Part one in a short series of problematic identification headaches....
Having scanned the gulls on the local patch here at Stoke Newington Reservoirs increasingly of late, and while it's by no means a premium site for Larids (see here for overview), there's been plenty of intrigue to keep the home fires burning. Aside from a succession of colour-ringed birds of the four commoner species, an apparent hybrid Herring x Lesser Black-backed Gull, regular Yellow-legged Gulls and omnipresent argentatus Herring Gulls, a couple of recent individuals have been less than straightforward.
Top of the list presently is this troublesome 'Yellow-legged' Gull, on the East Reservoir on the 14th December. Immediately standing out and looking 'wrong' for e.g. standard michahellis, it was distinctive enough to inspire the seeking of more opinions.
After receiving the views of several larophiles, a satisfactory ID seemed increasingly improbable, with the most popular punt initially being a hybrid with potential 'throwback' characteristics. Other possibilities mentioned but thought unlikely included a 'dwarf' (or exceptionally small) michahellis, or an odd Finnish / north-eastern yellow-legged argentatus.
However, a message from Martin Garner the other day raised a fresh possibility. Martin aligned several key features of the bird with lusitanius Yellow-legged Gull, a comparatively little-researched form from the north-west Iberian coastline, and suggested the Stoke Newington bird looked good for this race.....
Following web links and finding several other useful Spanish and European gull study sites as a consequence, it's becoming increasingly difficult to rule this form out of the equation. Most potently, returning to the Yellow-legged Gull text in Olsen, the (previously unnoticed) small print on lusitanius suddenly jumped out, for a variety of reasons.
Key features of this bird included:
size - smaller than the smallest Lesser Black-backed Gulls (of which a variety were present for direct comparison), and significantly smaller than all nearby Herring Gulls.
shape - distinctly low-carriaged, squat and short-legged (almost bizarrely so). Appeared particularly long-necked when alarmed, and hind quarters slender when relaxed.
head - rounded with a flattish forehead and slight rear-crown peak. Very light streaking on the crown and ear-coverts.
eye - iris pale, yellowish.
bill - proportionately heavy and medium-long, with large red gonys spot extending onto upper mandible.
legs - bright yellow; brighter than most recent winter-plumaged michahellis at the site.
upperparts - medium-dark grey. Obviously darker than argentatus, and darker than 'normal' michahellis (and indeed Common Gull); substantially paler than e.g. paler graellsii.
wing pattern - large white wing tips, and reduced black, on primaries. White mirrors on p9. Reduced black compared to e.g. michahellis extending up primaries (e.g. black not reaching primary coverts on p9). No black at all on p5.
Graellsii, argenteus and 'standard' argentatus are easily excluded, realistically leaving a hybrid, an exceptionally small and aberrantly-plumaged michahellis Yellow-legged Gull, a different race of Yellow-legged Gull, or a different race of Herring Gull.
If it's a hybrid, which is always a possibility (especially with large gulls), then of what? While offspring can show anomalous characteristics not indicative of either parent, and 'throwback' features can appear from much further back in the family line, it still seems like difficult to explain this bird's combined features, not least regarding its structure and primary pattern.
An exceptionally small (female?) michahellis also seems implausible - this wouldn't apparently explain the combination of the bird's primary pattern, distinctive structure, and indeed probably its size; in the photos above and below, the bird is stood to the right of a relatively small graellsii, which is clearly larger (easily visible with the naked eye). For comparison a 'medium' sized graellsii (and Common Gulls) is also to its right in the less cropped photos.
Re: the possibility of a NE European race of 'yellow-legged' Herring Gull being involved - which can have darker mantles also - but as MG points out, would expect to have more white in the primaries, and the size and structure would be at best unusual. (Again, being on a steep learning curve in this realm, I'd appreciate signposting to other relevent references of lesser-known Northern HG races).
Finally, the possibility of the bird being a Yellow-legged Gull of the race lusitanius. There's doubtless more information out there, most likely online, that I'm not yet aware of (and would welcome hearing about), but the following features described by Olsen (in comparison with michahellis) read compellingly:
Generally smaller than michahellis. Heavy-billed with flat forehead, 'snaky' neck, slender hindparts and short legs.
Compared to michahellis shows narrower black markings on p5.
Upperparts generally slightly darker (Kodak greyscale 6.5-8).
White-headed July-Jan or just with scattered dark markings.
Bare parts as michahellis, but generally brighter.
Far from being well-versed in the racial and ID complexities of LWHG's, I'd be interested in the views of those with more knowledge and insight, and comments are welcomed.