Heuglin's Gull (L. heuglini / antelius)
Amir Ben Dov (Israel)
Chris Gibbins (Scotland)
Hannu Koskinen (Finland)
Mars Muusse (the Netherlands)
4cy / sub-adult heuglini: May
Characteristics and moult of "Baltic Lesser Black-backed Gull" Larus [fuscus] fuscus
Fuscus and heuglini in immature plumages show some significant differences from western Lesser Black-backed Gulls (and most other large gulls): first, they develop faster into adult-like plumage (at least superficially), and second, they both show more or less significant variations in moult timing and moult extent (“arrested moult”, “StaffelMauser”). No other form of the Lesser Black-backed Gull group shown these phenomena, which are associated to different migration strategies (breeding areas are located far-north and birds follow a very broad, trans-continental migration route), and -probably- also associated to climatic conditions in the winter quarters (Jonsson 1998a, b). I will confine myself here to describe the phenology of plumage development and moult cycles of “immature” heuglini and fuscus based on summer observations in Finland and Russia.
SECOND CALENDAR YEAR (2CY)
At least a large part of 2CY Siberian Gulls spend the summer in or near the breeding areas. In Finland, they appear from the end of April with a peak between early May and mid June. Some seem to spend the complete summer on Finnish landfills.
Primary moult in second calendar year
Unlike Baltic Lesser Black-backed Gulls, the majority of 2CY Siberian Gulls arrive in the breeding areas with juvenile primaries. The first complete moult starts in late May to mid June, when P1 is dropped and continues throughout the summer months. By early September, the birds in Archangelsk have 0-4 (-6) old primaries left and the body feathers consisted predominantly of renewed feathers (“second winter plumage”). Some birds seem to finish the complete moult before departure in 2CY autumn, other birds have to interrupt moult prior to migration, as adults do. Figure 32 shows the moult strategy compared to 2CY fuscus.
The statements of Harris et all (1996), that the first complete moult takes place until late in the second winter, is probably a misinterpretation related to birds that renewed primaries in first winter. (At that time, it was unimaginable that large gulls could undergo a major plumage moult in first winter.) The statement of Eskelin & Pursiainen (1998), that 2CY spring heuglini show on average an “advanced flight feather moult”, is perhaps explained by generalization of a few advanced individuals.
Plumage in 2CY spring (April to early June)
Typical heuglini have a very characteristic look at this time of year. Head and underparts show a pure white base colour. Spots or mottling may be completely absent or only poorly developed, but if present, it is often concentrated in the hind neck and upper flanks. There is often a “make up line” behind the eye. Intensive spotting on head, chest and flanks occurs only rarely, but even in such birds, the center of the breast is pure white, and the dotted lines are sharper defined than in typical Herring Gulls.
The wing-coverts in spring can still be completely juvenile (in about 10-20% of individuals). On the other hand, 2CY birds can also have renewed all wing-coverts (approximately 10-20% of individuals). Most birds show a mix of old and new coverts. In general, the proportion of moulted feathers is lowest in the greater coverts and increase towards the lesser coverts.
Birds that renewed wing-coverts in first winter have also replaced some or all tertials. The new tertials show a pattern similar to same aged fuscus, but the proximal part is often paler greyish and the white tip is broader.
Plumage in 2CY summer (June to September)
Siberian Gull undergo a complete moult (or most of it) in summer and change rapidly. Moulting birds show very variable combinations of “spring plumage”, which increasingly deteriorates, and fresh “second winter plumage”. The upperparts in “second winter” are usually already very adult-like. Standing birds give an impression of plain grey upperpart without any brown markings. However, some birds have all upperpart feathers brownish with faint shaft streaks and transversal bars (except maybe some mantle feathers). Streaking on head and underparts is weak, at least by early September, which is late in the moult scheme.
Identification in 2CY
Separation from Herring Gull is easy in most cases. Some individuals may resemble Herring Gulls, when they have relatively extensive spotting or mottling on head and underparts, combined with brown scapulars with internal anchor patterns. But the upperwing in flight and the tail pattern should immediately lead to correct identification. By May, the moult stage usually already support a lot: Finnish Herring Gulls only extremely rarely renew coverts in first winter, prior to the start of the first complete moult.
The western form of Lesser Black-backed Gull graellsii strongly resembles Siberian Gulls at older age and cause serious identification problems; however as 2CY, graellsii usually looks quite different. The general impression is much darker than heuglini, the head and underparts show bold stripes, scapulars are uniform brown or show strong anchor patterns, the bill is usually predominantly dark, and the wing-coverts are usually almost completely retained juvenile feathers. The first complete moult starts late April or in May (Cramp & Simmons 1983). But graellsii is highly variable, and some birds can look very similar to heuglini. In the region of heuglini, such graellsii should occur only very exceptionally, but the possible identification of heuglini in central Europe is hindered considerably. The average timing of complete moult commencement is different, as already stated, but useless as reliable criterion on individual level. At current knowledge, completely renewed rectrices and secondaries seem not to occur to in 2CY graellsii / intermedius in spring, but it is extremely difficult to correctly interpret such renewed feathers under field conditions [we now know, in 2011, that a small proportion of intermedius Lesser Black-backed Gulls do moult rectrices and secondaries in first winter, MM]. Also, graellsii and intermedius on average renew considerably less wing-coverts in first winter, but advanced individuals can be extremely similar to heuglini in this respect too.
Baltic Lesser Black-backed Gulls clearly can cause very difficult identification problems in Finland and in Russia, as well as along the eastern migration routes. Of course, typical heuglini with clean-coloured grey scapulars, heavy-patterned wing-coverts and pale general impression can be distinguished pretty easily from dark and brown 2CY fuscus, but some birds are very similar. Fuscus can also show pale fringes on the scapulars, with even margin and broad greyish centres or shaft streak, like heuglini. Still, the shape of this dark centre is different: broad at the base and tapering to the tip in fuscus, narrower and of approximately same width in heuglini. The hue of the adult-like scapulars in 2CY fuscus is usually similar dark as in adults fuscus, but sometimes much paler, hence it can not be regarded as a reliable feature. Such pale fuscus often show typical dark tones on some wing-coverts, even if the scapular region has confusing blue-grey tones.
A major problem in identification of 2CY heuglini is the cachinnans group (Jonsson, 1998b), as appearance and variations in spring are only little known. Individual 2CY Siberian Gulls may perfectly resemble same aged Caspian Gulls L. [cachinnans] cachinnans both in coloration and structure. Birds in Finland were distinguished from typical western Caspian Gulls (“ponticus”) on the following criteria: dark inner primaries, often dark underwing coverts, a broader tail-band and often completely renewed rectrices (which so far have not been observed in cachinnans). Also, on size cachinnans can be distinguished, with heuglini much smaller than Herring Gulls. The absence of typical cachinnans structure in heuglini is only useful for experienced observers. Caspian Gulls on average have more juvenile wing-coverts, which also show stronger wear and abrasion than in Siberian Gulls. Also, the second generation scapulars are paler in cachinnans (own observations in Romania, April 1999). Probably the commencement of the complete moult will give useful clues as well, but I have no sufficient detailed information on the onset in cachinnans. A more reliable distinguishing feature is the colour of the first adult-like feathers (especially scapulars), which appear in summer in 2CY Siberian Gulls, and on average somewhat later in Caspian Gulls.
|Heuglini sub-adult, May 06-07 2011, Eilat, Israel. Picture: Avi Meir.|
|Heuglini 3cy/4cy, May 22 2008, Hämeenlinna, Karanoja dump, Finland. Picture: Hannu Koskinen.|
|Heuglini 4cy, May 01 2009, Tampere, Finland. Picture: Hannu Koskinen.|