Before departing to the wintering grounds, some juvenile fuscus moult several mantle feathers and scapulars to second generation feathers. Migration may start early, with juvenile birds arriving in Africa by late September, but most arrive in October, in some cases in complete juvenile plumage. Normally an extensive moult starts after arrival: scapulars, coverts, secondaries and primaries are often all included in the early complete moult on the wintreing grounds. By April, some 2cy birds migrate north to the breeding areas. On arrival, the wing may look anything from only a few new replaced feathers to completely 2nd generation primaries. Sometimes, moult is arrested during migration and a clear division is visible between outer old juvenile primaries and fresh 2nd generation primaries.

Primary moult of Baltic Gull during the first 15 months (in: Dutch Birding 28: 158-161)
Hannu Koskinen & Visa Rauste (Finland)

The moult strategies of the members of the Lesser Black-backed Gull complex have been studied extensively during the last decade. Jonsson (1998) was the first to describe moult patterns of nominate fuscus during the first winter and summer: the most frequent pattern is a complete moult before the first summer, but there are lots of variation. Rauste (1999) gave some quantative data of the subject. As these data comprised only 19 ringed birds, they left much room for discussion.

Subsequent studies have revealed that the difficulties of subspecific identification were severely underestimated in the 1990's (Gibbins 2004, Winters 2006, pers. obs), as moult patterns proved much more variable. It has also become obvious that unknown but possibly significant numbers of western taxa (L. f. intermedius, L. f. graellsii, hereafter intermedius and graellsii respectively) may occur in Finland in addition to the local subspecies fuscus and the fourth member of this complex, Heuglin's Gull, Larus heuglini (hereafter heuglini) (pers. obs.). This casts some doubts on the homogeneity of the data set used by Rauste (1999). Thus, the reliability of Rauste's results have been questioned with good reason.

To study the primary moult with more reliable material we have assembled a data base of birds which have been ringed as chick in Finland (n=49) or Eastern Sweden (n=4) and that were observed in Finland during their second calendar year. Table 1 shows the phase of primary moult of these birds. It seems to confirm the results of Rauste (1999): in addition to the early moulting majority there is a long ‘tail’ in the distribution of moult scores. The least advanced individual had not even started its primary moult before its arrival in Finland. 66% had completed the first primary moult before returning to Finland, which is comparable with the estimate given by Rauste (1999) of 60-70%. Most of the birds which had completed the first moult started the second primary moult during the summer and this moult can proceed to P4. Five individuals had two simultaneous moult waves in the primaries (‘Staffelmauser’ which is known from older age classes of fuscus since Stresemann and Stresemann 1966). It is also worth noting that the most advanced birds were more than one whole primary moult cycle ahead of the least advanced ones. Such wide variation has probably not been documented in any taxon with such a restricted range and homogeneous migration strategy.

The Finnish breeding population has been considered very homogeneous, all birds being typical fuscus. However, there are records of birds which have been ringed as chick in Southern Finland that have nevertheless developed a paler mantle in (sub)adult plumage. The palest of these birds have a mantle shade which is very close to typical graellsii or heuglini.

It may be a matter of opinion or definition what these paler-mantled birds should be called. In spite of their appearance they have nevertheless hatched in the core range of fuscus. They may be considered as variation of fuscus, or as genetic influence of other taxa, in which case it is unclear if western (graellsii/intermedius) or eastern (heuglini) (or both!) birds are involved. Based on structure and other clues we think that these birds are more similar to western birds than that they resemble heuglini (Koskinen at all 2001).

The frequency of such birds is low: some six pale-mantled birds have been seen among c 1700 ringed fuscus in Tampere (less than 0.5%). Quite surprisingly, at least two such birds have been included in table 1: CYHP and CEVC have been seen and photographed in Tampere in 2005 as third caledar-year and both showed a significantly paler mantle than a typical fuscus. It is worth noting that, apart from the moult, also the plumage of CYHP in its second calendar-year caused lots of debate (see Gibbins 2004).

If we apply the strictest definition of fuscus and accept only those birds of which both provenance and appearance as an adult are known, there are only 27 such birds in our data set that fulfill these criteria (marked ‘YES’ in table 1 in the column ‘controlled as 2+cy’). Based on this limited number, the variation of the primary moult during the first winter seems to be equally variable as it is among all ringed birds together. This indicates that the variation in moult patterns is not solely due to the inclusion of birds of other populations, but rather that the variation is likely to be normal among fuscus.

Markkhu Kangasniemi, Hannu Kettunen, Janne Kilpimaa, Harri Kontkanen and Veijo Turunen contributed details to the moult data base. Risto Juvaste has organised the Finnish colour ringing project which is the base of this study. Ruud Altenburg, Dick Forsman, Mars Muusse and Rik Winters have commented on the manuscript.

References

Gibbins, C N 2004. Is it possible to identify Baltic and Heuglin’s Gull? Birding Scotland 7: 154-186.

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

Koskinen, H, Kettunen, H & Kangasniemi, M 2001. Pale-backed Lesser Black-backed Gulls in Tampere. Linnut 36: 27-30 [in Finnish].

Rauste, V 1999. Kennzeichen und Mauser von ‘Baltischen Herringmowen’ Larus fuscus fuscus und ‘Tundramowen’ Larus fuscus heuglini. Limicola 13: 105-128, 153-188.

Stresemann, E & Stresemann, V 1966. Die Mauser der Vogel. J Ornithol 107, Sonderheft.

Winters, R 2006. Moult and plumage variation in immature Lesser Black-backed Gulls in the Netherlands. Dutch Birding 28: 140-157.

Tables