Ageing primaries in 3cy fuscus

Larus fuscus is a long distance migrant with an aberrant moult sequence compared to other large gulls in NW Europe. The primaries are already replaced within half a year, on the wintering grounds in a periodische Staffelmauser, or step-wise moult sequence, as described by Stresemann & Stresemann in 1966. As a result, most 2cy fuscus return in Scandinavia with a new set of (inner) primaries. Many 2cy birds even succeed in replacing the complete flight feathers set. Subsequently, primary moult continues both on breeding grounds and in winter quarters in immature birds, interrupted by migration. Moult can be arrested or suspended anywhere in the primaries, but in the field this phenomenon is most obvious in the outer primaries P5-P10, creating a clear division between fresh and old primaries. As moult progresses in "waves", it's often difficult to age the primary generation accurately.
On this page, we enlarge upon primary moult wave sequences in 3cy fuscus, by reconstructing moult sequences of returning ringed birds in SW Finland. We compare 2cy and 3cy moult stages to build a larger picture about the age of primaries, their appearance and the primary moult in immature fuscus.

Research methods

Best results may be expected from an annual observation cycle, but this is very hard to obtain in large gulls, and incredible challenging in fuscus. July 2003 and August 2002, we surveyed primary moult scores in 2cy & 3cy fuscus at Tampere dump in SW Finland, to construct a picture of primary moult sequences in immature fuscus. Sub-adult fuscus is not included in these surveys, as the differences between primaries moulted prior to or after suspension is often difficult to determine in 4cy fuscus. Therefore, we focused on 2cy and 3cy birds only.

In August 2002, 2cy and 3cy fuscus were identified using standard binoculars (8x40 and 10x40 Leica) and telescope (10-60x zoom Leica Televid). Once 2cy or 3cy ringed birds were determined, one person described primary moult scores through binoculars, while the other person tried to photograph the bird with open wings, to check the score and for back-up control of observations.
The method used in July 2003 was slightly different, as only one person was in charge of both describing and photographing birds, but in time, the 2003 period was almost twice as long.
When inner primaries are described on this web-page, this is always supported by pictures of birds with open wings (although not all pictures were good enough to publish here). Sometimes, the moult sequence in the left wing varies from the sequence in the right wing; if so, this is mentioned as well.
To be sure about correct ageing, we only describe colour-ringed or metal-ringed birds at this page. As can be seen in the captions, we received ringing information and life histories of nearly all birds. We believe there are enough clues to include the other birds as well in this 3cy section, and when available, more details on these ringed birds will be added on this page.

General primary moult sequence in immature fuscus

Fuscus leave Scandinavia as juvenile birds (1cy). Most birds migrate to the Mediterranean and Africa in complete juvenile plumage with fresh first generation primaries. In Scandinavia, fuscus are not seen in winter and absent until early summer of the next year. Ring recoveries suggest that only small numbers of 2cy birds return, the vanguard arriving on July 01. Many 2cy birds return with completely replaced primaries and secondaries, now second generation. Often, P10 is included in this moult as well. The second generation outer primary P10 has no mirror. While spending the summer months in Scandinavia, the next moult wave is started at the inner primaries. This moult wave is suspended for migration in autumn.
One year later, 3cy fuscus again return in Scandinavia in spring, often showing a clear division between fresh inner primaries and old outer primaries. The inner primaries show neat white tips, while the outer primaries are paler brown and worn. Most 3cy birds show a well-defined mirror on P10, indicative for third generation. Arrested moult in the primaries is often reflected in the secondaries and (to a less extend) in the tail-feathers as well. As was the case in 2cy birds, 3cy start a moult wave in summer as well.

Primary appearance in large gulls as reference

Since fuscus is a long distance migrant, spending "winter" under the equatorial sun, some birds return with strongly worn and bleached primaries. In many cases, the exact age of the primaries is hard to determine in this condition. As a reference, we first describe the ordinary appearance of primaries in graellsii / intermedius, which are closely related taxa with primary moult in summer in NW Europe. The general appearance of primaries in graellsii / intermedius is slightly different from fuscus, as graellsii / intermedius moult the primaries half a year later. It's interesting to see the differences in pattern (especially P4-P6) created by this time interval. Remember that the extend of the white mirrors on P9 and P10 is limited in fuscus and may further depend on sexes.

LBBG graellsii / intermedius 1cy, IJmuiden, the Netherlands, August 22 2008. First generation (juvenile) primaries in fresh condition in graellsii / intermedius.

LBBG graellsii / intermedius 2cy, IJmuiden, the Netherlands, June 30 2001. First generation outer primaries in worn condition in graellsii / intermedius. Primaries already 12 months old. Primary moult started; the 4 inner primaries now 2nd generation.

LBBG graellsii / intermedius 2cy, Brouwersdam, the Netherlands, September 02 2001.Second generation primaries in fresh condition in graellsii / intermedius. PMS (primary moult score): 43.

LBBG graellsii / intermedius 3cy, Maasvlakte, the Netherlands, May 31 2002. Second generation primaries in worn condition in graellsii / intermedius. Primaries already 10 months old. Note the bleached brown inner primaries and note the absence of black sub-terminal markings on P4-P6.

LBBG graellsii / intermedius 3cy, Maasvlakte, the Netherlands, September 02 2001. Third generation primaries in fresh condition in graellsii / intermedius. Note the black sub-terminal bands on P4-P6 and old 2nd generation outer primaries, P10 mirror absent.

LBBG graellsii / intermedius 4cy, Maasvlakte, the Netherlands, June 16 2001. Third generation primaries in worn condition in graellsii / intermedius. Primaries already 10 months old. Note the bleached brown outer primaries and abrasion on the fringes. The white tips wear away easily.

LBBG graellsii / intermedius 4cy, Maasvlakte, the Netherlands, September 17 2001. Fourth generation primaries in fresh condition in graellsii / intermedius. Note mirror on old P10.

Primary moult in immature fuscus: literature

The primary moult in immature fuscus has been described poorly in earlier papers, but much changed with the publications by Jonsson (1998) and Rauste (1999).

According to Jonsson, 2cy fuscus start to moult the fight-feathers after finishing the replacement of wing-coverts by late January or February. Jonsson describes ten 2cy fuscus from late January in Israel and none had shed any primary yet.
In museum collections, he found four birds: one 2cy fuscus from late January, growing P1 and P2. Two other birds from April, found in Angola moulted the inner primaries (moult scores 13 and 46) and another bird from Aden almost finished its primary moult by early May (moult score 39). For explanation of moult scores: see Topography Section. A summary of the Jonsson article can be found here.

Initiating of primary moult in February nicely fits the fuscus picture, as this replacement takes an average 4 months. Hence, 2cy birds are just in time to finish this moult to migrate north again by late-May – June.

From 1995 to 1998 Jonsson described 2cy fuscus after arrival at Gotland, south Sweden, and these birds showed a wide range of variation. Most 2cy birds finished moulting all of the flight-feathers (e.g. MC24 & C3VS) and a minority arrested moult in the primaries prior to migration (e.g. C5UJ). However, some 2cy fuscus return in Scandinavia with all flight feathers still juvenile (e.g. this bird).

This is in line with the findings of Rauste. He estimates that 60-70% of 2cy birds return in Finland with complete new flight-feathers. He examined 19 2cy fuscus in Finland; 15 individuals had replaced all the primaries. Second generation primaries show rounded tips (unlike the pointed juvenile primaries) and the P10 mirror is absent. A mirror on P10 is very rare, but can be possible in some 2cy birds. Rauste: "In einem Fall habe ich im Juni kleinen Spiegelfleck einer erneuerten H10 gesehen.". It is not clear whether these birds spend the winter in the Mediterranean or in Africa. After arrival in Finland, most 2cy fuscus (70-80%) initiate the next moult, now moulting the inner primaries to 3rd generation (e.g. CJUC).
A minority of 2cy fuscus return with several (20 out of 64, fig 32) or all (0 out of 64, fig 32) primaries still juvenile, and these birds continue/start primary moult in Finland (see 2cy July section). Interestingly, it may thus be possible to see two 2cy fuscus, both moulting the inner primaries. One bird moulting to 2nd generation, the other moulting to 3rd generation. Rauste examined one colour-ringed bird which continued moult in the outer primaries and in the mean time started moulting the inner primaries (P1-P2 fresh 3rd generation, P3 missing, P4-P8 2nd generation, P9-P10 growing 2nd generation). Such a moult wave is also showed by this 2cy bird.

Most 2cy birds remain at the wintering grounds or rich feeding grounds at stop-overs, and this is confirmed by colour-ringed Finnish 2cy fuscus. Rauste: "... mindestens 5-10% der Vögel im 2en KJ für kurze Zeit im Hochsommer in die Brutgebiete zurückkehren. Die meisten dieser Vögel wurden zwischen Mitte Juni und Ende August gesehen." Rauste stresses that, since such a large proportion of 2cy birds do not return in Finland, moult sequences may develop in a different way in these birds; different from what he described from his field observations in Finland.

The second renewal of the primaries start in the summer months in Scandinavia, but fuscus moult central and outer primaries in Africa again. Jonsson examined eight birds from museum collections and there is a lot of irregularity involved in moult sequences of autumn 2cy birds. In his opinion, 2cy birds in autumn are not moulting the primaries in the ordinary sequence (from P1 to P10), but in “waves”. Jonsson doesn’t further enlarge upon these waves, but the final result, when 3cy birds arrive in spring, are obvious visual moult steps in the primaries (normally at P6, P7, P8 or P9). This pattern of arrested moult is regularly found in 3cy fuscus and must be a common characteristic.

Jonsson examined two 2cy birds found in Africa in October; both were actively moulting the outer primaries. This is a nice example of “continuation after suspension”. Prior to spring migration, 3cy fuscus arrest primary moult again in the outer primaries, resulting in the obvious moult step when they return in Sweden by late-April and early-May. The obvious mirror on P10 can be explained by the late requirement of this second-generation P10, which is therefore more adult-like in appearance.

Rauste estimates that a large proportion of 3cy fuscus return to Scandinavia. He doesn't describe the exact generation of the primaries, but like Jonsson, he stresses the division in the primaries in 3cy fuscus: "Die Handschwingen stammen in der Regel aus zwei Generationen, was an stehenden Vögel nur zu erkennen ist, wenn die Mausergrenze innerhalb der äusseren 5-(6) Handschwingen liegt." The inner primaries are adult-like, the outer primaries are more or less worn and bleached brown.
In some individuals, primary moult simply shows a chaotic sequence. This probably results from the aberrant moult stage in which some individuals arrive at the wintering grounds and continue the moult after suspension. Many 3cy fuscus already start primary moult in the inner flight-feathers in Scandinavia (e.g. CNMR), only to suspend this moult during migration and continue moult on the wintering grounds as 3cy-4cy birds, just like adults.

Primary moult in 2cy fuscus: observations in Finland

In Finland we encountered only small numbers of 2cy birds in July and August (daily maximum about 30 birds), of which five ringed birds and one 2cy bird was ringed at the spot. Four of these birds show completely replaced flight-feathers: primaries P1-P10 all 2nd generation, with rounded tips and P10 lacking a mirror. Two birds arrested primary moult at P9, leaving P10 still juvenile in summer. One single juvenile outer primary seems to be no problem and the replacement of such remaining feathers may take another half year, at the wintering grounds. See e.g. CNMR, an individual which arrested moult at P9, which re-appeared with second generation P10 in 3cy. 
Out of the six ringed birds, two started a new moult wave to third generation in Scandinavia: C6XV and CJUC. Most 2cy birds leave Tampere dump between mid-August to September, and presumably 2cy don't initiate primary moult in Scandinavia in these months, just before migration.

From these data it may be concluded:
- Some 2cy fuscus arrest moult prior to migration in 2cy spring. 
- If they do arrest moult, this is no limitation to continue moult at P1, leaving P10 juvenile.
- Initiation of primary moult to 3rd generation may occur in 2cy fuscus in summer (July & August).

Primary moult in 3cy fuscus: observations in Finland

Brown inner primaries, outer primaries lack clear white tips, no mirror on P10. This is the ordinary appearance of the western taxa graellsii and intermedius and applies to 2nd generation primaries. There was only one such observation among ringed fuscus in Finland: C0NM. Apparently this is an uncommon moult strategy among fuscus and can be regarded as 'delayed' .

Brown primaries with obvious mirror on P10. P10 mirror explained by late moult of this primary on the wintering grounds, but actually all primaries still 2nd gen. In the checked group of 3cy fuscus, there were no such birds. Probably, birds that moult P10 in a late stage, continue with the inner primaries.

Outer primaries brown and bleached, no mirror or faint mirror on P10, while inner primaries are fresh, blackish with obvious tips. Combination of 3rd gen inner primaries and old 2nd gen outer primaries.

Outer primaries brown, second generation. No mirror on P10 Inner primaries obviously white tipped third generation. This is the largest class of birds. The division is often in the outer primaries, at P6, P7 or P8. Generally this can be regarded as 'common strategy'. 

Outer primaries blackish, obvious mirror on P10. P10 and probably all primaries 3rd gen. Only one such bird was found: C9NN. 

Outer primaries blackish with an obvious mirror on P10, probably third generation. The inner primaries are fresh, adult-like, probably fourth generation. Combination of 4th gen inner primaries and older 3rd gen outer primaries. A few birds show this strategy. To conduct such a moult pattern, these birds must have advanced moult in 2cy.

The last group include birds with 'really aberrant' moult schemes, which may be classified as waves. Illustrative for this group is C5XM.

References

Jonsson L. 1998. Baltic Lesser Black-backed Gull Larus fuscus fuscus – moult, ageing and identification. Birding World Vol 11 no 8.

Stresemann E. & Stresemann V. 1966. Die Mauser der Vögel. Journ f Ornith. (sonderheft) 224-232.

Rauste, V. (1999). Kennzeichen und Mauser von "Baltischen Heringmöwen" Larus [fuscus] fuscus und "Tundramöwen" Larus [fuscus] heuglini Limicola 13:105-128 (Part I) & 13:153-188 (Part II).

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