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LWHG at Moseley, West Midlands, February & November 2017 and November 2018


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A LWHG appeared at Swanshurst Park, at Moseley, West Midlands, during February 2017, when it was in 1W plumage. It re-appeared in late-October, still in its 2cy but having moulted into 2W plumage, and yet again in November 2018, in its 3cy and exhibiting  'advanced' 3W plumage  It proved to be an exemplar of controversial LWHGs. During its initial sojourn opinions were divided between Caspian Gull and Yellow-legged Gull and, in the field, the appearance did vary quite extensively, depending upon posture, light conditions and even which side of the bird was in view. Plumage tracts which provide clues to distinguishing between Yellow-legged Gull and Caspian Gull are at a maximum during 1W and become fewer in 2W and 3W, with primary pattern subsequently providing firm characteristics as adult plumage is obtained. The images below illustrate the gulls variability in appearance and discuss particular traits. After successive moults, it became clear that there was certainly michahellis in the gull's lineage, particularly evident in its bare parts. However, aspects of the structure and plumage remained enigmatic. The intention of this text is not to contest any particular diagnosis but to document the intriguing level of variability and make the case for caution in ascribing the gull to a 'thoroughbred' taxon. The possibility of 'mixed lineage' is discussed.


Second calendar-year, 1W plumage. Photos taken February 2017

LWHG, Moseley, West Midlands, Feb 2017

A. R. Dean

Plate 1. Here the neck is neither unduly extended nor settled into the shoulders and the posture appears relaxed.
The head appears proportionately small and has a slightly pear-shaped outline, with somewhat extruded
maxillary feathering leading into a quite pointed-looking bill. The eye is set well forward and close to the forehead.

 

LWHG, Moseley, West Midlands, Feb 2017

A. R. Dean

Plate 2. This plate illustrates the technique for measuring bill-ratio, as employed in the key paper on Caspian Gull identification
by Chris Gibbins et al (see References). Here the bill ratio is approx. 2.11 while the mean measured from several photos was 2.15.
This is low for Caspian Gull, where Appendix 1 in Gibbins et al. indicates a mean for 1W of c. 2.55 with SD of 0.25.
Conversely, a value of 2.15 is high for YLG, which has a mean of c. 1.90 with SD of 0.15 (own data from photos).
Thus, in its1W, the Moseley gull has a ratio somewhat 'in between'.

 

LWHG, Moseley, West Midlands, Feb 2017

A. R. Dean

Plate 3. Here the head is settled more towards the shoulders, with the neck compressed. The head appears proportionately larger
and slightly more bulbous. The bill shows an evident gonys. With the left-side in view, the upperparts appear quite coarsely marked,
with some second-generation scapulars showing broad, almost triangular, interior marks and a 'double barred' pattern.
The under tail coverts are quite coarsely marked.  When the gull was standing on the embankment,
retained central greater-coverts revealed a barred / notched pattern.

 

LWHG, Moseley, West Midlands, Feb 2017

A. R. Dean

Plate 4. On the right side, the second-generation scapulars were less-heavily marked, with several feathers showing a subterminal anchor
and a fine mesial streak but extensive and  'clean' areas of grey basally. Tertials also atypically variegated  for both cachinnans and michahellis,
though perhaps more likely in the latter.

 

LWHG, Moseley, West Midlands, Feb 2017

Graham Fellows

Plate 5. The inner primaries show grey on inner webs reaching the edge of the wing, creating a  'venetian blind' pattern.
There are pale 'lozenges' at the extremities of the inner webs. The outer webs are dark but have pale lines internally.
The underwing shows a pale and relatively plain centre and the underside of the primaries is also pale.
In combination, this produces  a rather 'hollowed out' appearance overall.  The tail has a black terminal band
of quite even but unexceptional depth and has black 'spots' internally, The bases of the outer feathers are more-cleanly white.


Second calendar-year, 2W plumage. Photos taken November 2017

In late-October what was evidently the same individual re-appeared. Still in 2cy but now in 2W plumage, its relatively small size and rather dainty structure were the same and, at both seasons, it was easy to find LBBGs which were larger than it.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 6.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 7. Compare plates 6 & 7 with plates 1 & 3. Again note how posture affects impressions of relative size and shape of head and bill.
In plate 7 note the evident gonys (YLG-like) but the lack of conspicuous streaking on ear-coverts (the corresponding lack of an 'eye-mask' is unlike most 2W YLG).
Although here the iris looks quite dark, there are indications of emerging colour.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 8. Here bill ratio is approx. 1.84, thus lower than as measured in February (in 1W) and too low for a Caspian Gull per se.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 9. At times the size and structure appeared very dainty, almost recalling an over-sized Common Gull.
Note the relatively small yet rounded head and slight bill and also the very attenuated rear end. Iris here appears grey.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 10. At other times there was a marked resemblance to the posture and structure of Caspian Gull, with head appearing relatively small
and forehead low, bill looking slender and posture upright, with a rather vertical line to breast and fore-belly.
Again note the lack of prominent streaking around eye but iris (here catching the sun) with a distinctly pale hue.
The extent of grey in the upperparts is extensive and more adult-like than in many 2W YLG of this age (Olsen & Larsson).

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 11. Demeanour here somewhat more-robust but note size and structure of both body and bill in comparison with LBBG (probably a male).

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 12. As evident in 'standing' images, there has been extensive moult of wing coverts
(as well as mantle, scapulars and flight-feathers), creating a much more-advanced appearance than in 2W HG and many YLG.
Note patterns of tail, gc and inner primaries all of which are more associated with Caspian than YLG, at least according to some sources.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 13. Again note the patterns of innermost primaries, creating a rather uniform pale grey 'window', a feature more often seen on Caspian.
Also the whitish underwing. Conversely, a small white mirror on p10 is a feature exhibited quite frequently by 2W Caspian but there is no sign of that here.

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 14. Note white appearance of underwing, with significant marks restricted to primary coverts.
Also evident pale inner webs to central primaries, creating a pale centre to the 'hand'.
(The gull has a leaf in its bill, so bill shape and gonys not evident)

LWHG, Moseley, West Midlands, Nov 2017

A. R. Dean

Plate 15. Pale underwing and unmarked axillaries again evident, as are uniformly pale-looking innermost primaries and pale 'veins' on central primaries.


Third calendar-year, 3W plumage. Photos taken November 2018.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 16. Now in 3W plumage. Note very white head, lacking 'eye-mask' and with evident streaking confined to a 'necklace' at base of hind-neck. Note also pattern of underside of p10.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 17.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 18.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 19. In plates 16 to 19 note yet again how head-shape and apparent  position of eye vary markedly depending on posture and demeanour.
In plate 19, there is a definite michahellis-like demeanour but this is less evident in plates 16, 17 and 18.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 20. Note flat-backed appearance, lack of 'tertial step' and very attenuated rear end compared with the LBB Gull.
(However, telephoto effect has exaggerated the apparent size difference, which was in fact much less than it appears here)

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 21.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 22.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 23. Note considerable variation in structure and relative proportions between plates 21, 22 and 23.
Note also the evident 'keel' behind the legs. Compare with plates 9 and 10, showing the gull in the previous winter.
This demonstrates that such variations were characteristic and not 'one off' aberrations.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 24. Here there is a suggestion of grey 'leads' into the base of the outer primaries (c.f. plate 26).

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 25. Even in 3W, there is still just a single mirror (on p10) with no sign of a mirror on p9.
Note very extensive moult, with pure white tail and pristine grey mantle, scapulars and secondary coverts (black streaking being confined to pc and alula).
The appearance could well be attributed to 4W were the history of the bird unknown.

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 26. In plate 25 the black in the primaries appears 'solid' and rather' square cut' across the 'hand'.
In plate 26, however, there are indications of grey 'leads' cutting into the base of the outer primaries (c.f. plate 24). 

LWHG, Moseley, West Midlands, Nov 2018

A. R. Dean

Plate 27. The underwing shows extensive black in the primaries and no sign of a mirror on p9.  While not excluding cachinnans, the pattern is typical of michahellis.

Discussion

The photos above illustrate why this LWHG generated so much debate and differing conclusions, particularly in its first winter. Its shape and structure were decidedly 'malleable', at times appearing slender with suggestions of a Caspian-like head and bill-structure, at other times appearing more robust with a squarer head-profile and a stockier and blunter-looking bill, more reminiscent of Yellow-legged Gull.  In terms of structure, 'classic' Caspian Gulls are, in truth, the 'most distinctive' and not necessarily the 'most typical'. The popular image of Caspian Gull is something of a caricature (most casual sketches of the species, for example, depict a wholly exaggerated bill ratio (length : depth) -see below). Any individual which does not match this overstated (or at least 'exceptional') appearance inevitably shifts its demeanour towards that of its congeners. However, in the field, the Moseley gull certainly did not match the 'classic' Caspian Gull image with any consistency, though it did so periodically

Assessing the features visible in the images of 1W plumage, the barred greater coverts and under tail coverts tend to favour Yellow-legged Gull. The whitish 'core' to the underwing and the 'venetian blind' and 'lozenge' patterns of the inner primaries are said in some texts to favour Caspian but the pale internal lines on the outer webs may be more frequent in Yellow-legged. Coarse cross-bars on second-generation scapulars are deemed to favour Yellow-legged while the plainer more-extensively pale grey feathers are associated with Caspian. The width of the tail-band better matches Caspian but the interior pattern favours Yellow-legged. The pattern of the tertials is more variegated than is usual in either Yellow-legged or Caspian.

There are fewer plumage clues to differentiating between Caspian and YLG in 2W and 3W plumages but four features evident when the gull re-appeared at the end of October 2017 and again in November 2018 showed that it is not a Caspian Gull per se.

Several of these features are 'percentage' characters rather than invariable and diagnostic but, in combination, they are not readily compatible with a diagnosis of Caspian.

When it returned in November 2018, in 3W plumage, the legs were a pale straw hue and appeared quite stocky (not long and spindly as in 'classic' Caspian). There had been an extensive moult, with mantle, scapulars and secondary coverts all pristine grey and with a pure white tail, with no sign of a residual tail-band. On the spread wing, black streaking was confined to the pc and alula. Were the history of the gull unknown, it could easily be diagnosed as 4W. Although sub-adult Caspian is sometimes stated to appear more 'advanced' compared with Yellow-legged of comparable age, some michahellis can appear as 'advanced' as the Moseley gull (pers obs). Thus, plumage tracts providing firm 'clues' were limited but there was still no sign of a mirror on p9.

So, can a diagnosis of Yellow-legged Gull now be adopted? At times, the overall appearance of plumage and bare parts certainly indicated michahellis in the lineage. However, it is very easy to be selective when arguing the case for a particular identification and to dismiss traits which do not accord with a 'preferred' diagnosis. There are several features which suggest adopting a cautious approach. Features which suggest restraint before diagnosing the Moseley gull as a thoroughbred YLG include :

While by no means diagnostic, these features are associated with Caspian Gull in the current literature. Two other 2W Caspian Gull traits which have been mentioned by Peter Adriaens (in earlier discussions of this species in December 2011) are :

A plethora of photos of first and second calendar year Caspian Gulls and Yellow-legged Gulls is available on the Gull Research Organisation website.  Even a cursory examination there shows that, in practice, many features noted in the photo-captions and bulleted lists above are no more than 'percentage' features. Most of the cited characters are within the range of variation of both species. It is unwise to be dogmatic about the identity (or at least the 'lineage') of an individual when several features are less than ideal. The character of both Yellow-legged Gull and Caspian Gull derives from a combination of features and is more than 'the sum of its parts'. A forensic analysis will often fail to provide a convincing diagnosis.

The problems arising from character variation and convergence are exacerbated by the issue of hybridization, which besets LWHGs more than any other group of birds. Part 2 of the paper by Chris Gibbins et al (2011) was dedicated to the issue of hybrids and the paper developed a 'scoring system' by which a 'hybrid index' could be calculated from a combination of traits. However, this applied to hybrids between Caspian Gull and Herring Gull (hitherto the most common source of problems) and most of the trait-scoring system does not apply to 1W Caspian versus Yellow-legged - where many features are much more convergent. Additionally, plates 213 - 215 in Neubauer et al. (2010) show a known adult hybrid Caspian x Herring Gull which looks remarkably like a Yellow-legged Gull. More recently, hybridization between Caspian Gull and Yellow-legged Gull has been on the increase in central Europe. See details in Neubauer et al. (2010)  - where plate 216, showing a hybrid 1W Caspian x Yellow-legged Gull, is also instructive. Thus, the identity of the Moseley gull is not of necessity a straight choice between Yellow-legged Gull and Caspian Gull. The possibility of hybrid origin also needs to be 'in the frame'. To my eye, even after extensive observation in the field, the appearance of the Moseley gull did not 'resonate' convincingly and consistently as either thoroughbred Yellow-legged Gull or Caspian Gull. That said, were it encountered for the first time as it appeared in its 3W, then, despite the structural variations, I would very likely have diagnosed it as a 3W / 4W female Yellow-legged Gull with an unusually white head. However, its full history and plumage development have to be taken into account and signal a more cautious approach.

I asked Chris Gibbins for his opinion, both in February, relating to 1W appearance, and subsequently when the gull returned in 2W plumage and extracts from his replies follow.

In relation to the photos in 1W plumage:

'This is a tricky bird. [For a Caspian] the problem is the bill.  I would suggest that the general structure of the bird is more like a female. Females obviously tend to have smaller bills that can look very weak, slender and contourless but this bird has a very strong gonys. It has a strong angle and bulges. This I find most odd for a female. Perhaps in Romania or Ukraine or around the Caspian I would forgive this anomaly, but in UK then its a different matter.  
I was at the IGM
[International Gull Meeting] in Germany last spring and we visited several colony areas with mixed pairs.  It is really alarming what is happening there; Herrings, Caspians and YLGs all together and all combinations involved in mixed pairs. Many of the birds we were seeing were birds that were outwith my experience of Caspians from Azerbaijan, Ukraine etc where Ive visited breeding colonies, but nor could I fit them comfortably into Herring or YLG. A high % of assumed hybrids. It was fascinating but very alarming. So personally Id be cautious with birds like this.'

In relation to the photos in 2W plumage:

'The bill is not right for Caspian  and one or two other things, although they can be seen in second cycle Caspians (e.g. no p10 mirror, pale-ish eye) are less typical in this species. Nor does it look like a YLG. While I think we can never be sure, there seems to be enough here (and along with what it looked like in 1W) to suggest mixed parentage, though exactly what I'm not sure.' 

Thus, Chris Gibbins too counsels 'caution' in attributing it to a given taxon. Such gulls will always divide opinion. Many problematic gulls are debated on internet discussion groups, particularly on the American 'Id-Frontiers' site. Few discussions achieve unanimity of opinion. My own conclusion is that the Moseley LWHG exhibits a number of definite michahellis features but its full lineage remains questionable ..... That's LWHGs.


Thanks to Graham Fellows for his photos showing the wing and tail patterns in 1W plumage, to Des Jennings for providing information about the return of the gull in subsequent winters and to Chris Gibbins for his informative comments.

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