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The probable identity of the gull Larus sp. at King’s Lynn, UK

Images taken on 30th July 2000 by Brian J. Small. or Brian Small would be grateful for any feedback offered, or opinions about the identity of this gull. At the moment we have an open mind about its identity.

Discussion below by Brian J. Small

All images taken using a Sony PC-3 through a Leica APO Televid 20X or 30X eyepiece.

Just when you begin to think that you have got large white-headed gulls sorted, along comes one that totally confuses. The gull at King’s Lynn (hereafter KL gull) is one of these. In my opinion it is a Caspian Gull, but one that is, for most birders, outside the current accepted norm for this species, and I intend here to put my reasons for this.

The confusion surrounding or, rather, questioning of the gull’s identity, is largely a result of a lack of information on the plumage of this, and other, species in late summer. Most images present in the birding press are taken in the winter or spring, and many birders have an impression of Caspian in autumn based upon Lars Jonsson’s photo in Alula, taken in September. We are still ‘pushing the envelope’ in gull identification; the KL gull has certainly stretched it a bit.

I feel that I am lucky, and have had a bit more insight, as a result of finding a typical first-summer Caspian Gull at the Blyth estuary, Suffolk, UK, only two days after seeing the KL bird. Also, I have had much correspondence from people and I am grateful to Richard Millington, Rik Winters, Peter Adriaens, Visa Rauste and Dick Newell for their informative and knowledgeable comments. The general consensus is that it is certainly not a Heuglin’s, most likely a Caspian, but (a remote possibility in my opinion) possibly a hybrid cachinnansxmichahellis or xargentatus.

A detailed study of the plumage of both gulls showed so many similarities that I feel that they have to be the same. In fact the only ‘odd’ or different thing about the KL gull is its structure (in particular head and bill structure) - Rik Winters who has had more experience than I in Holland, says that it matches the largest Caspian Gulls seen there.

Let’s look at the plumage of the King’s Lynn gull closely (what follows is out of necessity very detailed)-:
The head and body were notably white - there was some slight smudging around the eye, highlighting two white crescents above and below the eye and some smudgy sepia-grey marks on the hind-neck; the body had some diffuse marks and crescents along the flanks. Nothing odd here – compare the images of the two gulls.

The under-wing coverts and axillaries were basically white, but on the marginal and lesser coverts and the axillaries faint brownish barring was present. Although second generation under-wing coverts should be totally white, were these old or new and can some show markings like this? The Caspian at Blythburgh had unmarked white under-wing coverts, but a hint of barring on the axillaries. Again, nothing odd here.

Now we are getting to the confusing features. The mantle and scapulars were an admixture of dark spots or diamonds (forming rows), greyish or brown-grey feathers, with, or without, a slightly darker sub-terminal line, and (particularly towards the rear scapulars) those that had a darker brown-grey base, and two bars across white or grey. Most of these tended to look quite worn – these were similar in pattern to that shown by the Greater coverts. Compare this with the Blyth Caspian and we can see the similarities: the rows of dark diamonds on basically grey or (along the lowest scapulars) brown-grey with thin darker sub-terminal lines; the rearmost scapulars had dark bases with two darker bars across white. The only thing which we expected the KL gull to have were the rather contrasting white scapulars, with large black diamonds at the base and thin black lines extending to the tip. These feathers are those which have been used to support the hybrid theory, but my observation would show that they in fact normal for Caspian in late July or early August. Why seek a confusing answer when a more obvious one is already there?

The greater and median coverts were identical in each bird and obviously normal for Caspian at this time of year, but by being not what we had expected, they caused confusion. If you look closely at the pattern of the rear greater and median coverts on the KL and Blyth gulls you can see the largely dark base, with faint barring, but a black shaft streak extending towards the tip and becoming an anchor mark, behind which are two distinct white marks coming from each edge almost forming a bar. The outer greater coverts are almost uniformly moth-grey, but the outer medians more barred near the tips. On both gulls the lesser and marginal have retained some very worn and bleached juvenile feathers. The tertial pattern, black or grey-black with white edges and white barring near the tips is identical, and presumably typical at this age.

The new inner eight primaries on each wing showed from above a ‘Venetian blind’ effect, with darker grey outer edges contrasting with silvery white inners, darkening towards the outer edge where the dark outer edge hooked back to form darker tips. From below the effect was to produce a virtually white ‘hand’ with a dark or blackish trailing edge. Compare this with the (admittedly poor) images of the Blyth bird and the same pattern is visible.

An examination of the moult is interesting, because we do not really know the process by which Caspian attains its second-winter plumage. The KL gull showed one retained tail feather from juvenile (the left outer) and had started to grow the majority of the central rectrices. The pattern of the remaining juvenile rectrix was a wholly white base with a black terminal band; the new feathers showed black with white marbling along the edge. The wing showed: eight fully grown new inner primaries; P9 growing, but P10 retained from juvenile; one new outer secondary and about six inners retained from juvenile – the rest were missing. The wing moult of the Blyth Caspian was identical when first found, but two days later (when the flight shots were taken) it had shed the juvenile outers; the tail was slightly more advanced in having lost the old and the new inners extended further beyond the upper-tail coverts – as it flew feathers fell off it.

Finally, the bill, another feature used to support the hybrid theory. I personally have not seen a bill on Caspian looking like this – they are variable in shape, but from certain angles there is no denying the fact that The KL gull had a big one and a prominent gonydeal angle. Mind you, sometimes (in profile) it didn’t look that odd (cf. close up of head). I suspect, and Rik Winters has supported my idea, that Caspian Gull can have a bill like this, that it not outside the range of large male.

I have purposely not discussed Heuglin’s as a possibility, although there may be some features that look a bit like those on heuglini, as Visa Rauste has indicated it is rare for a first-summer not to show adult-like mantle/scapular feathers; the pattern of the immature rear scapulars and coverts is not that of Heuglin’s; and that the moult is too advanced.

The King’s Lynn gull is obviously not Heuglin’s and in my opinion not a hybrid. It is a typical Caspian Gull, albeit a large one, in late summer. Now I have expressed my opinion let’s hear yours.

Please send us your comments and opinions .

e-mail Brian J. Small

1st-summer Caspian Gull L. cachinnans photographed by Brian Small on the Blythburgh Estuary on August 1st, 2000. Apart from the bill, note the similarity in plumage to the Norfolk bird above.

This flight shot was taken on August 3rd. All other Caspian shots taken on August 1st