In the June 2003 bulletin we discussed the problems of identifying and recording Yellow-legged Gulls in Northumberland. We also announced that we would present a guide to the features we find most useful when attempting to assess claims. Publication of the first part of these guidelines here coincides with the reinstatement of Yellow-legged Gull of the subspecies michahellis (hereafter ‘YLG’) as a full description species.

The guidelines will focus on two main age categories, adults and first-years. We cannot pretend to fully understand the complexity of the issues surrounding the identification of other ages and in relatively few cases will we be able to offer a fully informed judgement on such claims, at least for the time being. In any case, adults and first-years are likely to represent the most frequently occurring age classes. It is not our intention to produce a comprehensive identification paper here, that would run to very many pages (if not volumes!), but rather to draw attention to some key features that we are looking for in claims and to pinpoint some potential pitfalls

These guidelines will be published over two issues of the bulletin, in a future issue we will discuss first-year birds but here we will concentrate on some general points and on the features we feel to be most useful when dealing with adults.

General Points.

The identification of scarcer large gulls represents something of a Catch 22 for observers, on the one hand relevant experience is very useful indeed but on the other it is hard to come by such experience due to the actual scarcity of YLG in Northumberland. Even observers who are familiar with YLG in other areas are not necessarily used to seeing them at comparable times of year, in comparable plumage states, or alongside locally common species. However, most would-be rare gull finders have ample opportunities to familiarise themselves with the taxa that are common in their local area; in Northumberland this means Herring Gulls (hereafter HG) of the races argenteus and argentatus, Lesser Black-backed Gulls (hereafter LBBG) of the races graellsii and intermedius (and perhaps intergrades between these two) and Great Black-backed Gulls. It probably goes without saying that when assessing claims we require detailed comparisons to be made with familiar species, preferably through side-by-side comparison.

A full understanding of gull topography is essential when attempting to identify and describe large gulls, the best available reference for this remains the paper on Yellow-legged Gulls in Britain by Garner and Quinn (British Birds 90: 25-62) which provides useful sketches showing where key feathers lie and how these can best be viewed in the field. We assume that observers rising to the challenge of identifying YLG are familiar with this important paper. Similarly, an understanding of moult patterns is desirable, as is an awareness that the features that hold good at one season are not always useful at other times.

The value of photographs, video or digital images in the assessment process cannot be overstated. In fact, some key plumage features that may be almost impossible to see in certain field conditions can often be readily seen in such images, for example, details of the open wing pattern.

Perched birds tend to show certain features much more clearly than swimming birds. Leg colour is usually concealed in swimming birds, as are important wing-covert details on first-years. For these reasons observers keen to make a contribution to our understanding of the status of scarcer large gull forms in Northumberland might benefit from concentrating their efforts on feeding areas rather than roost sites.

‘Jizz’
Details of the size and structure of a large gull are often one of the first things to attract attention to something unusual but can be very difficult to convey in words. Photographs, or other images, can overcome such difficulties though we appreciate that such ‘captures’ will not always be possible. Some indication of the difficulties of describing jizz differences in words is provided by the fact that highly experienced birders and competent authors have variously described the head shape of YLG as both ‘domed’ and ‘blocky’. Surprisingly, both of these seemingly contradictory descriptions are accurate in their own way, as are general descriptions of YLG as both ‘robust’ and ‘elegant’! We urge observers to try to be as objective as possible when assessing jizz and committing this to paper.

When describing jizz details, comparisons with a range of birds actually alongside a YLG candidate are more useful than more general comparisons between species. It is not difficult to find individuals of common species that show some, or even all, of the published jizz characteristics of a rarer form. Structural differences between individuals of the same species are sometimes greater than those between different species and the CRC, although acknowledging the value of jizz characters do not consider them a substitute for precisely described details of plumage; actually, both are required.

ADULTS
In addition to ‘jizz’ details, we would like to stress the following ‘concrete’ features as particularly useful when assessing claims of adult YLG’s.

Mantle Colour:
The literature describes the upperparts of YLG as intermediate between those of HG and LBBG. To a certain extent, this is true but consideration must be given to the racial identity of the comparison species. Northumberland is a major wintering area for HG of the migratory subspecies argentatus originating from Arctic Norway and Russia. Many of these birds are significantly darker above than British breeders (subspecies argenteus) and can easily match the upperpart shade of YLG. Although most argentatus do not arrive here until October or later, it is certainly possible to find a few individuals in August. Not all argentatus show the prominent white primary tips often associated with this form, especially in late summer, when they are also still white-headed. The superficial resemblance of some early autumn argentatus to YLG can be surprisingly strong.

Because of such factors, the actual colour (rather than just the shade) of the upperparts can be very important. Unlike similarly dark-backed argentatus HG, YLG’s have a very neutral ash-grey upperpart colour and lack any of the bluish colour often visible in HG.

In addition, because LBBG of the dark backed subspecies intermedius are quite frequent visitors to our area it is quite conceivable that even a fairly normal LBBG of the form graellsii could appear intermediate in upperpart tone between a darker intermedius and a nearby HG. Experienced observers are unlikely to fall into such traps very often (and the brain is surprisingly good at calculating grey tones, even on lone birds) but caution is obviously necessary.

By far the best lighting conditions for assessing mantle tone are bright but overcast conditions and the worst conditions are bright sunlight, it is also important to ensure that comparisons are being made with birds at similar angles as even two British HG facing in different directions can look surprisingly different in upperpart tone. If present, Common Gull is the ideal comparison species; a genuine YLG should be very close in upperpart tone to this relatively consistently coloured species.

Primary Pattern: YLG shows a substantial area of black on the open primaries, especially compared to a HG of the pitfall form argentatus. This difference is well described and illustrated in the popular literature (i.e. the Collins Bird Guide) and we would expect it to be seen and described in any claims.

Because of the as yet unquantified problem of HG X LBBG hybrids we feel that more detail than a relatively subjective assessment of the amount of black in the primaries is currently required. Details of the precise patterns on both P5 and P10 (numbered descendently, P10 being the outermost feather) are of key importance.

Neither feather is necessarily easy to see in field conditions but they certainly can be seen, at least with patience and practice. Having established that a bird under scrutiny looks like a good YLG candidate in other ways it is well worth spending the time to try and record these features, they may well make the difference between a bird being regarded as ‘not proven’ or a YLG ‘beyond reasonable doubt’.

P10: Although genuine YLG can (rarely) show a combined white tip and mirror on p10 we feel that when dealing with out of range birds a black subterminal divide between these features should ideally be present. P10 is usually concealed on the upperside of the folded wing because it is similar in length to P9 (thus the wing-point is normally formed by P9 and P10). It is, however, normally quite easy to determine whether or not the mirror on P10 is divided from the white tip in flight views or through reference to the pattern on the underside of the far wing of a settled bird.

P5: Almost all genuine YLG show a complete, broad black subterminal bar on P5 whereas quite a number of birds that can masquerade as YLG’s do not. The pattern, or lack of it, on P5 is often hard to judge accurately in flight views and this feather is largely concealed below the tertials on the folded wing. However, with practice, patience and a full understanding of wing topography, it is possible to see this feather on both folded and open wings. The pattern on P5 is often revealed when a bird is preening or stretching or when lower tertials are missing through moult and it is easily seen in good photographs of the open wing.

Even in combination, the patterns on these feathers cannot be used as conclusive proof that an individual under scrutiny is a YLG but anomalous patterns on these feathers may strongly suggest that a bird is not one. If the patterns on P10 and P5 are consistent with the typical appearance of YLG and everything else fits too we feel that a bird can be identified as a YLG beyond reasonable doubt.

YLG-like birds that show anomalous patterns on these feathers may lead to a better understanding of the number of hybrids at large. We encourage observers to submit details of such birds.

Moult stage
It is a popular misconception that moulting birds are more difficult to identify than birds in full plumage. In fact, moulting birds may actually offer more clues than non-moulting individuals. Also, given the known occurrence patterns of YLG in southern England it is to be expected that most adult birds that do occur will be in active moult. Indeed, one of the main reasons why YLG’s from Mediterranean breeding areas visit northern Europe in late summer is to take advantage of the excellent feeding conditions that power this energy draining annual moult.

Adult large gulls commence a complete moult very soon after breeding. Because different taxa breed at different times (largely influenced by breeding latitude) the timing of the moult therefore differs between species. Being an early breeder, YLG generally shows a more advanced moult stage than more northerly breeding forms at comparable dates, it may actually complete its primary moult as much as ten weeks or more earlier than some Arctic argentatus! With some practice, it is relatively easy to determine how far moult has progressed in the wing. Assessing the number of old primaries remaining on the folded wing is usually straightforward, especially once moult has progressed beyond P5 and can therefore be read on the folded wing. The more precise the moult details provided the better, though we are happy to receive more impressionistic descriptions of moult progress, i.e. ‘just beginning’, ‘half complete’, ‘nearly finished’ etc.

It is worth remembering too that when outer primaries are missing, the pattern of the underlying feathers is temporarily revealed on the folded wing, notes on the precise patterns visible on any primaries thus exposed could be of value in assessment.

Conclusion
In brief, an acceptable record of an adult YLG in the county should contain as much detail as possible (size, structure, bare part colour, extent of head streaking etc.) and in particular an accurate description of:

• Mantle colour
• Primary pattern, particularly p5 and p10

Moult details (when applicable) may represent important supplementary evidence.

County Records Committee (September 2004)
Last updated 17 October 2004