Amir Ben Dov (Israel)
Chris Gibbins (Scotland)
Hannu Koskinen (Finland)
Mars Muusse (the Netherlands)
4cy / sub-adult heuglini: July
Characteristics and moult of "Baltic Lesser Black-backed Gull" Larus [fuscus] fuscus
and “Heuglin’s Gull" L. [fuscus] heuglini
|(1, 2) adult heuglini.
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The plumages and moult of Larus [fuscus] fuscus and L [f] heuglini are described and their identification discussed, mainly on the basis of extensive experience in Finland and two trips to Archangelsk (Russia). The description of fuscus is mainly based on ringed Finnish birds of known age. Comparisons are also made with the western members of the "fuscus group" (graellsii and intermedius).
STRUCTURE: Fuscus is structurally at the extreme end among the European large gulls (argentatus-cachinnans-fuscus-complex): it is the smallest and has the proportionately longest wings and a thin bill. Heuglini is between fuscus and (Finnish) argentatus in size and often more similar in structure to fuscus. It has rather long wings, a small head, a long neck, a long bill and long legs. Graellsii is somewhat smaller compared to heuglini, and has a larger head, thicker bill and more compact structure. However, individual variation is extensive and structural features
are not very useful when identifying a lone bird, especially a suspected vagrant.
ADULTS: When identifying adult fuscus outside the normal range, the problem is to separate it from the darkest intermedius. The upperpart tone alone is not sufficient. The moult pattern and head pattern can support the identification of adult fuscus during autumn migration. Fuscus moults 0-2 (-3) of the innermost primaries on the breeding grounds and suspends the moult before migration, whereas the western forms usually moult continuously throughout the autumn. Fuscus usually has a whitish head in autumn, whereas the western forms show extensive dark mottling.
It is extremely difficult or impossible to reliably identify an adult heuglini or graellsii outside their normal ranges, especially in spring. The structure can offer some clues. There are small average differences in the wing tip pattern (heuglini has more primaries with black patterning and smaller mirror[s] on the outermost primaries, but the overlap is extensive). The iris is sometimes rather dark in heuglini. In winter, the head streaking is more extensive and more evenly distributed in graellsii and there is a distinct average difference in the timing of primary moult (June to March in heuglini, suspending the moult during migration, May to January in graellsii, usually continuous), but here, too, there is overlap. In addition, it may be risky to pay too much attention to the moult phase when identifying a vagrant, as untypical moult patterns occur in all gulls.
|(3, 4) 1CY heuglini.
FIRST CALENDAR YEAR: In juvenile and 1st winter plumages all members of the fuscus group can be separated from other large gulls (except some michahellis) on the basis of the dark inner primaries. The plumage of fuscus is the most contrasting in the fuscus group: the dark centres to the upperpart feathers are blackish brown and the narrow fringes whitish. The white ground colour is prominent on the head and the underparts. There are often one or two whitish areas on the underwing coverts.
The general appearance of juvenile heuglini is variable: some are rather
dull and lack contrast, and therefore similar to Herring Gull on the ground, while some are virtually identical to a typical fuscus. Usually the whitish fringes of the upperpart feathers are broad, giving a pale and cold greyish tone to upperparts. Compared to fuscus, the greater coverts are more extensively barred, the dark tail band is narrower, the underwing coverts are paler with finer patterning, and the inner primaries are often slightly paler.
Both fuscus and heuglini can moult some scapulars before migration. In fuscus the new 1st winter scapulars are usually rather uniform dark brown with paler fringes, whereas in heuglini they vary from dark brown to rather pale with an anchor pattern similar to that in Herring Gull scapulars. Because of extensive individual variation, I suggest refraining from identifying members of the fuscus group in juvenile plumage outside their normal ranges until the variation in each form is better understood.
|(5, 6) 2CY heuglini.
SECOND CALENDAR YEAR: Some data about the primary moult of fuscus and heuglini during the 2nd cy is presented here. ln fuscus, the moult pattern is very variable. A majority of the birds which occur in Finland during summer (this is, however, only some 5-10% of the age class, the majority staying in more southerly regions) have moulted all their primaries. A minority has moulted none or only a few primaries. ln these, the moult starts or continues in summer, and a majority of those which have completed the 1st moult start a new moult cycle. In late summer, the most advanced birds are more than one whole primary moult cycle ahead of the least advanced birds.
First-summer heuglini usually return to their northern breeding grounds with juvenile
primaries and start to moult them in late May or June, about one month later than the Herring Gulls of same age. In some birds the moult is completed during the summer, whereas others apparently suspend it during autumn migration. A small proportion (5-10%) irregularly moults some (sometimes perhaps all?) primaries during the first winter.
The plumage of first-summer fuscus is usually rather contrasting. Head and underparts are pure white with variable brown streaking and mottling. The mantle, scapulars and wing feathers show two types of patterning: rather worn dark brown feathers with diffuse paler fringes, and (almost) uniform, blackish ones, which are usually of the same tone as in adults, but sometimes distinctly paler. The most advanced birds are already surprisingly adult-looking. In contrast to graellsii and intermedius, even the birds which have not moulted primaries have moulted all
secondary wing coverts during the winter. The colour of the bill is very variable and the tail usually shows a broad dark tail band.
In spring and early summer first-summer heuglini shows a pure white head and underparts with a variable amount of mottling and streaking, which is strongly concentrated on the hind neck. The scapulars are variably brownish grey with broad paler grey tip and dark patterning,which is typically restricted to a shaft streak. In some birds the blackish markings are missing, while in others they may be more or less anchor-shaped, as in Herring Gull. Anything from 0% to 100% of the secondary wing coverts and tertials have been moulted during the 1st winter. Usually there are wing coverts of both types. The basal half of the bill is often pale.
After the complete moult to 2nd winter plumage in the autumn of the 2nd calendar year most birds already show uniform adult-type greyish upperparts but have a variable amount of brown mottling on the wing coverts. In some, the scapulars are still of a brownish type with black markings.
Separating first-summer heuglini from Herring Gull is usually straightforward. Typical
graellsii should also be separable from heuglini. Graellsii shows more dark patterning on head and underparts, darker upperparts with different patterning, a darker bill, and has usually moulted only a few wing coverts, at the most, during the 1st winter.
In spite of being typically very different, some fuscus appear very similar to some heuglini, and the most difficult individuals are best left unidentified. Birds of the cachinnans group may also cause major identification problems. Larger size, typical structure, paler inner primaries, narrower tail band, whiter underwing, paler 1st winter scapulars and less extensive wing covert moult during the winter should separate most individuals. A more reliable basis for separating these two is the tone of the first true adult-type feathers (distinctly darker in heuglini), which appear during the first summer or the following winter.
|(7, 8) subadult heuglini.
OLDER "SUBADULTS": As early as the spring of the 3rd calendar year many fuscus look very much like adults, although others still have brown feathering on the wing coverts (only rarely on the scapulars) and variable dark patterning on the tail. Many show a distinct moult contrast in the outer primaries (outermost being more worn, innermost new). The least advanced 2nd summer birds can be aged accurately, the rest should be referred to as "older subadults", as some signs of immaturity can sometimes be seen in 4th and even 5th calendar year birds.
Many 2nd summer heuglini can also be reliably aged in spring on the basis of a combination of adult-type grey saddle, dark mottling on the wing coverts, some mottling on the tail and a yellow bill with a dark tip. Because data on ringed birds is lacking, the details of plumage development from the 3rd calendar year onwards are not known, but the progress seems to be very similar to that of fuscus.
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|heuglini 4cy, mid-July 2003, Tampere, Finland (61.33N 24.59E). Arrested
moult: P1-P4 replaced, P5-P10 old. Slate-grey bird.