Amir Ben Dov (Israel)
Chris Gibbins (Scotland)
Hannu Koskinen (Finland)
Mars Muusse (the Netherlands)
adult heuglini: April
Is it possible to identify Baltic and Heuglin's Gulls?
By Chris Gibbins, IN: Birding Scotland 7(4), December 2004.
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THIS IS PART 2
Context and aims of the paper
To summarise the above discussion, it may be argued that both fuscus and heuglini are
potential visitors to Scotland. Both have been discussed previously in this context (Gibbins
and Golley, 2000). A number of papers published in the 1990's improved our knowledge of the identification of fuscus and heuglini. In particular, they suggested that moult could
be used to support identification, including that of immature birds. These papers set new
standards. In the case of fuscus, they re-awakened interest and initiated a new and more
rigorous search for this taxon in Western Europe. In the case of heuglini, they brought the
details of its field appearance to our attention for the first time and raised the possibility of
its occurrence in the UK. Most recently, the gulls monograph (Malling Olsen and Larsson,
2003) was an opportunity to synthesise and consolidate knowledge of these taxa. Unfortunately, because of the mislabelling of so many plates in the first edition, this book
has not proved a reliable reference point. More particularly, critical errors remain in the heuglini section in the revised edition (discussed on page 172).
The remainder of this paper discusses current ideas on the identification of fuscus and heuglini. It is based on the author's observations of both taxa in Finland (2001, 2002 and
2004), the United Arab Emirates (2004) and Israel (2000 and 2001) and of graellsii and intermedius in Portugal (2001, 2002, 2003 and 2004) and the UK. The paper also draws
upon field studies being undertaken by gull enthusiasts around Europe, the results of
which have yet to be formally published. The focus is on the extent to which this recent
work has affected our perceptions of fuscus and heuglini; in this sense, the paper is
essentially an update to the work of Lars Jonsson and Visa Rauste.
Because of differences in the way ideas on their identification have evolved, each taxon is
treated in a slightly different way. For fuscus the discussion concentrates on the criteria
given by Jonsson (1998a) and the extent to which these are still seen as holding true. For heuglini the discussion is based largely around some individual birds ('case studies') and
whether they could be identified with certainty if encountered outside of the normal range
of this taxon. Overall, it is hoped that the paper improves awareness of these taxa among
Scottish birders who have no previous field experience of them, or not had access to the
literature. It concentrates on identification in the spring to autumn period as this is when
they are perhaps most likely to be encountered as vagrants in Scotland. No firm identification
criteria have so far been suggested for juvenile (first Calendar-year [1 cy]) heuglini,
while there are no known diagnostic features for 1 cy fuscus. The paper therefore concentrates
on birds in their second calendar year and older. As is now the convention for gulls,
primaries are numbered outwardly, with the inner primary being P1 and the outer primary
P10. As far as possible, ringed individuals (therefore of proven age and origin) are used to
illustrate identification features.
Identification of heuglini
Although treated by Grant (1986) as a subspecies of Herring Gull, heuglini is essentially a Lesser Black-backed Gull. As its separation from Herring Gull is therefore not a real
problem, the following discussion concentrates on identification relative to the other Lesser
Black-backed Gull taxa.
There are contrasting statements in the literature about the appearance (particularly the
size, structure and upperpart tone) of Heuglin's Gull. It seems most likely that, as with all
other large white-headed gulls, it is rather variable. However, some of the apparent
variability of Heuglin's Gull may actually reflect descriptions based on misidentified birds,
while some results from the inclusion of taimyrensis as the eastern form of Heuglin's.
There are relatively few photographs of heuglini in the English language literature. This and
the contrasting statements about its appearance have lead to some uncertainty among UK
birders as to what the field characteristics of heuglini are. The following text therefore
attempts first to build a general image of the appearance of heuglini and concentrates on
the issue of its apparent variability. This is followed by a discussion of the identification of
some individual birds – the 'case studies'.
(i) Size and structure
Grant (1986) described heuglini as large and long legged, "readily separated from fuscus by (its) much larger size and heavier build". The structure of taimyrensis was described by
Grant as being "much like heuglini". However, biometric data indicate that taimyrensis is
appreciably larger than heuglini, and in fact is larger than many Herring Gulls (Table 1).
Kennerley et al. (1995) and Garner (1997) separated wintering heuglini and taimyrensis
based on these differences in size and structure, as well as upperpart tone (as per Figure
1). Despite the field identification by these authors, the question of what 'taimyrensis'
represents is controversial. If heuglini is the western and taimyrensis the eastern subspecies
of Heuglin's Gull, then Heuglin's is an unusually variable species (evident from Figure 1 and
Table 1). However, much of this variability results from the classification of taimyrensis as
the eastern form of Heuglin's, since taimyrensis is both different to the 'statistical average'
heuglini and is itself highly variable. Yèsou (2002) uses the marked variability of taimyrensis
to argue that it does not exist as a taxon. To summarise his argument, Yèsou suggests that
'taimyrensis' comprises either birds from a hybrid zone between western heuglini and Vega
Gull L. vegae (as argued by earlier workers), or yellow-legged individuals that were identified
as taimyrensis but were actually either pure vegae or pure heuglini. Yèsou's argument is a logical interpretation of the extremely variable descriptions of taimyrensis given in the
literature. If correct, his argument helps simplify matters as it means that taimyrensis has no
taxonomic validity and so should be left out of the Heuglin's Gull equation. Thus, to
understand what Heuglin's Gull looks like, it is necessary only to consider the western birds
– heuglini. So what is known of the field characters of these birds?
The average western heuglini is larger than fuscus, intermedius and graellsii (Table 1).
While it is most similar in size to graellsii, the average bird has rather more elegant
proportions, typically appearing to have a smaller, sleeker head and a slimmer neck (e.g. HERE, HERE). Unfortunately there is marked individual variation in the absolute size and
relative structure of heuglini, such that overlap with the other taxa is extensive. Structurally,
some heuglini appear very similar to intermedius and female graellsii (HERE, HERE) while
others appear large and even hulking (HERE, HERE). A particularly small, delicate heuglini
seen in the UAE (March 2004) was very similar to fuscus. Conversely, some fuscus can be larger and more robust than some heuglini. Bill size and proportions are also
variable in heuglini, as evident in the images on the right hand side.
(ii) Field characters of adult heuglini
The average and range of upperpart tones shown by heuglini match almost exactly those
of graellsii (Figure 1). Like graellsii there is individual variation, such that the darkest
heuglini overlap with paler intermedius and the palest birds are only fractionally darker than
the darkest argentatus. Thus, heuglini does not have a diagnostic upperpart grey tone.
Usually heuglini is described as having a white mirror only on P10 (e.g. HERE); this is
unlike the average graellsii which has mirrors on both P9 and P10. Harris et al. (1996)
suggested that in heuglini the white mirror on P10 is smaller and further from the feather tip
than in graellsii. Eskelin and Pursiainen (1999) found that this was the case with most of the
heuglini they encountered. It is not unusual to find graellsii in which the P10 mirror is merged
with the spot at the feather tip to form an extensive white tip to the feather, unlike the pattern
described for typical heuglini. However, there is variability and overlap between these taxa in
the pattern of white in the primaries. For example, intermedius (probably female) usually has
only 1 mirror, while heuglini (probably male) can sometimes have two. Moreover, heuglini
can have a large mirror on P10, as shown by the bird in Plate 18 of Rauste (1999). The
implication of this overlap is discussed with respect to the case study birds.
Figure 1 in Buzun (2002) illustrates what is described as "the most common wingtip
pattern" in heuglini. The figure shows a bird with black on 8 primaries, with the black
extending as a complete band across P4 and to the outer web of P3. From the limited
published data available, it is clear that both heuglini and graellsii can have black on a
total of either 6, 7 or 8 primaries (heuglini data published in Panov and Monzikov, 2000;
graellsii data in Rauste (1999), given in Hario, in litt.). These data indicate that black on 7 primaries is the most frequent pattern in both taxa (58% of graellsii, 54% of heuglini),
but that a larger proportion of heuglini (23%) have black on 8 primaries than do graellsii
(18%). However, sample sizes are so small (n = 38 and 26 respectively) that this
apparent difference in the frequency of black on 8 primaries may not be representative;
in fact BWP states that 25% of graellsii have black on 8 primaries. Even if a larger
sample supported the values of 23% (heuglini) and 18% (graellsii), a bird with black
on 8 primaries is only fractionally more likely to be a heuglini than a graellsii. Clearly, a
measure of the number of primaries with black pigmentation does not provide a firm
basis for field identification. Note that in Malling Olsen and Larsson's book (2003), the
text describing the frequency of black on P4 in heuglini appears to be erroneous
(revised edition p 395). They state that "Less than 5% black markings on P4". This
implies that more than 95% of heuglini lack black on P4 and so have black on only 6
primaries (P10-5 inclusive). This is at odds with both the published literature (Rauste,
1999 and Panov and Monzikov, 2000) and personal observations.
As with the pattern of white in the wingtip, it is clear that there is much individual
variation and overlap between taxa in the extent of black in the primaries. It is also
apparent from differences in published values that larger samples are needed before
we can be confident about the significance of small apparent differences between heuglini and graellsii. The problem that individual birds can show differences between
their right and left wing should also be borne in mind when using primary pattern to
help identify individual birds; indeed, Plate 562 in Malling Olsen and Larsson (2003)
shows one such graellsii. Unpublished studies also continue to show that the extent of
white in the wingtip of British graellsii varies with age (adult birds continue to develop
more white as they get older) and with sex (males typically have more white than
females). While on average heuglini shows more black and less white in the wingtip
than graellsii (Rauste, 1999), overlap with graellsii and intermedius is extensive. Adult heuglini have a greater tendency to have dark marks on the primary coverts (visible HERE) but as with other features, frequency statistics are needed before its value in
field identification can be assessed.
In general terms, the bare part colouration of adult heuglini is similar to other Lesser Blackbacked
Gull taxa: legs are typically yellow, the bill bright yellow with a red gonys spot and
the orbital ring is red. Some gull species (e.g. cachinnans) have dark iris spotting
('peppering') such that in the field their eyes can look dark. The dark-eyed appearance of
some heuglini has been mentioned by several authors (e.g. Lindholm,1997) and this has
been suggested as something that might be useful for separating heuglini from graellsii and intermedius. However, data do not support the use of this feature. Rauste (1999)
found that around 10% of adult heuglini have eyes which have brown iris peppering but
analysis of unpublished data collected by Mars Muusse (n = 137) indicates that a very
similar proportion (11%) of graellsii also have some degree of iris spotting.
END OF PART 2
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CONTINUE AT PART 3
|Heuglini adult HT-168224, April 17 2006, Tampere, Finland. Picture: Hannu Koskinen.