Heuglin's Gull (L. heuglini / antelius)

(last update: 6-3-2011)

4cy / sub-adult heuglini: May

Characteristics and moult of "Baltic Lesser Black-backed Gull" Larus [fuscus] fuscus
and “Heuglin’s Gull" L. [fuscus] heuglini

Visa Rauste

(1) 2CY heuglini & (2) fuscus in spring.

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IMMATURE PLUMAGES
Siberian Gull Larus [fuscus] heuglini

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)
Siberian Gull Larus [fuscus] heuglini

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.

(3) spring 2CY heuglini juvenile primaries.

(4, 5) spring 2CY heuglini with new primaries.

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.
Most Siberian Gulls follow a complete moult strategy which closely resembles the moult in other large gulls and starts about one or two months earlier than in adults. Table 3 provides information on the moult stage of 2CY Siberian Gulls in June. In the period 10.-12. June 1999, about half the individuals had not started the primary moult in Archangelsk, Russia.

A small minority (5-10%, ten birds of the approximately 150 individuals sampled) of the birds show renewed primaries (in rare cases, perhaps all?), replaced in the first winter. The number of renewed primaries varies between one and eight, the sequence appears to be very variable, and sometimes new primaries are mixed seemingly random between old feathers, but the scenario is always at least approximately symmetrical. All these individuals had also replaced all the wing-coverts, which are visible in a standing bird.

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.
The secondaries (they are part of the “flight feathers”, together with primaries and rectrices) are renewed regularly in the winter quarters. Many times, all secondaries are about the same age, and this can cause extreme difficulties -even in photos- to distinguish juvenile feathers from those of the second generation, even if the whitish tips are wider in the second generation.
The rectrices can be replaced completely or partially, but some birds return in spring with a completely juvenile tail. Apparently, the first alternative is the most common.

(6) white underparts. (7) grey upperparts.

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 scapulars are slightly to moderately worn and at least part of them were replaced a second time during late winter or spring. The base colour of these feathers is pale brown grey. The distal part is extensive whitish-gray due to wear and fading. In extreme, these parts can wear away almost completely, resulting in a darker scapular panel. The scapulars usually have a dark shaft, sometimes a dark center, or rarely dark anchor patterns. The pattern is very variable, and in some plain individuals it is very weak or absent. Such birds can look very similar to adults regarding the upperpart tone. Few individuals seem to grow in grey feathers in first winter. Normally such adult-like feathers do not appear before May of the 2CY.
By mid-June most 2CY Siberian Gulls (approx. 80%) obtain adult-like scapulars, while only a minority of the new feathers is brown or dark with internal markings. The grey tone of feathers usually correspond to adults, but, like in fuscus, it can also vary. The palest individuals show a shade of grey reminiscent of nominate argentatus.

(8) juvenile coverts worn, (9) or surprisingly fresh.

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.
Renewed wing-coverts are similar in colour to the scapulars, but usually show more extensive dark markings. The greater coverts are often quite plain and darker than the rest, but they can also be clearly barred. Retarded juvenile coverts are sometimes strongly faded and worn, but can also look surprisingly fresh.

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.
The innermost juvenile primaries are usually as dark as in 1CY autumn birds, but sometimes they become paler and brown, which in some individuals accentuates a pale window. The outer primaries are sometimes heavily worn (as in fuscus), but usually not very worn, sometimes even surprisingly fresh. Therefore, it can be difficult to distinguish renewed primaries from juvenile feathers, and hence, ageing and interpretation of primaries in the field, deserves great care.

The tail-band is usually wide, sometimes covering the whole tail-feather. Very rarely the tail-band dissolves into one or two narrow bands at the tips of the rectrices, while exceptionally some tail-feathers may be pure white. Rump and upper-tail coverts are usually pure white. The underwing coverts on the other hand, show more or less extensive dark markings but may also appear unmarked white.

The bill may be blackish with a pale tip and diffuse paler at the base, but it is significantly more common to find two-coloured bills: a dark tip and greyish, yellowish, greenish or pink base; rarely already with a red gonys spot. The eyes are almost always very dark, only exceptionally slightly paler amber.

(10, 11) 2cy heuglini in autumn.

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.
The innermost second generation primaries are also highly variable; some birds have these feathers blackish with some grey, very similar to fuscus. The common pattern is a clear bluish-grey tone and blackish sub-terminal markings. Most advanced birds show uniform grey inner primaries with a white tip.
The outermost second generation primary (P10) may already show a mirror, which is small and usually diffuse. The bases of the secondaries are dark brown. Brown markings may be almost completely absent on the upperwing, but remains obvious and extensive on the greater primary coverts.

(12) 2cy Finnish argentatus. (13) 2cy British graellsii.

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 commencement of primary moult may be a useful clue as well: by early June exceptionally few Herring Gulls still have all juvenile primaries, while this is often the case in Siberian Gulls. Most difficult individuals ask for detailed analyses, but it is usually always possible to distinguish between these taxa (except when inner primaries are missing and wear and tear dramatically change the appearance of body feathers). Herring Gulls in 3CY, that show no adult-like scapulars (in Finland quite commonly), can sometimes be mistaken for 2CY Siberian Gulls. Normally such birds show paler wing-coverts and they lack the head streaking concentrated in the hind-neck; added to this, the primaries in 3CY Herring Gulls show rounded tips.

(14) 2cy Dutch intergrade. (15) 2cy Swedish intermedius

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.

(16-18) barred 2cy fuscus.

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.
Any 2CY bird, that has replaced all of its primaries before late-summer (or early, on the wintering grounds, MM), is most probably a Baltic Lesser Black-backed Gull according to current knowledge, but this can even occur in extreme Siberian Gulls probably.
A rather more reliable criterion is the distribution of mottling and spots on head and underparts: the extent of dark head streaking is highly variable in both forms, but Siberian Gull shows a very strong tendency to concentrated stripes in the hind-neck. The impression of the overall colour is certainly a good criterion, but it is often difficult to assess in the field. Structural features are sometimes helpful. In late-summer the colour of the new inner primaries can serve as a characteristic feature.
Most problematic are some birds in mid-summer, when 2CY heuglini is worn and can look very dark, and these birds show unusual combinations of features due to moult. It actually seems possible to find overlap in all characteristics, so that not all individual birds can be assigned to one of the two forms. At current knowledge, about 10% of the individual birds should probably safely remain unidentified.

(19, 20) cachinnans-like 2cy heuglini.

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.

Even more limited is our knowledge on the eastern forms of the cachinnans group, especially barabensis. This taxon should be very similar to heuglini and is also smaller than western cachinnans. Descriptions of the plumage of 2CY barabensis have not been published yet. The occurrence of barabensis in Western Europe can not be fully excluded, as it has been observed near Moscow, already west of the Urals (Jonsson, 1998b).

In Finland and Russia, and in general in the normal distribution range of Siberian Gull, where Caspian Gulls occur only very rarely, it seems justified to determine birds as “heuglini“ as long as they do not show distinct features of Caspian Gulls. However, there remains a slight possibility, that I included one or a few Caspian Gulls among the observed birds (both in Finland and in Archangelsk), not to mention other less-known forms. If one encounters such a bird in Central Europe, then the uncertainty increases considerably, since cachinnans is here much more abundant than in Finland. Hence, all other alternatives should be eliminated, which may be a difficult task considering the current state of knowledge.

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