Monday, 4 August 2014

Two for the price of one ...

It's been a while since I've had time to post on the blog, largely because I've been working hard to finish a new book on Western Palearctic birds (more anon). As that nears completion and I have more opportunity to get out into the field and post about it, the plan is for birding news to appear here again more regularly. Let's start with a subject which is a personal favourite: gulls.

Note the tail pattern and also the greater coverts of this colour-ringed juvenile gull.
I spent a day birding on the Suffolk coast yesterday, a fair amount of it looking at and photographing gulls. I first picked up this interesting juvenile large gull distantly in flight over the mouth of the River Blyth at Walberswick, and thought it was probably going to be a juvenile Yellow-legged Gull when I got better looks. On closer inspection, however, the black tail band seemed rather broad and not typical for that species. Intriguingly, I also noticed the bird was colour-ringed (white on green XDEE), so I kept an eye on it and got a few images when it came within range.

The bird was colour-ringed XDEE white on green as a chick in a mixed Caspian x Herring colony in east Germany ...
Eventually the mystery gull landed not far away and perched shots were briefly possible. As I took them, the bird suddenly struck me as looking somewhat Caspian Gull-like in standing profile. If the tail pattern wasn’t right for Yellow-legged Gull, however, it certainly wasn’t typical for Caspian; yet at the same time I noticed it looked ‘leggy’, and it was also already moulting in some first-winter feathers on the upperparts, more or less ruling out later-breeding local species such as Herring and Lesser Black-backed Gulls.

... so it could be Caspian or Herring, or more likely a bit of both, as its mix of characters appears to indicate.
A better view of the upperside, showing the rather heavily marked tail and a wing pattern resembling michahellis.
My curiosity was well and truly piqued, and this morning I sent images to Peter Rock, who co-ordinates colour-ringed gull sightings in the UK, to request information on the scheme that marked this gull. He responded quickly to say: “This is a German bird … you and [Ronald Klein] will, I'm sure, have an interesting correspondence.” He wasn’t wrong. Ronald made contact shortly after and explained that “it is a gull from the mixed area (Caspians and Herrings) in East Germany”, and confirmed that the metal ring code (which I’d only been able partly to read) is Hiddensee EA-068143. “The problem is,” he continued, “the parents of all the ringed birds are unknown, because [they are] caught in ‘chicken-nurseries’.”

Note the bird's leggy jizz but somewhat 'truncated' rear-end profile, plus the tertial pattern and greater coverts.
Ronald believes this bird is a hybrid Caspian x Herring Gull, a view which helps explain my early impression of atypical and perhaps mixed characters. From looking more closely at the images, the slightly ‘truncated' look of the bird’s rear end, the speckled pattern on the tip of the second tertial and the more heavily chequered greater coverts are also suggestive of Herring Gull influence in a bird with some Caspian-like qualities. It will be interesting to see what it looks like if it returns in subsequent years, and I’d welcome news of any further sightings of this bird (and also other comments from observers with first-hand experience of juvenile Caspians and hybrids).

If this bird returns to Walberswick regularly to winter, it will be interesting to monitor its plumage progression.
It was one of several interesting larids in a day which also produced a second-winter Caspian Gull and a possible juvenile (too distant) in the pig fields at Walberswick, and which gave me a final Suffolk day total of a very respectable 10 gull species – the other nine being Black-headed, Little, Mediterranean, Common, Herring, Lesser Black-backed, Great Black-backed, Yellow-legged and Kittiwake. Autumn is definitely here …

2 comments:

  1. Hi Dom

    I think that all these green-ringed LWHG are tainted with the possibility of hybridization. The CR website describes the colony as mixed where Caspian dominates, but hybrids are possible. I saw the attached gull at Hedgerley Landfill, Bucks in Feb this year, which although it had a lot of features within good Casp variation, had a green ring and so raised the spectre of doubt (I never was able to read the ring).

    http://www.goingbirding.co.uk/bucks/show_photo.asp?photo_id=3560

    Cheers, Adam

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  2. Thanks for the interesting feedback, Adam. I agree that in theory all the gulls in these colonies *could* be tainted with the possibility of hybridisation, but not that they necessarily all are. Assuming that Caspians do indeed dominate (and I've read elsewhere that they do), then it seems logical to assume that there will be a significant number of pure Caspian pairs, with pure offspring. There will be hybrids and backcrosses too, but unless an individual gull shows some kind of indication of the influence of another species in its appearance (as with XDEE above), it is effectively not a hybrid. I realise that's a simplification, but I don't like to use the hybrid tag without good reason; your bird looks pretty Caspian-like to me in terms of jizz and general appearance, and assuming that with better views the detail of the markings on the mantle, wing coverts, tertials and tail all fit, I would be tempted to call it just that. It would be interesting to know how many generations it would take to 'breed out' any visible hybrid influence if, say, a Caspian x Herring hybrid and its future offspring only mated with pure Caspian; maybe only two or three?

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