Mystery Gull in Katwijk aan Zee, The Netherlands

Mystery Gull in Katwijk aan Zee, The Netherlands

I have uploaded 2 pictures of a first-summer plumaged gull that was present at Katwijk aan Zee (ZH), The Netherlands on 21 August, 1999. It was quite a distinctive bird that stood out from other gulls present by its buff tones to the plumage and bold markings on scapulars and greater coverts.


Picture 1 (Jaap Dijkhuizen)	Picture 2 (Jaap Dijkhuizen)

I have no clear idea what species it is.
Just to make things perhaps a little more clear: at this site, of the larger gull species, Lesser Black-backed Gull Larus (fuscus) intermedius and Herring Gull L. argentatus argenteus are very common, followed (numberwise) by Greater Black-backed L. marinus Yellow-legged L. michahellis and Caspian L. cachinnans. The latter two are quite rare, actually. Please have a look at the pictures, which show much of the main features, that - what I thought - were rather different when compared to (the similarly sized) intermedius and argentatus: a marked buff tone to (the older feathers of) the upperparts, with large dark (almost black) markings. Head, breast and belly were generally white, with some streaking/spotting at the breast sides. The vent was mainly white with quite a sparse dark brown banding. The tail was white, with a dark terminal band and narrow white edge. Please note that only the ultimate primary is old, while P9 and P8 are not yet full-grown.
I would be interested to hear any suggestion on the (sub-)specific identity of this bird!

Comments are welcome at teus.www@tiscali.nl

The tips and suggestions I received thus far have been summarised in a message posted at EuroBirdNet (19 November 1999). It read as follows:

I received eleven answers to my question as to what (sub-)species birders thought this bird could belong.

Generally, those who responded thought that the choice was between Herring L. argentatus and Yellow-legged Gull L. michahellis. Two thought Lesser Black-backed L. fuscus graelsii was another possibility and even Greater Black-backed was suggested, but then more as a candidate for one of the parents of this bird, thus being a hybrid. The scapular pattern could indeed suggest some Greater Black-backed influence.

But as I stated, most went for argentatus or michahellis. Some were just sure that it was the latter species (see the pictures published in Limicola: Gruber 1995, Klein & Gruber 1997). Most others, however, were cautious and warned for the obvious difficulty that exists when trying to identify birds of this age. Apparently, both species are notoriously variable in this (first-summer/second-winter) plumage, although the knowledge of argentatus appears to be somewhat more extensive under gull watchers than that of (1S/2W) michahellis.

The bird's jizz, although it can be a useful character, was considered right for michahellis by some, but just as right for argentatus by others. Plumage details were hardly mentioned in the discussions, most probably because of the variability in both species.

People from areas where michahellis is common were quite positive that it was this species, although it was acknowledged that a michahellis should show more grey in the upperparts by this time.

Slightly as a surprise to me, Lesser Black-backed Gull L. fuscus graelsii was also mentioned as a candidate (by 2), but others pointed out that this species would show darker and less patterned upperparts. However, a picture that has been published in British Birds (vol. 90, 1997, no. 9, p. 374, pl. 110) depicts a graelsii of similar age that shows a striking similarity to the mystery bird. Although the underparts and the greater coverts are quite darker than those of the mystery bird, the pattern and ground colour of the scapulars and especially tertials are just right. It was stressed by one observer that these rather pale greater coverts generally fit argentatus better than any other of the candidates.

Another birder described ths plumage to be not uncommon under argenteus Herring Gulls. This is sometimes retained throughout the whole of the second winter. They often also produce very colorful bare parts (e.g., brighter pink bill base). Too bad the bird wouldn't stay around to check this!

In conclusion, I cannot but say that the puzzle just hasn't been solved.

Thanks everyone who responded!! And keep on gulling!

Teus

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