Tuesday, 18 January 2011

More musings on Slaty-backed Gull

Another shot of last week's putative Slaty-backed Gull at Rainham, showing key details of the wing-tip pattern.
Photo and text © Dominic Mitchell - do not use without permission. Fees apply.
After two days on my patch I wasn’t back at Rainham over the weekend, but apparently more than 1,200 birders turned up on the Saturday hoping to see the Slaty-backed Gull. There was no mass sighting, however, even though the tip was working in the morning – I felt sure it was going to put in a reappearance. I’ll be looking for the bird again later this week but, in the meantime, I’ve been liaising with others who have had experience of the species, both in its native range and in the context of extralimital vagrants.

Head comparison of birds from Rainham (left) and Newfoundland (right). Note the close similarity in the extent and patterning of streaking, which in both individuals covers the entire head and most of the neck; the streaking is more concentrated around the eye, where it forms a short line running backwards, and both birds also show a diffuse darker patch at the rear corner of the ear coverts. The bill patterns of these birds also have similarities, being a very washed-out yellow from base to gonys, and then much brighter yellow to the tip. According to Olsen and Larsson 2003 darker eyes occur in 30 per cent of birds.
Photos © Dominic Mitchell (left) and Jared Clarke (right) - do not use without permission. Fees apply.
Osao and Michiaki Ujihara are renowned for their first-hand knowledge of Japanese gulls, and have established an excellent online image resource (see below). From their extensive experience they have provided expert opinion on numerous extralimital birds, so I asked for comments on the Rainham individual. I was pleased to hear that, like me, they felt it was a Slaty-backed Gull, saying they “can't find any wrong feature. Its overall appearance including the short-winged structure, wide tertial crescent, moderately dark mantle shade and ‘string of pearls’ look very nice. What a great find!”.

Slaty-backed Gull (foreground) with American Herring Gulls (right) and Great Black-backed Gull (behind), Connecticut, USA, 21 November 2008. A useful comparison of mantle shades.
Photo © Mark S Szantyr - do not use without permission. Fees apply.
The comment on the tone of the mantle is interesting, given that this has been cited as potentially problematic. This is nothing new, the same issue having been discussed at length on vagrant birds in both Connecticut (see above) and Newfoundland. On the former, the Ujiharas have previously commented: “In our experience, mantles of Slaty-backed Gulls are not as dark as American birders often expect”. There is clearly variation from more blackish birds to greyer individuals, and this is borne out by the range of Kodak grey scale values quoted in Howell and Dunn 2007, compared here with the two black-backed species:
  • Lesser Black-backed Gull (subspecies L f graellsii): 9-11
  • Slaty-backed Gull: 9.5-11.5
  • Great Black-backed Gull: 13-15
Note how Slaty-backed overlaps extensively with graellsii Lesser Black-backed Gull, but not at all with Great Black-backed. Here are a few images which help illustrate this extensive variation (reproduced with permission from the photographers, to whom many thanks):

Slaty-backed Gull (foreground, with Glaucous-winged Gull), Juneau, Alaska, 17 January 2011. This stop-press image of a bird found yesterday was sent in overnight by Gus van Liet, and to my eye shows upperparts recalling graellsii Lesser Black-backed Gull in tone. This also seems to be a fairly dark-eyed (for SBGU) individual.
Photo © Nick Hajdukovich - do not use without permission. Fees apply.
Slaty-backed Gull, Kuril Islands, Russia, 6 June 2010. Even allowing for photographic vagaries, a very pale bird.
Photo © Dick Newell - do not use without permission. Fees apply.
Slaty-backed Gull, Japan, 24 February 2010. An obviously darker individual, with prominent underwing 'shadow'.
Photo © Ian Lycett - do not use without permission. Fees apply.
Slaty-backed Gull, Newfoundland, 13 January 2008. Same bird as in the head comparison montage above.
Photo © Jared Clarke - do not use without permission. Fees apply.
Slaty-backed Gull, Japan, 28 February 2008. The black bill band is almost the only remaining sign of immaturity.
Photo © James Lidster - do not use without permission. Fees apply.
Jared Clarke, who has good field experience of vagrant Slaty-backed Gulls in Newfoundland (having seen 14 out of 16 individuals), commented on the Rainham gull as follows: "I think your bird is a pure SBGU since all other features (with the exception of the slightly pale mantle) are dead-on and show no indication of a hybrid. I agree whole-heartedly with Nick Bonomo's recent comments [on ID-Frontiers] on this issue, and that unless there is more evidence of mixed blood than just the pale mantle (with some very experienced people saying that it is within range) than the best conclusion is a solid Slaty-backed Gull. Case closed."

A feature also mentioned as a possible pitfall for the Rainham bird is the underside of the ‘hand’, the dark show-through being less prominent than expected. While this may partly be affected by camera exposure, I suspect it could possibly also be related to the intensity of pigmentation on the upperparts. An accepted vagrant Slaty-backed from Connecticut shows a similar pattern (see this instructive image on James P Smith’s excellent blog).

Enough for now, save a few links for those still interested:
  • For a Japanese bird which somewhat recalls the Rainham individual, see the fourth image down on this page on the Ujiharas’ excellent website.
  • Mark S Szantyr has a great selection of shots of a Connecticut vagrant on his website .
  • Steve Huggins has a nice darker, lemon-eyed bird here.

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