Thursday, 13 January 2011

A candidate Slaty-backed Gull - in London!

Adult gull at Rainham Marshes this afternoon showing characters of Slaty-backed Gull. 
Photos and text © Dominic Mitchell - do not use without permission. Fees apply.
I would like to have posted this news earlier today, but it's been one of those days ...

I paid a short visit to Rainham Marshes today, originally with the intention of trying out a new camera. However, when I got to the site there were so many gulls present that I decided to focus on gulling, so left the scope on the tripod and instead slung my trusty old 50D, 500mm f4 lens and 1.4x extender over my shoulder, foregoing using the new body while doing some serious birding.

Having worked through the gulls in the Coldharbour Lane area for about an hour and failed to find any Caspians and just a single Yellow-legged Gull, an adult, I began to wonder if the session would pay off. Bar a couple of interesting-looking Herring Gulls there seemed little else worthy of scrutiny. I was panning back through the massed ranks for about the eighth time when I stopped dead in my tracks. My gaze fixed on a large, dark-mantled gull that was immediately distinctive, and instantly I said to myself "adult Slaty-backed Gull!" Then I suspended belief in what I was thinking and looked at it again, feeling I must be kidding myself - yet, to my eyes at least, it still seemed to scream 'Slaty-back!'.

As I started to look at it more closely, another gull dropped in to land almost on top of it, flushing the bird. I panicked, reaching for the camera while trying to keep an eye on the bird. Fortunately, it came back down only a few metres away, so in case it flew again I fired off a short series of record shots (unfortunately, and unavoidably, directly through chain-link fencing, so they are not as clear, sharp and bright as the bird was 'in life'; they are all similar to the best two in the series, published here).

Note relative size and upperpart coloration of bird (left) compared to nearby Herring and Great Black-backed Gulls.
Photos and text © Dominic Mitchell - do not use without permission. Fees apply.
Here is a summary transcription of the points I noted at the time (please note this is not a full write-up, submission or discussion):

Size: almost size of nearby Great Black-backeds (possibly larger than one individual), and clearly significantly larger than Lesser Black-backed Gull and 'typical' argenteus Herring Gull. One nearby argentatus of similar size (all these taxa present in numbers in close proximity today).

Structure: bulky and solid, though different in 'character' to Great Black-backed Gull. Head less 'powerful' than the latter, more like Herring Gull, and in profile appeared more pot-bellied and a little shorter legged than Great Black-back.

Plumage: the combination of slaty-grey upperparts with very broad white tertial/secondary edges was immediately distinctive, especially taken in combination with the pink legs and large size. These broad white edges then continued, more narrowly but still conspicuously, along the folded edge of the wing. The upperparts were uniformly and solidly grey, with no trace of browner remnant immature markings; if anything, there was the slightest bluish tone to the upperparts, very subtly distinct from graellsii Lesser Black-back (this feature being mentioned to several others in discussion of the bird). The primaries were black, with four conspicuous white tips visible beyond the tertials at rest. The tail was white. The head and neck were diffusely and extensively streaked pale brown, these streaks combining to form a hood which extended at the front to the breast, becoming less boldy marked in this area. Streaking was densest around and especially below the eye. The rest of the underparts were unsullied white.

Bare parts: iris seemed pale yellow. Bill dull yellowish, dullest and more colourless towards base, brightening towards tip, and with small red spot near tip of lower mandible; not especially bulbous at the gonys. Legs shorter than nearby Great Black-back and pink, rather stronger in tone and darker than they appear in these 'uncontrasty' images - while watching through the scope, at times they seemed almost bubblegum-pink.

After watching the bird and noting the above points, I started to ring some local birders in a bid to get others to see it. While I was on the first call, still watching the gull through the scope one-handed, it took off and seemed to fly towards Wennington Marshes. In the very brief moment between when it took flight and when I lost it, I was unable to note detail on the underwing and wing-tip, as it was quickly hidden in a flurry of wings.

Eventually, three other observers joined me to help try and refind the bird, which we failed to do before dark. I remained reasonably happy that it was probably a Slaty-backed Gull, a species I have seen in Asia previously (though not for about nine years), but as always with large gulls, caution must be exercised. On seeing one of the other observers' copy of Howell and Dunn 2007, I was surprised that some adult-type Slaty-backeds can look almost black above, and that fact caused me some concern as this bird was closer in tone to graellsii Lesser Black-backed Gull (if anything, the grey appears subtly lighter in these images than some graellsii, but the quality of the images may also have affected this slightly).

I therefore welcome opinions on this bird, whatever its true identity - please leave your comments below. In the meantime, there is much research to do on this in the days ahead.

Thanks to those who have taken the time to discuss this bird during the course of the evening. Early feedback, including from Martin Garner and others, is encouraging towards the identification of what would be the first British record of this species. The next thing to do is refind it, observe the open wing and hopefully photograph it. Having been slightly concerned initially about the upperpart tone, I have this evening checked Olsen and Larsson 2003 and other references, and also discussed the bird with Chris Gibbins (who found the Western Palearctic's first individual in Latvia two years ago), and it seems that the Rainham bird is unproblematic in this respect, falling within the likely variation for Slaty-backed Gull. But more comments have been solicited from other gull experts and online to try and gain a full range of opinion.

There are many thousands of gulls at Rainham at the moment, and they favour the RSPB reserve, the adjacent Wennington Marshes (very large numbers of loafing birds), the river/foreshore (partly tide dependent) and the tip (strictly out of bounds at all times). If you are planning to look for the bird, it is essential to follow these instructions, provided this evening by the RSPB:

Parking options:
  • Rainham Marshes RSPB main car park, off New Tank Hill Road, RM19 1SZ. Park and walk along river wall to the west and then on the cycle path on the inland side towards the tip, stopping to scan Wennington Marshes on your right.
  • Rainham Riverside car park off Coldharbour Lane. Only room for about 20 cars. Do not park in or obstruct entrance road or disabled spaces. If parking here walk back down the path to Coldharbour Lane and follow the cycle path east past the tip to Wennington and then Aveley Bay.

Viewing options
  • Scan the fields of Wennington Marshes, where all the gulls come down to rest during the day.
  • Enter the reserve via the visitor centre and check Aveley Pools and the Target Pools, where gulls also come down to bathe.
  • Check the foreshore to the east of the Stone Barges, accessed via the Rainham Riverside car park as birds also commute here to bathe.
  • Check Aveley Bay for similar reasons.
  • DO NOT ATTEMPT TO ACCESS THE LANDFILL SITE UNDER ANY CIRCUMSTANCES. It can be viewed more distantly from the cycle path, but the gulls are much easier to check on Wennington Marshes, where they linger for longer and are far less active.


  1. A very interesting bird, and from initial impressions almost certainly a Slaty-backed Gull. The overall impression, wide tertial crescent, slate grey back (more on this later), large white apical spots, pot-bellied/short-legged appearance and especially the brown head streaking extending down to the chest to form a bib are all very strong points in favour of that identification. It is difficult to tell from your photos just how bright pink the legs were, but based on the colour I can see and your description it sounds good. Really, the only thing missing is a confirmation of the wingtip pattern (which, of course, is important).

    We have had 16 records of Slaty-backed Gull in Newfoundland since our first in January 2006. Combined with the increase in sightings elsewhere in northeastern North America, it is clear “something” is happening … so I’m not too surprised by your bird. While I certainly don’t have the experience of a birder who has spent significant amounts of time in Asia, I have been fortunate enough to have seen 14 of our 16 individuals, and studied the species extensively. We have seen a variation in mantle colour on our adult birds (14/16 records), and your bird certainly appears to fit within the reported range for Slaty-backed Gull. No real worries there, as you have already pointed out.

    I was a little concerned about your size description, as it sounded a little on the large end for Slaty-backed Gull – but you mentioned it did seem to match at least one argentatus Herring Gull so that should be fine. I don’t think that any of the potential hybrid combinations (all of which would presumably include GBBG as on parent) would produce a bird with this combination of features – especially the wide tertial crescent, bright pink legs, apparently extensive white in the wingtip and pattern of head streaking. Your bird is a much much better fit for Slaty-backed than any hybrid combination I would expect.

    All that being said, I think you have a bona fide Slaty-backed Gull on your hands. Congrats!! Now, just go back, nail a shot of the spread wings, and put this thing to bed!

    Jared Clarke
    Jared_jjc AT
    St. John’s, NL, Canada

  2. Dominic,

    Have put this on the ID section of Bird Forum.

    Hope it gets you some feedback, and good luck with it!

    Really hope that you score with it,


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  4. Dom, looks a great candidate and a striking bird- well done.

  5. Dominic

    I think you've pulled a blinder! The pale iris, pale bill, massive white on tertials & secondaries all look spot on. I can't imagine any form of hybrid that would have the third-mentioned feature. Well done! I'll be down at dawn tomorrow if not sooner.

    James Lowen

  6. Fingers crossed it gets accepted - you deserve it! I can't imagine the emotions you must have gone through when you realised you had potentially discovered something special.

  7. It certainly has that squashed in look about it;-and one of the first things I noticed in the close-up photo was the outstanding tertial step.It reminds me of Glaucous-winged in shape and structure in your photos.
    Never seen this species,but it certainly looks a distinctive gull.

  8. Cracking bird. Here's hoping!

  9. WOW!! what a superb find Dominic, very well done indeed

  10. Wow a superb find Dominic, well done

  11. Hi Anonymous Dave from Anonymous Jim! :-)

  12. Thanks to everyone for their comments and kind words, and especially to Jared for feedback from his experience of so many vagrant individuals in Newfoundland (any chance of emailing suitable images of adults from either end of the mantle tone spectrum, for a comparison post I'm preparing? If so, thanks in advance). I've also seen images of Connecticut's first, which was also comparable in mantle tone to the Rainham bird. More to follow, doubtless!

  13. Awesome Dominic!!!!Well done and what just reward for such dedicated patch work.



  14. Look at my blog post of a possible Slaty-backed Gull in Minnesota.

    Most folks responding think it is not a Slaty-backed Gull, but I am less sure of their opinion after seeing your blog.



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