The Netherlands, with nine accepted records up to and including 2000 (Van der Vliet, Van der Laan & CDNA; 2001).

However, both articles were primarily written from a Swedish/Finnish point of view, i.e. telling British or Scandinavian Lesser Black-backed Gulls Larus (fuscus) graellsii /intermedius from local fuscus. Applying the described characters the other way round, may perhaps be a dangerous thing to do. Recent observations from The Netherlands (Muusse 2001; Winters & Bakker, in prep.), Belgium, and Great Britain (Brian Small, pers. comm.) seem to indicate that studying much larger numbers of intermedius and graellsii reveals not just a few, but a surprising number of birds with odd characters, both in plumage and crucially as well as moult.

This article should be seen as a word of caution, and illustrates examples of such "odd" birds, all photographed in Belgium: a number of graellsii/intermedius and one possible fuscus are shown. It may well be that many 'odd' birds are not likely to cause identification problems, since other features they show are clearly those of the more common taxa. What they do show, however, is that care, as always, must be taken when applying the criteria as described by Jonsson and Rauste and that, in fact, moult is not as diagnostic of fuscus as first thought.

Short note on Ageing

The first step in identifying (sub)species of Lesser Black-backed Gull, is correctly ageing the bird. However, this in itself can be a problem at times, since immature Lesser Black-backed Gulls are highly variable and some birds look more advanced than others. When trying to age such birds, it is therefore important to look not only at the colours and patterns of the scapulars, wing coverts, and bare parts, but also to examine all of the primaries to correctly determine their age.

A. Second summer

By far the most "odd" birds observed to date have been second summer. For this reason I have chosen to start this article with this age. Typically, such birds show a plumage that resembles intermedius or graellsii, but their moult state is significantly advanced, much like fuscus.

Examples are shown below. Figures 1-3 show a colour-ringed bird (left leg, blue; white code "JUH6"), which was caught as a pullus in southern Norway in the summer of 1999 and is therefore certainly a 2nd summer intermedius. In February 2001, it was seen in Morocco. The bird shows an arrested primary moult, with 9 new primaries (3rd generation); only P10 is 2nd generation. The moult of its wing coverts, tertials, scapulars, and mantle feathers is also advanced.

Fig. 1: Second summer intermedius, Zeebrugge, Belgium, 27 May 2001

A colour-ringed bird. Nearly all visible primaries are third generation; also, notice the large amount of 3rd-generation wing coverts and tertials (adult-like).

Fig. 2: Second summer intermedius, Zeebrugge, Belgium, 27 May 2001

The outermost primary (P10) is browner and more worn than the others, indicating it is one year older. It lacks a white mirror and white primary tip. Primary moult has been suspended; there is no active wing moult - P2 is simply displaced here.

Fig. 3: Second summer intermedius, Zeebrugge, Belgium, 27 May 2001

The difference between the two generations of primaries is well visible. From below (on the far wingtip), it is easily seen that P10 lacks a white mirror, showing an obvious pale shaft instead.

Figures 4-6 show another bird that has an arrested primary moult, which seems to occur in two different waves: one from the inner-most primary on, and one in the middle of the outer wing. The colour of the upperparts does not suggest fuscus.

Fig. 4: Second summer Lesser Black-backed Gull, Nieuwpoort, Belgium, 25 May 2001

Fig. 5: Second summer Lesser Black-backed Gull, Nieuwpoort, Belgium, 25 May 2001

P1 and P5-7 new (3rd-generation); all others retained. No active moult in the primaries.

Fig. 6: Second summer Lesser Black-backed Gull, Nieuwpoort, Belgium, 25 May 2001

In the field, the difference between 2nd and 3rd generation primaries can be really striking which, to a certain extent, probably explains why the majority of "odd" birds recorded, are from this age group. Note also the white outer tail feathers.

In Figures 7 & 8, a very advanced 2nd summer is shown. No old, brownish feathers are retained on the upperparts, nor on the upperwings. The bird basically resembles a 3rd summer, but reveals its true age by its retained old outer primaries (2nd-generation), which are clearly brown and lack white tips. The upperparts of this bird were rather blackish-grey (quite dark).

Fig. 7: Second summer Lesser Black-backed Gull, probably intermedius, Nieuwpoort, Belgium, 25 May 2001

The retained, brownish and plain P8-10 can just be seen here.

Fig. 8: Second summer Lesser Black-backed Gull, probably intermedius, Nieuwpoort, Belgium, 25 May 2001

Just like the bird in Fig. 4, there are two different moult waves: P1 is new, and so are P5-7.

The bird in Figures 9-10 is perhaps more complicated. Its moult pattern seems to be asymmetrical, with one new primary in the middle of the left outer wing (P7), while in the right wing, only P1 seems to be really 3rd generation. The colour of the upperparts does not suggest fuscus. The outermost primaries (P8-10, both wings) are very worn.

Fig 9: Second summer Lesser Black-backed Gull, Nieuwpoort, 25 May 2001

Fig 10: Second summer Lesser Black-backed Gull, Nieuwpoort, 25 May 2001

In this bird the outer primaries are heavily abraded which may perhaps indicate that the bird spent the winter far south.

It should be stressed that birds like these are not rare in spring; indeed, figures 4-10 all show birds that were part of one and the same flock of gulls, containing about 500 Lesser Black-backed Gulls. Apparently, in the north of the Netherlands, up to 30 second summer Lesser Black-backed Gulls with arrested primary moult are sometimes recorded in one single day (Rik Winters, pers. comm.).

For a detailed discussion of Lesser Black-backed Gulls (with arrested moult), see also Mars Muusse's website: http://www.geocities.com/muussesite1/miscellaneous/discusfus

B. First summer

Several 1st summer Lesser Black-backed Gulls showing arrested primary moult have been recorded in the Netherlands in spring. Some resemble fuscus, others don't.

The difference between first and second generation primaries is usually subtle, the latter being slightly darker and more rounded. Good views and, preferably, good photographs are therefore necessary to establish the presence of two different generations of primaries in 1st-summer birds and this may partly explain why they are much less often recorded than 2nd-summer birds. It is also possible that 1st-summer birds remain on their wintering grounds more often.

Note that, even in spring, the (juvenile) primaries of 1st-summer Lesser Black-backed Gulls may look surprisingly fresh and dark, making it even more difficult to correctly assess any moult patterns.

The plumage of 1st-summer graellsii/intermedius is also highly variable. I used to think that they are rather unlikely to show any 3rd-generation (i.e. adult-like) scapulars and/or wing coverts in spring and summer, unlike 1st-summer fuscus which often has those, but I am not so sure any more. It seems that such birds turn up occasionally.

The bird in Figures 11-13 is a good example. It was filmed at Zeebrugge, Belgium, in early August 2001. Even though it has a surprising amount of adult-like scapulars, mantle feathers, and wing coverts, it can be confidently aged as a 1st summer because its newly acquired inner primaries (P1-4) are only 2nd generation, its retained outer primaries (P7-10) are still juvenile, and its underwings (not visible in the the photographs presented here) and bill are still largely dark. The uniform, very dark upperparts/upperwings with many 3rd-generation feathers (adult-like), contrasting with mainly whitish underparts and head may suggest fuscus, but in flight the bird revealed active primary moult with four retained juvenile outer primaries - a moult score that was matched by many of the local 1st-summer graellsii/intermedius too.

Most 1st-summer fuscus that return to the breeding grounds in spring, will show a complete or almost complete set of new, 2nd-generation primaries. Some birds even start a new moult wave already in July or August, acquiring some 3rd generation primaries. The bird in Figures 11-13 is certainly less advanced. Moult, however, is very variable in 1st summer fuscus, and some birds return with some or many (even all) juvenile flight-feathers retained. A moult pattern as shown by the Zeebrugge bird is therefore not impossible for fuscus, though the juvenile primaries are normally extremely abraded in such birds (cf. Jonsson, 1998; Rauste, 1999), which is not the case here. Oddly enough though, the bird has a complete set of new tail feathers, which appear to be of two different generations - 2nd and 3rd generation, the latter being entirely white. This advanced tail moult is quite surprising in combination with the normal primary moult, and this combination is said to be unique for 1st summer fuscus (Jonsson 1998, p. 305). From a Western European point of view, however, it is perhaps better to leave this bird unassigned.

Fig. 11: First summer "Lesser Black-backed Gull", Zeebrugge, Belgium, 4 August 2001

The upperparts and upperwings already show many adult-like feathers; note e.g. the blackish tertials with relatively broad white tips. The degree of darkness of this bird's upperparts can be appreciated by comparing it to the adult Lesser Black-backed Gulls to the left of it.

Fig. 12: First summer "Lesser Black-backed Gull", Zeebrugge, Belgium, 4 August 2001

It may seem difficult to believe, but both birds in this photograph are of the same age.

Fig. 13: First summer "Lesser Black-backed Gull", Zeebrugge, Belgium, 4 August 2001

Primary moult matches that of 1st summer graellsii/intermedius at this time of the year, but the tail moult is much more advanced. There appear to be no juvenile rectrices left, though one tail feather with a somewhat disproportionate amount of black is remarkable.

C. Adult

Much emphasis has been placed on moult by those defining differences between adult fuscus and adult intermedius, since all other characters are known to overlap. Moult, however, is variable in large gulls, and to a certain extent also influenced by variable factors, such as climate/weather conditions (both on the breeding and wintering grounds), migration, state of health etc. There have been several reports of adult graellsii/intermedius moulting as late as late January or February to even early March. So far, nothing has been published on such birds, but clearly, more research into the variation of moult patterns in intermedius is needed to establish whether there is overlap with fuscus or not. On current knowledge though, it seems more likely that there is some overlap, and that it may be better to put the identification of adult suspected fuscus in Western Europe on ice, until we know more.

Figures 14-15 below are just an example of a small, dark Lesser Black-backed Gull, which is most likely an intermedius. This bird spent two consecutive winters (winter 1999-2000 and winter 2000-2001) at the same nature reserve in Belgium. Its moult had always been completed when it turned up.

Fig. 14: Adult intermedius, Harelbeke, Belgium, 17 February 2001

Note the rather blackish upperparts and entirely white head compared to the accompanying adult Lesser Black-backed Gulls. The white tips on the primaries and tertials are much less distinct, even though these parts were quite fresh. Also, note smaller size and slimmer build.

Fig. 15: Adult intermedius, Harelbeke, Belgium, 17 February 2001

This individual was easily located in both winters by means of its damaged right wing (P8 missing). There is no white mirror on P9, and the mirror on P10 is fairly small.

Figures 16-19 show an adult Lesser Black-backed Gull with all relevant characters of fuscus, which was filmed in September 2001. When more is known about moult in intermedius, this bird might get accepted as fuscus, but for the time being Baltic Gull officially remains off the Belgian list.

Fig. 16: Adult possible fuscus, Het Zwin, Belgium, 23 September 2001

Compare the colour of the upperparts with that of the Lesser Black-backed Gull in the foreground. The bird had quite an elongated profile, though it was not really smaller than other Lesser Black-backed Gulls present.

Fig. 17: Adult possible fuscus, Het Zwin, Belgium, 23 September 2001

The upperparts are about as black as the primaries. Note the very worn state of plumage, with primary and tertial tips much reduced.

Fig. 18: Adult possible fuscus, Het Zwin, Belgium, 23 September 2001

Wing moult has not started yet; all primaries were still present and were worn, as can be told by the brownish tinge on the inner ones here. There is also some brownish tinge on the secondaries and inner primary coverts.

Fig. 19: Adult possible fuscus, Het Zwin, Belgium, 23 September 2001

All ten primaries are still present. There is a thin but complete black subterminal band on P4, which can also be shown by some dark adult intermedius (and some graellsii).

Conclusion

Identification of fuscus in Western Europe seems clouded by great variability of graellsii/intermedius. It also appears that timing and progress of moult, especially in intermedius, is not yet fully understood. More research would certainly be very useful and, in this respect, colour-ringed birds may be of great help.

This article does not deal with Heuglin's Gull Larus (fuscus) heuglini, but variation in moult and plumages of Lesser Black-backed Gull clearly make identifying this taxon in Western Europe equally challenging.

Acknowledgements

Many thanks to Nils Helge Lorentzen for providing ringing data on the 2nd summer intermedius from Zeebrugge (code JUH6). Also, many thanks to Mars Muusse, Rik Winters, Visa Rauste, Brian Small, Richard Millington, Gunter De Smet, Diederik Kok, and Laurent Raty for discussing "Lesser Black-backed Gulls".

Bibliography

Jonsson, L. 1998. Baltic Lesser Black-backed Gull Larus fuscus fuscus - Moult, ageing and identification. Birding World 11,8: 295-317.

Muusse, M. 2001. Immature LBBGs resembling fuscus in Western Europe (so-called discusfus LBBGs). http://www.geocities.com/muussesite1/miscellaneous/discusfus.

Muusse, M. 2002. Suspended moult in adult graellsii in summer. http://www.geocities.com/muussesite5/arrestedmoult/peterlbbg.

Rauste, V. 1999. Kennzeichen und Mauser von "Baltischen Heringsmöwen" Larus [fuscus] fuscus und Tundramöwen Larus [fuscus] heuglini. Teil 1. Limicola 13,3: 105-128.

Rauste, V. 1999. Kennzeichen und Mauser von "Baltischen Heringsmöwen" Larus [fuscus] fuscus und Tundramöwen Larus [fuscus] heuglini. Teil 2. Limicola 13,4: 153-188

Van der Vliet, R. Van der Laan, J. & CDNA; 2001. Rare birds in the Netherlands in 2000. Dutch Birding 23,6: 315-347.

Informed comments are welcome: p_adriaens@yahoo.com