Heuglin's Gull (L. heuglini / antelius)
Amir Ben Dov (Israel)
Chris Gibbins (Scotland)
Hannu Koskinen (Finland)
Mars Muusse (the Netherlands)
4cy / sub-adult heuglini: April
Characteristics and moult of "Baltic Lesser Black-backed Gull" Larus [fuscus] fuscus
THIS IS PART II
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
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.
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.
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  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).
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’”.
JUVENILE PLUMAGE - FIRST CALENDAR YEAR
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.
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.
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).
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 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.
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.
|Heuglini 4cy, April 29 2007, Tampere, Finland. Picture: Hannu Koskinen.|
|Heuglini 4cy, April 08 2011, Ashdod, Israel. Picture: Amir Ben Dov.|
|Heuglini 4cy, April 02 2011, Ashdod, Israel. Picture: Amir Ben Dov.|
|Heuglini 4cy, April 01 2011, Ashdod, Israel. Picture: Amir Ben Dov.|