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. The combination of state of moult, extensive plain grey in scapular and coverts, and the pattern of the outer tertials do not equate with Herring Gull but there are indeed features which do not match a classic Caspian Gull.

Conversely, in the photos above taken by Graham Evans, there are several features which are indeed associated with cachinnans:

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 indicators of cachinnans.

So, which features raise concerns that the bird is not a straight-forward 2cy Caspian Gull? Most reservations perhaps focus 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 rather 'hooked' tip, features more associated with Yellow-legged Gull or a large male Herring Gull. On a 'classic' 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 attenuated 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) Some of the new, second-generation tertials are 'double-barred' with little evidence of a solidly dark base. 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).

In individuals such as the above, it can be difficult to discriminate between natural character variation within pure-bred Caspian Gull and hybrid influence. Is it conceivable that its lack of 'classic' appearance is simply due to 'individual variation' and the sometimes surprisingly hefty appearance of large male Caspian Gulls? There is an example of a comparably robust bird on the Surfbirds photo gallery, photographed at Southwold, in Suffolk, in September 2005 (http://www.surfbirds.com/cgi-bin/gallery/display.cgi?gallery=gallery9.htm then enter a 'Search' for 'Caspian Gull' and scroll down). The Suffolk bird was apparently ringed as a Caspian Gull in the Ukraine. 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 colour-ringed gulls of hybrid origin have been observed in the UK. At landfill sites in Cambridgeshire, for example, Dick Newell has encountered several Caspian-like gulls with 'mixed' characters. For a colour-ringed example proven to be a hybrid Caspian x Herring Gull see www.magikbirds.com/image.asp?title_id=1102.

The problem of natural character variation versus hybrid influence was explored in detail by Gibbins, Neubauer and Small (British Birds, 2011, 104: 702-742). They generated a numerical index to determine which individuals could be assigned with confidence to Caspian Gull. However, this numerical index (currently) includes only first-winter and adult individuals and deals only with Caspian Gull and Herring Gull (it does not include Yellow-legged Gull). Although providing some useful guidance in a field-context, accurate evaluation of the index requires that all plumage tracts and bare-part structure & colour are discerned unequivocally; this generally means that clear photographs are a pre-requisite. From the regions of overlap in the individual trait scores, it was possible to define index values which would embrace pure Caspian Gulls while excluding hybrids. However, this also means that a percentage of 'true' Caspian Gulls will be excluded. For example, the data indicate that so-called 'classic' individuals in all respects may not comprise much more than 60% - 70% of the population. In first-winter individuals, for example, 32% displayed coarse makings (rather than fine anchors) to the second-generation scapulars while 16% had quite evident streaking to the head and body rather than the archetypal clean, white appearance.

Nevertheless, with proven hybrids reaching the UK, there is a strong case for diagnosing as Caspian Gull only those individuals with a full suite of 'classic' features.

Pure or hybrid Caspian Gull?

December 20th 2008, Stubbers Green, West Midlands