Possible Caspian Gull hybrid

A second-winter gull photographed at Stubbers Green, West Midlands, on December 20th & Chasewater, Staffs, on December 21st, 2008. Initially identified as a Caspian Gull, its identity was questioned when photographs were published on the Chasewater Wildlife Group website (see www.chasewater.org.uk/diarymonths/2008/December%202008.htm). Some birders apparently suggested that this bird might be simply a variant Herring Gull but there are clear cachinnans features evident in the photos above, taken by Graham Evans:

In photos 1 & 2, the relatively small head in proportion to body-size is evident, while in photo 2, in particular, the 'snouty' appearance is emphasised, resulting from the long bill and extensive maxillary feathering. In these two photos note also: (a) the rather deep, 'boat-shaped' body, with appearance of 'keel' to rear of legs, (b) that streaking is concentrated at the rear and base of the neck, creating a 'shawl' contrasting with the otherwise much cleaner and whiter head and foreneck; (c) the retained tertials have a solid black base, sharply demarcated from a neat white tip; (d) the legs are quite long with an evident tibia and their colour is a rather washed-out greyish-flesh. In the lower two photos note: (e) the generally rather plain and dark outer greater coverts and (f) the strongly contrasted 'black and white' tail, with a zone of vermiculations separating the tail-band from the clean white base. These are all strong indicators of cachinnans.

Is there, in fact, anything to raise concerns that the bird is not a straight-forward 2cy Caspian Gull? Most reservations have no doubt focussed on the overall configuration of the bird. It does lack that classic 'attenuated fore and aft' shape of a classic cachinnans. This results from three features in particular: (1) The bill is rather heavy-looking with an expanded and blunt-looking tip, with an obvious gonys angle. On a convincing cachinnans the bill is long, slender and rather parallel-sided, with little evident gonys and a rather pointed tip. This contributes significantly to the characteristic attentuated shape of the loral area and bill. (2) The primary extension is a little short - in the second photo note how it is looks less attenuated at the rear than the Lesser Black-backed Gull. (3) The underwing, while pale with extensive white, is arguably a little coarsely marked for a late 2cy cachinnans, where a cleaner and more uniformly white underwing is characteristic, even if not invariable. Many 2cy cachinnans acquire a small white mirror on the outermost primary (p10). This is another variable feature but a useful additional indicator of cachinnans when present (being rarely encountered in 2cy Herring Gull).

There are few if any unequivocal argentatus features evident on this bird and it is quite conceivable that its lack of 'classic' appearance is simply due to 'individual variation'. All large gulls display a considerable (and at times bewildering) range of individual variation and Caspian Gull is no exception. However, when dealing with scarce and extralimital species, there is clearly a need for enhanced care in identification, especially in cases where intergrading/hybridisation with similar taxa is known to occur. Caspian, Yellow-legged and Herring Gulls are now regularly hybridising in parts of eastern Europe and the possibility of such birds reaching the UK is real. At landfill sites in Cambridgeshire, for example, Dick Newell has encountered several Caspian-like gulls with 'mixed' characters: several of these have borne rings and each has proven to be a hybrid or to come from mixed colonies: see www.magikbirds.com/image.asp?title_id=1102 for an example of a Caspian x Herring Gull and further comments. Also see photos of a gull at Dosthill Lake, Warks, with some characters of Caspian Gull but with other features suggestive of Herring Gull influence. Conversely, it would be an over-reaction to conclude that any Caspian-like gull which is not absolutely classic is thereby a hybrid. A balanced approach is needed.

Pure or hybrid Caspian Gull?

December 20th 2008, Stubbers Green, West Midlands