Identification of Caspian Gull

27 Jan 2003 | Paul Bright-Thomas

Introduction

This article describes the identification of Caspian Gull, a bird that occurs regularly at inland rubbish tips and gull roosts, but is perhaps under-recorded due to limited awareness of identification criteria. The distinguishing features separating Caspian Gull from commoner gulls are somewhat subtle, and there are pitfalls due to the variability of gulls in size, plumage and bare part colouration, but when good views are obtained it is a distinctive gull and is not exceptionally difficult to identify. Caspian Gull is, in my opinion, one of the most handsome of gull species and well worth a look, and hopefully this article will help more people to see them in Berkshire. I have seen a few adult Caspian Gulls in the UK, and found them distinctive and yet challenging at the same time. The information in this article is gleaned partly from personal observation of birds in the UK, partly from reading several articles in the popular birding journals and partly from studying pictures of Caspian Gull taken in Europe in winter and published on the Internet.

Caspian Gull is largely untreated in most field guides and even major works, such as BWP*, lack details of identification criteria. There have been several good articles in journals and magazines and the forthcoming Helm book Gulls of Europe, Asia and North America* by Olsen and Larsson will hopefully put the identification criteria, plus some decent plates and photographs, on many people's bookshelves. There are also a large number of good photographs on the web, and I have provided links to web sites featuring a number of pictures of Caspian Gulls at the end of this article; links to specific pictures are placed in the text below in order to illustrate identification features described in the text.

As a postscript, I include a few gull-watching tips that I have picked up over the last few years.

What is it?

Caspian Gull is part of the taxonomical open question that covers the large white-headed, grey-backed gulls of the Northern hemisphere. By “Caspian Gull” this article refers to gulls of the form cachinnans that is variously treated as a subspecies of Herring Gull (Larus argentatus cachinnans) or Yellow-legged Gull (Larus michahellis/cachinnans cachinnans) or a species in its own right. The Caspian Gull (sub-)species is sometimes referred to as "Pontic Gull", particularly by Dutch sources, and some authorities recognise two subspecies, ponticus in the west and cachinnans in the east. “Caspian Gull” and “cachinnans” are used synonymously in the following.

Most gull-watchers and taxonomists now agree that Caspian Gull is a species (Larus cachinnans) distinct from Herring (L. argentatus) and Yellow-legged Gull (L. michahellis) - it differs in range, plumage, structure and vocalisations - and that hopefully the review by the BOU taxonomic committee that is currently underway will soon clarify the position. Caspian Gull breeds around the Black and Caspian Seas, east to Kazakhstan and west though the Ukraine as far as Poland (in small numbers). In winter, immatures disperse to the south and south-east, but are regularly found in Poland and north-east Germany. It is becoming increasingly clear that small numbers winter further to the north-west, with birds of all ages regular in the UK, Netherlands and northern France.

I believe the BBRC and BOURC are still trying to establish the first record for Britain ¹, although several birds have been trapped and colour-ringed birds from Eastern Europe have been observed, but it is now clear that most winters bring UK records well into double figures and many are long-stayers and well-photographed. Caspian Gull occurs predominantly in the South-East and the Midlands, and frequently penetrates inland to tips and reservoirs. There have been several records in Berkshire since the late 1990s. Favoured UK sites where birds have been seen in several years include the Blyth estuary in Suffolk, Milton and Godmanchester tips in Cambridgeshire, Dungeness in Kent, and various sites along the Thames.

Adult

General impression
Caspian Gull is a large gull but slender in its proportions. Compared to a Herring Gull (typical bird of the British race argenteus), it is of similar size or slightly larger (some males very large) but appears longer winged, longer necked, longer legged and longer billed. Its adult plumage is composed of clean white, rich mid grey and long black primaries, which makes for a smart and elegant ~~appearance~~, and generally close to michahellis. Adult Caspian Gull is sometimes reminiscent a huge Common Gull, due to the soft grey of the mantle, long black primaries, dark eye and gentle expression, although the dull yellow bill is more substantial and has the red gonys spot typical of large white-headed gulls.

Cachinnans has a characteristic ~~posture~~, standing tall on long legs with the wing-tips often dropped below the horizontal, and the breast rather high, with long neck sometimes erect (although of course other gulls may adopt a similar stance). Caspian Gull often seems to have a trim belly and a full high breast bulging just below the throat. On a standing bird, there is a smooth line along the back and into the long narrow blade-like primaries, and the profile of the back is unbroken by a “step” between the tertials and primaries, in contrast to Herring Gull. The upright posture and smooth line of the back result in a “ski-slope” profile apparent in a number of photographs, although in cold and/or windy weather, gulls adopt a more ~~hunched~~ and horizontal posture.

On the water, the long neck and the long narrow wing tips that are often held elevated and well clear of the water, give a profile that is very “high” at both ends. In flight, cachinnans is long-winged and buoyant, cruising lightly on bowed wings and the long legs often dangle below the bird on take-off or landing.

In winter adults the head lacks the coarse dark streaking on face crown and neck of both argenteus/argentatus Herring Gull races (which become whiter from January onwards). They therefore stand out in a winter gull flock in the same way as adult michahellis, due to their clean bright plumage, particularly on the head, and the extensive black primaries with small white tips on the folded wing. Like michahellis, the mantle is darker grey than argenteus (British) Herring Gull, although cachinnans is paler than michahellis, and mantle colour may be approached by dark argentatus (Scandinavian) Herring Gull. From lightest to darkest the ordering is argenteus : argentatus : cachinnans : michahellis : graellsii, although some argentatus may be as dark as, or darker than, cachinnans. A comparison of mantle shades between michahellis, cachinnans and argenteus can be found here. In direct comparison I have found Caspian Gull a fractional shade darker than Common Gull.

Head
Although subtle differences in proportions of the head are rather difficult to portray in words, it is clear from photographs that Caspian Gull has a distinct “look” about its face and head. The classic Caspian Gull has a small “pear-shaped” head on a long slim neck, with a long sloping forehead, flat crown and angular nape. The small head, small eye, slender bill and long gape line give it a gentle smiling “~~facial expression~~”, without the sharp and aggressive look of a Herring Gull. The crown may appear rounded, but with a long sloping forehead, and not domed like michahellis, which tends to show a steep forehead and an even curvature, peaking over the eye, and lacking the angular nape of cachinnans. Caspian Gull also lacks the square brow and broad bulky head of Herring Gull.

Caspian Gulls in winter are usually very white-headed but often show fine grey streaking on the nape forming a well-defined but faint “shawl” (see first-year birds below). They usually lack obvious dark streaking on the crown and around the face (although a few pencil thin streaks on the crown may be apparent at close range), and seem to lack the “brow” that helps give Herring Gulls such a stern expression. An otherwise classic bird with some smudging around the eye, perhaps a sign of immaturity together with the dark bill band, is shown ~~here~~.

The feathering extending forward to a point on the upper mandible is often very prominent on Caspian Gull, and the distance between eye and feather point is large. This is somewhat balanced by the usually long slim bill of Caspian Gull, particularly the straight basal section before the gonydeal angle on the lower mandible; the base of the bill often seems to be stretched, but this may be less apparent on small-billed, possibly female, birds. The combined effect gives a “long-faced” look, which to some people is reminiscent of Great Black-headed Gull (L. icthyaetus) or even Slender-billed gull (L..genei), although this latter comparison has struck me on only a couple of the slighter-billed first-winter birds.

Bare parts
Caspian Gull is more likely to show a dark eye as an adult than other large gull species in the UK, although pale-eyed birds do occur, as well as intermediates with varying shades of brown through to gold. On close views a dark eye is often found to be dark brown through to dull amber, i.e. the pupil is visible in contrast to the iris. The usually pale eyes of Herring and Yellow-legged Gull are noticeable at surprisingly large distances in good light and contribute to a facial expression very different from that resulting from the small, beady dark eye of Caspian Gull. Beware apparently adult Herring and Yellow-legged Gulls that have retained dark immature eye colouration, which is not uncommon in fourth or subsequent winters. The orbital ring of bare skin around the eye is red or reddish-orange in cachinnans, as for michahellis, rather than yellow to orange in Herring Gull. Caspian Gulls have a long gape line extending horizontally to almost below the front edge of the eye, and the edge of the gape may be visibly reddish.

Caspian Gull in winter typically has a dull pale yellow bill sometimes with a slight greenish tint, not unlike winter Common Gull. The gonydeal spot on the lower mandible is usually a dull pale red, and often has a black bar at the far edge of the spot, extending and narrowing onto the upper mandible but not reaching the culmen. Black in the bill spot may be a sign of immaturity, although is often the only apparent immature character in an otherwise adult-looking bird. The dullness of bill and bill spot colouration may be shared by Herring Gulls, particularly of the race argentatus, although the proportions of the Herring Gull bill should be distinctively different to all but the most huge-billed male cachinnans.

As already mentioned, the bill of Caspian Gull is long, with a slim straight basal section; on some birds it is almost ~~ludicrously long~~. In some cases the bill appears to have only a slight gonydeal angle, more reminiscent of the smaller gulls, whereas in others the gonydeal angle is more distinct. The bill tip is distinctly slighter and more gently curved than the hooked, almost square-ended tip of many Herring Gulls, particularly argentatus, and can contribute to the bill looking slightly droop-ended. The bill of michahellis gulls is typically rather thick in the final 1/3 with a pronounced gonydeal angle, and is distinctly brighter in colour, often attaining a marigold-yellow hue with a bright poppy-red gonydeal spot, even in mid-winter. Bill sizes and shapes are variable amongst the large gulls, and as well as ~~small-billed~~ (probably female) Caspian Gulls and ~~large-billed~~ birds, other gull species may show bills of atypical proportions that suggest Caspian Gull. Finally, the nostril of a Caspian Gull is supposedly narrower and more parallel-sided than the triangular nostril of a michahellis gull (broadest towards bill tip) …this is unlikely to be of much use in the field.

The legs of Caspian Gull appear particularly long, especially the tibia, between belly and joint. They usually appear pale, slender and often of an indeterminate hue. Despite the frequent classification within Yellow-legged Gull, the legs of an adult Caspian Gull are often not yellow, in winter especially, and rarely attain the bright yellow of michahellis. The legs vary in colour from pale butter ~~yellow~~, through flesh tones to greyish-greenish, and may rarely be quite ~~pink~~.

Wings
An important feature to support the identification of an adult Caspian Gull is the wing pattern, which requires analysis on an individual-feather basis. Caspian Gulls have extensive black in the outer primaries, almost as much as michahellis, but the pattern is distinctively different. Numbering the primaries ascendantly, with P1 innermost and P10 the longest primary, then Caspian Gull has black on P10-P5, the latter with a complete black band across the feather tip, and sometimes a black comma on P4. The black is more extensive on the outer (leading) web, almost reaching the primary coverts on P10 and P9, and covering about half the exposed length of P8 and P7. The inner web has a reduced amount of black with the basal section fading from grey to whitish grey (most white on P10 and P9), with a sharp division from the black. This gives an overall impression of interlocking fingers of black and white(ish) on the leading 4 primaries, rather than, as on michahellis, an extensive black tip whose (more or less) sharp boundary from the grey curves diagonally across the wing-tip. Caspian Gull shows more black on P6-P4 than argentatus/argenteus, which shows at most a small black mark or narrow band on P5, and no black on P4.

An adult Caspian Gull has large white mirrors on P10 and P9, although the P10 mirror usually merges with the white feather tip to give an extensive white tip several centimetres in length; if not merged, the black between the white of tip and mirror is a very narrow bar. The P9 mirror is better separated from the white tip but is sometimes split by a narrow black bar along the primary shaft. On the underwing there is little contrast between the secondaries and the white underwing coverts, unlike in michahellis, where the tips of the secondaries and primaries form a grey band along the trailing edge of the underwing.

In the folded wing P9 and P10 may be of almost exactly the same length, and so the upper surface of P10 is effectively hidden. All the primaries are tipped with white, so that on the folded wing four (five if P10 revealed) small semicircular white tips are visible against the black primaries, and three (four if P10 visible) of these primary tips are usually beyond the end of the tail. The tips of the primaries are very narrow and pointed, and the profile of the primary projection forms a more acute angle than the primaries of other gulls.

The tertials of adult Caspian Gull are narrow and do not form a step between back and primaries. They often seem to fail to cover the lower edge of the primaries and allow the black of the outer primaries to be visible below the tertials. The white tips are broad and crescent-shaped and melt slightly into the grey of the feather bases. The white scapular crescent further up the back is often very small and may not be visible.

It is often on the folded or semi-folded wing that the best opportunity comes to examine the primary pattern, particularly the underside of P10, which is an important feature in separation from michahellis, although a very similar pattern can be shown by argentatus. The P10 pattern is shown in Figure 1. The base of the P10 primary features a white “tongue” that projects down the inner web covering at least 50% of the exposed feather, and up to 75%. This is separated from a large white mirror by a sharply defined black bar across the inner web of the feather, with the black extending back up the feather on the outer web. The white mirror may be separated from a small white tip by a narrow black line, but equally the entire feather tip may be white. The key part of the pattern is to establish the width of the black bar along the upper (inner) edge of the feather, which is often less than the distance from the farthest edge of the black to the feather tip, and that the inner web is largely white on the basal part leading up to the black bar, and extends more than halfway along the exposed feather. The primary P10 pattern is well shown ~~here~~ and ~~here~~. When viewed in profile, the underside of P10 may be viewed clearly (broader inner web is uppermost) whether above or below the tail. Views of more than just the feather tip are required to establish this pattern, and may require an inactive bird to be studied for a considerable period before it opens its wings sufficiently, although from the right angle the narrowness of the black on the inner web can usually be determined on the closed wing.

Figure 1. Underside of adult P10: top two cachinnans; middle two michahellis, bottom argentatus resembling cachinnans.

The pattern of P10 - white tongue, narrow black band and white tip –is an important distinction from michahellis, which may otherwise resemble cachinnans quite closely. A michahellis gull has less white on the basal part of P10, with the black band being broader than cachinnans, as shown in Figure 1. However, the cachinnans pattern on P10 can be replicated by an ~~argentatus~~ Herring Gull when only the distal portion of the feather is visible on the closed wing, although usually the black fades out towards the base of the feather rather than being sharply defined. A Herring Gull that resembles Caspian Gull in other ways –dark mantle, dull yellow bill –may easily be mistaken for Caspian Gull if P10 is viewed in isolation. It is therefore important to note the extent of black on the other primaries and to examine the full range of identification features for Caspian Gull.

Additional Features
Caspian Gull is reported to have a distinctive posture when calling, which birds may do when squabbling on rubbish tips. Herring and Yellow-legged Gulls keep their wings closed when giving the “long” call, with heads either raised or lowered. Caspian Gulls have a strong tendency to partially open their wings when calling.

First Year

General impression
Helpfully, first-winter Caspian Gull is also quite a distinctive bird compared to Herring and Yellow-legged Gull. In common with michahellis, cachinnans seems to advance towards adult plumage rather more quickly than argenteus and argentatus, possibly as a result of an earlier moult cycle, but also through whiter head and underparts and a greater predominance of adult grey in the earlier plumages. Caspian Gull in first winter plumage is a rather neat gull (compared to other LWHG species) with a zoned plumage which is usually mostly white on the head neck and breast, largely grey on the back (although mantle feathers and scapulars have dark shafts or sub-terminal crescents and browner juvenile feathers may be retained), brown and white on the wing coverts, and dark black-brown on the primaries. The structural characteristics of cachinnans are also apparent in first-winters, so the small head, long white neck, long legs, long wings and long slim bill are all likely to attract attention compared to other immature gulls. Like other large gulls, first winter Caspian Gulls are variable in plumage, and structural features are extremely useful in the identification of atypically plumaged birds.

Head
In common with adult Caspian Gull, first-winter birds usually show limited head-streaking, being very white on the forehead, crown, cheeks, throat and upper breast. The lower breast belly and flanks are more smudged with greyish brown markings but are still predominantly white. There may be a light dusky patch made up of diffuse streaking around the eye, and the nape frequently shows narrow longitudinal streaking that can be very sharply defined in extent, forming an obvious “shawl” on the nape and neck sides. Younger birds in late summer may have more diffuse brown on the head, but in general the whiteness of the head is an obvious feature that may allow a first-year Caspian Gull to be picked out at medium-long range. A largely white head is shared by michahellis at this age, but michahellis usually has heavier streaking around the eye forming a large smudgy eye-patch.

Bare parts
The bill is usually all-black on first-winter birds, although the base may start to fade to dull greenish-black in late winter; the bill may rarely show some pink in the base, but this is uncommon. The very tip of the bill may be yellowish on the culmen. The bill is often very slender on first-year birds and may show little gonydeal angle, and its dark colour possibly enhances the slender impression. The long gape line and long-billed can allow the bird to look very ~~big-mouthed~~ when the bill is open.

The eye is always dark, but this is also true of other large gulls at this age. However the small eye and its positioning on a white head still gives Caspian Gull its characteristic gentle expression. Long-legs are again characteristic and can result in a very skinny, almost shank-like impression. The legs are usually dull pinkish-grey, but can be quite pinkish at this age.

Mantle
Caspian Gull in first-year plumage shows a lot of grey in the feathers of the mantle and scapulars, usually with narrow dark-shaft streaks and a brown sub-terminal crescent, forming an anchor mark. The markings on the mantle can be quite broad and ~~bold~~ but they are often rather fine and delicate, allowing the scapulars in particular to appear largely grey. First-winter michahellis shows strong double anchor markings on mantle and scapulars and therefore usually look browner and more boldly marked on the mantle than cachinnans.

Wings
The wing coverts in first-winter Caspian Gull are often a rather uniform earth-brown, with fine pale fringing and tips, and lacking strong patterning or notching. The outer greater coverts in particularly are often solidly brown with pale outer fringes and slightly wider white tips and this can produce a quite solid block of brown on the closed wing, demarcated by a line of white tips from the usually ~~blackish primaries~~. The median and outer greater coverts have more obvious white outer edging and produce a lined “venetian blind” effect. The tertials are blackish brown with narrow white edging and a somewhat broader white tip, sometimes referred to as a “thumbnail”, often split by a dark shaft. They do not show the notching along the edges that is a feature of Herring Gull. Beyond the tertials on the folded wing, the primaries are black without any white tips or mirrors and form a long narrow projection giving the gull a tapered streamlined appearance and the folded primaries form a very sharp narrow point.

Flight
In flight, the dark primaries and secondaries are separated by a weak “window” made up of pale grey on the inner webs of the inner primaries. This is more obvious than on michahellis, which appears rather dark-winged, but less clear than on argenteus or argentatus Herring Gulls, which have very obvious pale inner primaries. The brown outer greater coverts form a second block of brown in front of the brown secondaries along the trailing edge of the wing, separated by a distinct white line made up of the pale tips to the coverts.

The underwing of first-year birds is usually very pale, appearing white from a distance, although in fact the coverts are intricately marked with pale brown. Even where the coverts may be slightly more heavily marked, the axillaries (armpit) are mostly white. This is a good distinguishing feature from first-winter michahellis, which have dusky underwings heavily marked with brown. As with michahellis, the tail of first-winter birds is white at the base with a strong black terminal band, which may “fray” into finer black barring towards the white tail base. On fresh birds there may in fact be narrow white tips to the tail feathers, and indeed to the trailing edge of the inner wing.

Second and Third year birds

Caspian Gull continues to advance to adult plumage more quickly than Herring Gull in its second and third winters. By the ~~second winter~~ the newer mantle feathers will be largely plain grey, although some first-winter feathers with dark shafts and crescents will remain. Some coverts and perhaps the inner ~~tertials~~ will have been replaced with grey. The belly and breast sides will be whiter, although the face may retain some grey smudging around the eye, and the shawl of grey streaks on the nape is likely to remain prominent. Second-winter birds frequently develop a faint white (or dusky brown) mirror in the longest primary, which is highly unusual in other gulls of this age. The bill base usually begins to pale to greyish-flesh colour, retaining a dark tip. The division base and tip them is usually uneven rather than a sharply defined vertical line. In flight the inner wings appear patchily grey but the primaries are still largely dark, and the tail band is a little narrower and more ragged.

Third winters appear very adult-like, but with a few residual brown marks on coverts and tertials. Head and neck streaking is more prominent than on adult birds, and the dull yellowish bill is likely to have a dark band rather than a red spot. In flight the wings are basically grey with black outer primaries, but the primary coverts are also black and traces of a tail band remain. Traces of black on the primary coverts and smudging of the tertials are retained into the fourth winter, when the bird is otherwise largely adult in appearance.

Pitfalls

As in other gulls, Caspian Gull is quite variable in appearance; sources of this variability include sexual variation, since some female gulls are often significantly smaller and with smaller bills than males, racial variation, since there may be distinct eastern and western forms of Caspian Gull (cachinnans and ponticus) and possibly hybridisation, since the degree of breeding isolation of westernmost populations of Caspian Gull from other taxa is not yet clear, and is the main barrier to recognising Caspian Gull as deserving of full species rank. Not all Caspian Gulls exhibit the classic long-billed, long-winged, gentle-faced “look”.

Gulls of other species can strongly resemble Caspian Gull in a number of ways. Yellow-legged (michahellis) gulls can sometimes appear very slender in proportion and suggest cachinnans when viewed at a distance too great to see the details of primary pattern (in adults) or upper-/under-wing pattern (in first years). Herring Gulls can appear very white-headed from mid-winter onwards and may stand out from neighbours exhibiting more typical head-streaking. Furthermore some Herring Gulls (particularly argentatus) may have a paler yellow bill and a combination of pale colour and slim proportions might put one on the wrong track. The possibility of gulls of other species exhibiting traits that correspond to Caspian Gull can only be dealt with by looking at the whole range of characteristics described above, including proportions and physical structure, specific plumage features and bare part shape and colouration, and requiring a high degree of correspondence across the board, taking into account the variability of all gull species. As in many difficult groups, some birds will have to go unidentified.

Summary

The main characteristics of cachinnans for separation from other taxa are summarised in Table 1, but refer to text for details of criteria and variability.

Table 1: Summary of separation of *cachinnans* from other large gulls.
Age	Adult Winter	1st Winter	2nd/3rd Winter
michahellis	Sloping forehead, flat crown, slim bill, darker eye (some birds), longer and paler (pinker) legs; Paler grey mantle, P10 pattern, white/grey on inner webs of outer primaries.	Less head streaking, fainter smudge around eye, slimmer bill, more grey on mantle, narrower crescent markings, pale tips to greater coverts. Paler underwings, more obvious primary “window”.	General proportions, particularly bill. Whiter head and underparts. Earlier development of mirror on P10.
argentatus	Slender proportions, whiter head, streaking on nape only, darker eye (some birds), slim bill with finer tip, black in gonys spot, red eye-ring and gape, longer and yellower legs; Long narrow primaries, narrower tertials, more black on outer primaries, black on P5, often P4.	Whiter head with limited steaking, mostly on nape. More grey in mantle, tertials with narrow white edges (no notching) and broader white tips, plain brown on bases of greater coverts, blacker primaries, darker bill. White base to tail.	General proportions. More advanced plumage with more adult grey in mantle and coverts and whiter head. Earlier development of mirror on P10.
argenteus	Larger and slimmer, darker mantle, white head, slim bill, dark eye, long legs; More black on primaries.	Whiter head, no notching on tertials, more grey on mantle, plainer coverts. White base to tail.	See argentatus.
graellsii	Larger size (most birds), paler mantle, whiter head, larger, longer bill, longer legs, dark eye. Larger mirror on P10 and P9.	Size, less head-streaking, slimmer, longer bill, pale grey mantle, less barred coverts, broader tertial tips, narrower black tail band.	Size and proportions, longer slimmer bill, whiter head, paler grey mantle, more grey in coverts.

Gull-watching tips

At the risk of stating the obvious, here are a few simple observations that are useful in watching gulls in the UK.

Gulls are difficult ... persevere!
Gulls are surprisingly wary when they know they are being watched, and may flush at long range. They are more confident when on water than on land, although this may hide some features, such as leg colour, but bathing and preening activity can give opportunities to view individual flight feathers well. They are also more static in cold weather, when the cost of disturbance is higher, although this may affect posture and apparent shape.
Gulls are extremely variable in appearance, given the progression of plumages as birds age, plus racial and sexual differences in size and colouration. Comparison of two individuals for size is not necessarily helpful, and an unusual gull should be compared to a number of representatives of a commoner species, where possible.
The exact shade of mantle grey is rarely crucial to an identification but relative shades of grey can be important, and can also distinguish a bird enough to allow it to be found in the first place.
The grey of adult gulls is very dependent on ambient light conditions, and gulls are best observed in dull, flat light conditions, although beware of dusk conditions when birds may appear darker. Gulls are best viewed in profile and a slight breeze will cause resting gulls to orient themselves to face into the wind, both on land and on water, allowing the majority of birds to be viewed at the same angle and reducing the amount of light-dependent variation.
Ageing of gulls can be important to identification and shows familiarity with the commoner gull species. The smaller species reach adult plumage more quickly than the larger species; gulls may be classified as two-year, three-year or four-year gulls depending on how many years after hatching they will reach adult plumage. Some species of apparently similar size also progress more quickly towards adult plumage than others.
The timing of moult can be important, since birds from southern areas moult earlier than birds with more northerly ranges. For example, first year michahellis birds may be three months older than northern Herring Gulls, and will therefore show a more advanced moult timing.
Bare part colouration is often a useful identification feature but obviously requires good views at reasonably close range.
When there are no rare gulls to be found, watch the commoner species, since there is always something to learn.
Gulls are difficult (again) and some birds will have to go unidentified.

Glossary

argentatus: Herring gulls of the large dark-mantled Scandinavian race
argenteus: Herring gulls of the pale British/North-west European race
armenicus: Small dark yellow-legged gull of Armenia, wintering in Israel
barabensis: Dark yellow-legged gull of central Asia, wintering in Middle East
cachinnans: Caspian Gull (or Yellow-legged Gull, together with michahellis)
culmen: Outer edge of bill (upper or lower)
distal: Part (e.g. of bill or feather) farthest from base
eye ring: Ring of feathers around eye
fuscus: Darkest race of Lesser black-backed Gull, a.k.a. Baltic Gull
gape: Inside of mouth, and folds of skin visible when mouth closed
gonydeal angle: Angle 2/3 along lower mandible
gonys: Straight-edged section of lower madible beyond gonydeal angle
gonys spot: Orange-red spot around gonydeal area of lower mandible
graellsii: Pale-mantled UK and NW European race of Lesser Black-backed Gull
intermedius: Larger and darker-mantled N European race of Lesser Black-backed Gull
mantle: Upper central part of the back, between nape and rump
michahellis: Mediterranean race of Yellow-legged Gull
mirror: White spot near (but not at) tip of outer primaries
omissus: Yellow-legged Herring Gulls from the Baltic
orbital ring: Ring of coloured skin around eye
ponticus: Suggested race of Caspian Gull from around Black Sea
Pontic Gull: Synonym for Caspian Gull, commonly used in Dutch references
primaries: Longest and outermost flight feathers
proximal: Part of bill or feather closest to base
scapulars: Two outer rows of back feathers, overlapping wing coverts
secondaries: Flight feathers along inner part of wing
shaft: Stiff tubular centre to feather
tertials: Innermost flight feathers, covering secondaries and primaries at rest
web: Feather vanes on either side of the shaft
window: Pale inner primaries between dark outer primaries and secondaries

Various UK Caspian Gulls

References

Garner M, Quinn D and Glover B (1997). Identification of Yellow-legged Gulls in Britain. British Birds, 90 (Jan), 25-62; 90 (Sep), 369-383.
Garner M (1998). Gulls: Another piece in the jigsaw. Birdwatch, 78 (Aug), 25-32.
Jonsson L (1998). Yellow-legged gulls and yellow-legged Herring Gulls in the Baltic. Alula, 4 (3), 74-100.

Acknowledgements

Thanks to Mike McCarthy and Dick Newell for comments in the preparation of this article, and to Marek Walford for hosting it at www.berksbirds.co.uk.

All linked photographs and articles remain copyright of the original photographers and/or authors.

Postscript

Caspian Gull was added to category A of the British List on 29 May 2003 following the acceptance of a bird seen at Mucking, Essex on 4 September 1995.