Heuglin's Gull (L. heuglini / antelius)

(last update: 6-3-2011)

4cy / sub-adult heuglini: April

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

Visa Rauste

Heuglini in Finland. Compare to argentatus & fuscus.

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ADULT PLUMAGE
Siberian Gull  Larus [fuscus] heuglini

In Finland, adult Siberian Gulls can be identified rather easily when observation conditions are good: they have the upperparts significantly darker than Herring Gulls, but paler than Great (L. marinus) and Baltic Lesser Black-backed Gulls (obvious, even without a direct comparison). Eskelin & Pursiainen (1998) analysed the occurrence of these birds in Finland, which appeared to be concentrated in early May in the north and east, and this is at the same time or slightly later than the main passage of fuscus. So far, the largest group seen together was seven to eight birds. The pattern of increasing observations continued in spring 1998 and 1999, a detailed analysis is in preparation (T. Eskelin & J. Pursiainen). Based on location and timing, it can safely be assumed no adults of western forms are involved. Adult graellsii / intermedius should already be breeding this time of year. Obviously, the western boundary of the migration route of heuglini currently runs through Finland.

[Addendum: apart from the evidence in the east and north-east, intensive observations in south-west Finland in Tampere, between late April and early June 1999, resulted in "over 100 Siberian Gulls" (including 50 2CY ind.) (H. Kettunen and H. Koskinen, pers. comm.). Whether the phenology has been misrepresented in previous years (because of incomplete knowledge), or whether numbers has changed within the last few years, can not be determined at present. Similar intensive monitoring, as were carried out in Tampere, should provide data from the northeast, and very likely more evidence will come up about increasing numbers of Siberian Gulls. ]

Still, in individual cases it remains difficult or even impossible to fully exclude western Lesser Black-backed Gulls completely. Siberian Gulls, the taxon, has been added to the official Finnish Avifauna List recently - predominantly based on the clear phenological evidence - but the Rarity Committee has not been able to accept any single individual. (Hence, also the photos of Siberian Gulls shown here, photographed in Finland, should be interpreted as “probable
heuglini”.) In Sweden, the Rarity Committee has similar problems (Hirschfeld & Kjellen, 1997). By the way, graellsii is not on the Finnish list up to 1999.
The grey tone of the upperparts in both forms is far too similar to be of any help in field observations. In June, a large flock of Siberian Gulls in Archangelsk showed a limited, but clearly discernible variation in the degree of darkness in upperparts in adult birds. This variation was clearly larger than found in Finland or larger than in Herring Gulls or fuscus. A few fuscus were present in this group in Archangelsk, and they could be identified easily; not a single bird was intermediate in any respect.

Heuglini and graellsii. Head streaking.

Possibly, the extent of winter streaking on the head may serve as sufficiently reliably basis to distinguish the forms (coarse and extensive streaking in graellsii, sparsely and concentrated in the hind neck in heuglini); and another clue can be the timing of the end of moult (Beaman & Madge, 1998). However, these features can not help indentifying birds in spring; this time of the year we have to rely on structural differences and primary patterns.

Shirihai (1996) pointed at the primaries patterns to be diagnostic: graellsii should have a clear mirror on P10 but also on P9, while heuglini often only has a small P9 mirror; and graellsii should have six or seven primaries with sub-terminal black, while heuglini seven to eight; the sub-terminal medial band on the inner-web of P8 should be 70-90 mm wide in graellsii, but broader in heuglini, 95-118 mm.
According to Hario (1986) however, 49% of graellsii examined show only one mirror in P10 (cf. P9 & P10 patterns in Moerdijk data, MM). Hario (in corr.) examined 38 museum specimen of adult graellsii in the Zoological Museum of Copenhagen (most birds from of the Färöer): nine (24%) showed black on six primaries, 22 (58%) on seven primaries and seven birds (18%) on at least eight primaries. According to Cramp & Simmons (1983) graellsii can have “a variable amount of black" on P4 (see also number of primaries with black in Moerdijk data, MM) and 25% of the birds had black markings on P3 (i.e. eight primaries).
There is hardly any detailed information for primary patterns of heuglini (but see Buzun 2002, MM). The size of the P10 mirror is on average smaller and positioned further from the top than in graellsii (Eskelin & Pursiainen 1998), but there is large overlap. And also heuglini can occasionally have a mirror on P9. The number of primaries with black varies between six and eight (35% have eight primaries with black, Cramp & Simmons 1983); individuals with black on less than seven primaries seem to be rare in heuglini.

Heuglini and graellsii. Primary patterns.

The difference in the extent of the sub-terminal black patterns may be diagnostic, but still this should be investigated on larger sample sizes. Still, to substantiate such small differences in field observations seems impossible (hence this may appear a useless feature), and using these criterion remain very problematic even on good photos. Currently, therefore, wingtip patterns do not offer diagnostic features.

Dark irides should occur only rarely in adult graellsii, and more commonly in heuglini (Fig. 3). In June 1999, under very favorable circumstances, the eye colour of Siberian Gulls was scored (from 20 m distance, with 60x telescope). About 80% of the adults showed varying degrees of small dark spots in bright yellow irides. For most adult birds, the general impression was that of pale eyed birds at larger observation distance; or proved to be only slightly darker than resting adjacent Herring Gulls. However, some adults (<10%) had conspicuously darker irides and in the group of “older subadults" (predominantly 4CY) this was very common.

When dealing with head streaking of heuglini in winter, I have to confine myself to the beginning of September, at which time the head of adults is still pure white; only single birds had very faint pencil streaks. By this time of the year, most graellsii show significant head streaking (see here, but many are white-headed in September as well, MM).

Moult commencement shows a significant mean difference between the forms. After Stresemann & Stresemann (1966) heuglini starts the complete moult in July/August in the breeding areas, and interrupt moult for migration, the moult stage has then reached P5 (see here). A very small part of adults in the Arkhangelsk (2-5%) already started to drop the one or two innermost primaries on 10.-12. June 1999. Graellsii start moulting between mid May and early August. (However, Verbeek [1977] reported that some birds still had to begin moult on 05 August. see also Moerdijk data for breeding adult graellsii in May. And late moulting 9CY bird end-August, MM) Even if the moult in graellsii usually progresses gradually the autumn, still it can be arrested for migration (Glutz of Blotzheim & Bauer 1982).
Table 2 presents some data about the state of moult in adult graellsii and heuglini in September in Finland. This table clearly shows the overlap of the period of primary moult. (Note that the heuglini sample was scored more than two weeks earlier than graellsii data.) On the other hand, we can clearly separate the most advanced graellsii from the most retarded heuglini. The overlap as shown in table 2 make it uncertain if we may use moult timing as a determinant feature in any out-of-range bird.

The most distinguishing features between heuglini and graellsii and are perhaps structural features, but practically this is subjective and often difficult. Many of the birds seen in Finland better match heuglini rather than graellsii "without being able to use diagnostic features other than ‘jizz’”.

Heuglini upper and underwing.

JUVENILE PLUMAGE - FIRST CALENDAR YEAR
Siberian Gull  Larus [fuscus] heuglini

Only very little is known about the variability of juvenile Siberian Gulls. The following description is based on birds observed between 01. to 04. September 1998 in Archangelsk. Identification of these birds was predominantly based on older Siberian Gulls, which were present at the same places. Most birds on the beach (approximately 100 individuals, of which 50% juvenile) were still close as family, and the young birds still attending and begging their parents. On the Archangelsk landfill the family ties were broken, but the variability among the young birds was very similar.
Older fuscus were not observed here in September, so it is expected that also juvenile fuscus were not present in large numbers. (In June 1999 however, two adults, one 3CY and at least one 2CY fuscus were observed together with 400 Siberian Gulls at the Archangelsk landfill.)
It can not be excluded with absolute certainty that a small proportion of the young bird (possibly also photographed) were actually fuscus. On the other hand, Archangelsk is significantly outside the normal range of fuscus, and no birds were seen showing all the characteristic features of fuscus. So we can probably assume that the variability of the juveniles in Archangelsk nicely represents the variability of juvenile heuglini.

Heuglini like fuscus. Heuglini like argentatus.

Juvenile heuglini in flight are easily recognized as member of the Lesser Black-backed Gull group: the inner primaries are dark -as in fuscus- or only slightly paler than the outer primaries. Some show a clearer “pale window" as fuscus sometimes does; yet this is never as pale and obvious as in northern Herring Gulls (this pale window may be less apparent in southern Herring Gulls, e.g. from the Wadden Sea, but also from S Finland). Despite the variability in this feature, juvenile heuglini are still easily picked out a group of juvenile Herring Gulls, their sympatric neighbour.
On the other hand, dark Yellow-legged Gulls L. (cachinnans) michahellis in 1CY and 2CY can look very similar regarding primary patterns, but michahellis show already a worn plumage in autumn and a more advanced post-juvenile moult (cf. Jonsson 1998a).
In all other characteristics juvenile heuglini prove to be as variable as any other of the large gulls, and describing an “average individual” is therefore very difficult. Standing birds may appear very similar to fuscus, whereas other birds tend to remind you of Herring Gulls (except maybe the tertials).

The upperparts are often similar contrasting as in fuscus, but many birds have the centres slightly paler and often internal patterns on the feathers, like a shaft streak. The scapulars have creamy fringes, which are usually of even width as in fuscus, but they can also be pronounced and notched as in Herring Gulls. The tips of the scapulars and wing-coverts are often much broader than in fuscus, and from a distance the entire upperparts may appear paler, with a cold grey tone, something already referred to by Eskelin & Pursiainen (1998).

Heuglini covert patterns.

The greater coverts show on average much extensive pale patterns than in fuscus and are less uniform dark, but the colouration is very variable: some are very similar to typical chequered piano-key pattern of Herring Gulls (except perhaps the outermost coverts). For others the pattern is diffuse or irregular, but usually the bases of the greater coverts are uniformly dark as in fuscus. This creates a dark “wing-bar", most prominent on the outer greater coverts, but often extends to the innermost coverts. Typically a reverse broad pale band of light tips runs from the inner greater coverts narrowing towards the outer feathers. These pale covert-tips can be barred or pretty plain patterned. In fuscus such pale patterns usually are confined to the very tip of the feathers, with exception of about (less than) ten innermost greater coverts, which may show strong patterning.
The pattern on tertials corresponds to the fuscus-type. The pale part is confined to the tip and usually not very complex patterned - usually only a pale line. This pale tip is broader than in average fuscus, and often continues (as notching) along the fringe. Birds with very pale tertials have such pattern extending on the greater coverts.

Fuscus tail patterns.

The blackish tail-band is on average narrower than in fuscus and the base of the tail less barred, sometimes the tail-band resolves into several narrow lines. On the other extreme, the tail can be almost completely dark. Rump and upper tail coverts are normally sparsely dark patterned, and they contrast often conspicuously with the dark tail-band.
The underwing coverts are on average paler and finer patterned than those in fuscus. Some birds show a broad, plain whitish field, usually the whitish areas are similar located in fuscus, but narrower and lighter. Some birds have also pretty dark underwing coverts, similar to typical western Lesser Black-backed Gulls.
The start of the post-juvenile moult has the same timing as in fuscus. Early September in Archangelsk, nearly half of the juvenile birds had renewed some scapulars. The new feathers (2nd generation / “first winter plumage") are highly variable: some were pale cream coloured with dark anchor patterns, very similar to those of Herring Gulls; others were dark-brown with paler edges, as in fuscus. A more abundant type was relatively pale brown with a fairly narrow, dark shaft streak; this pattern seems not to occur in fuscus, nor in Herring Gulls, and neither in western Lesser Black-backed Gulls. The Archangelsk material is not sufficient to allow final statements; only relatively few individuals had at least several fully grown new scapulars which allowed firm statements about colour and pattern.

Fuscus variability.

Distinguish juvenile heuglini from juvenile fuscus

Despite several “average" differences between fuscus and heuglini, both in structure and in coloration, it remains very difficult to define diagnostic features or combinations of features. Probably only the extremes can be identified.
The situation in Finland is illustrative for the problem. Small numbers of juvenile birds of the Lesser Black-backed Gull group can be observed until the end of October on landfills.
For long it was assumed these were all fuscus. This assumption was partly based on the fact that in this time of year older heuglini occur only rarely, while they are significantly more abundant in spring and summer. Secondly, the assumption was based on the complete ignorance of the juvenile plumage of heuglini. In autumn 1997 and 1998 several of these birds were tentatively identified as heuglini, due to the increasing level of knowledge about heuglini; and similar birds were documented well by photos from previous years. An exact estimation is hardly possible, given the current state of knowledge. But it is not unlikely that juvenile heuglini occur frequently in fall, just like older birds do in spring and summer; yet to prove this remains difficult. The main reason for this uncertainty is -perhaps surprisingly- the lack of knowledge of the variability of fuscus. There are not many photos or detailed descriptions from breeding colonies; most of the material comes from landfills. With the exception of a few ringed birds (see 1CY September & October), the identification on landfills is doubtful too. For instance, it is interesting that most juvenile gulls, which show extensive pale wing-coverts, are birds seen by the end of September and in October. If one takes a very critical position, all “juvenile Lesser-Black-backed Gulls” in Finland (as well in Central Europe) should remain tentatively unidentified.

END OF PART II
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CONTINUE AT PART III