Field Identification of West Palearctic Gulls P

British Birds VOLUME 71 NUMBER 4 APRIL 1978 Field identification of west Palearctic gulls P. J. Grant The west Palearctic list includes 23 species of gulls: more than half the world total. Field guides—because of their concise format—provide inad equate coverage of identification and ageing, which has probably fostered the indifference felt by many bird watchers towards gulls. This five- part series aims to change that attitude nterest in identifying gulls is growing, as part of the recent general I improvement in identification standards, but doubtless also stimulated by the addition to the British and Irish list of no less than three Nearctic species in little over a decade (Laughing Gull Larus atricilla in 1966, Franklin's Gull L. pipixcan in 1970 and Ring-billed Gull L. delaivarensis in 1973). The realisation is slowly dawning that regular checking through flocks of gulls can be worthwhile. Just as important as identification is the ability to recognise the age of individual immatures. This is obviously necessary in studies of popula tion, distribution and migration, but is also a challenge in its own right to the serious bird-identifier. Indeed, identification and ageing go hand- in-hand, for it is only by practising his recognition skills on the common species—of all ages—that an observer will acquire the degree of familiarity necessary for the confident identification of the occasional rarity. The enormous debt owed to D.J. Dwight's The Gulls of the World (1925) is readily acknowledged. That work, however, has long been out of print and its format was designed for the museum and taxonomic worker; the present series of papers will provide a reference more suited to field observers. Each of the five parts in this series will bring together species which share similar characters, especially in their immature plumages where the possibility of confusion is greatest. [Brit. Birds 71: 145-176, April 1978] H5 Fig. i. Sequence of plumages from juvenile to adult of typical small (A), medium (B) and large (C) gulls (Laridae) Field identification of gulls 147 PART ONE Small to medium size, with extensive Black-headed Gull Lams ridibundus white on leading edge of outer wing at Slender-billed Gull L. genei all ages, least developed in Grey headed. All except Slender-billed have Bonaparte's Gull L. Philadelphia dark hood in adult summer plumage, Grey-headed Gull L. cirrocephalus reduced to a dark ear-spot in winter. Medium size, with first-year wing PART TWO pattern of dark outer primaries and Common Gull L. canus secondary bar, and tail pattern with Mediterranean Gull L. melanocephalus more or less defined dark terminal Laughing Gull L. atricilla band. Adult Mediterranean, Laugh Franklin's Gull L. pipixcan ing and Franklin's have black hoods Ring-billed Gull L. delawarensis in summer when other two are white- headed. PART THREE Large size. All have four-year period of Lesser Black-backed Gull L. fuscus immaturity, producing wide variety of plumages, which are particularly Great Black-backed Gull L. marinus similar in first-year, when separation Herring Gull L. argentatus of Lesser Black-backed, Great Black- Audouin's Gull L. audouinii backed and Herring provides very Great Black-headed Gull L. ichthyaetus difficult identification test. PART FOUR Small to medium size, all except Ivory Gull Pagophila eburnea Ivory having, in first-year, striking W pattern across wings in flight, Sabine's Gull L. sabini formed by dark outer primaries and Kittiwake Rissa tridactyla inner wing-coverts. Ivory, Sabine's Little Gull L. minutus and Ross's among most sought-after Ross's Gull Rhodostethia rosea and beautiful of arctic rarities. PART FIVE Glaucous and Iceland: large size, Glaucous Gull L. hyperboreus wings and tail without black at all ages. White-eyed and Sooty: medium Iceland Gull L. glaucoides size with extensively dusky plumage White-eyed Gull L. leucophthalmus at all ages and black hood in summer Sooty Gull L. hemprichii adults. White-eyed confined to Red Sea, while Sooty also found on north east African coast and east to India. Part Five will also include comment on albinistic, leucistic and hybrid gulls. This series will undoubtedly focus attention on gaps which still exist in our knowledge: any suggestions for revisions or new photographs will be greatly appreciated. It is intended to publish these papers as a book at a future date, when such new material may be included. The wealth of information and photographs received in response to published appeals have been of great assistance, and will be fully acknowledged in part 5. General information The identification of adult gulls—especially those in breeding plumage— is relatively straightforward. Immatures are more difficult, since they can look quite unlike the adults and the specific differences are much less obvious. Each species takes a different length of time to acquire adult plumage, largely depending on its size. Most small species reach adult plumage in their second winter, most medium-sized species in their third winter, and most large species in their fourth winter. Fig. 1 shows the i48 Field identification of gulls sequence of plumages from juvenile to adult of typical small, medium and large gulls. Use of the age-terminology shown here is recommended. It avoids the imprecision of such terms as 'immature' or 'sub-adult', which are unhelp fully used in many field guides and local bird reports: even 'fast-year' (instead of the more exact 'first-winter' or 'first-summer'), 'second-jyear', and so on, are inadequate in late summer or autumn, since they do not indicate whether the bird has undertaken its complete autumn moult and, therefore, do not fix its exact age. When a gull is in a transitional stage of moult, it is useful to record this fact. A thorough understanding of the sequence of immature plumages and the terminology used to describe them is essential when identifying and ageing gulls. Familiarity with the mechanics of gull moult and the effects of plumage wear and fading is also most important, since it will greatly clarify a seemingly complex field situation. Gulls have two moults annually. In spring, a partial moult renews the head and body feathers, usually a few inner wing-coverts, and some times the tertials and central tail feathers. In autumn, there is a complete moult, but juveniles replace only their head and body feathers. Franklin's Gull is exceptional in apparently having a complete moult in both spring and autumn. Adults usually start the complete autumn moult near the end of breed ing activity; immatures begin the autumn moult earlier. Large species can take as long as four months to complete this moult, whereas small species take four to six weeks, but there is much individual variation, both in the length of moult and in the starting time. Some immature large gulls may start their wing moult as early as May, while some adults may not complete it until December or January. The moult periods given in the text refer to extreme dates between which moult can be expected: most individuals will start and complete their moult within this period. The timing of the moult sequence is related to the timing of the breeding season. In the southern hemisphere, this is the reverse of that in the northern hemisphere. In equatorial regions, the seasonality of breeding is much less marked: the moult sequence there does not always follow the same temporal pattern as in Europe and northern Asia. Broadly speaking, the moult of the primaries provides the yardstick by which progress of the autumn moult is measured. (Gulls have eleven primaries, but the vestigial outer one is ignored here.) The moult com mences with the shedding of the inner (10th) primary, and progresses outwards. The moult of the rest of the plumage takes place mainly within the period when the primaries are being renewed, so that the full growth of the outer (i st) primary comes at or near the end of the moult. The rate of renewal of the primaries is slow, with usually only one or two adjacent feathers growing at any one time. The secondaries and tail feathers are moulted in a much less regular pattern, and large gaps are often visible where groups of feathers have been shed simultaneously. Wing-coverts are shed in groups, revealing the whitish bases of the underlying feathers, producing the whitish patches Field identification of gulls 149 and lines on the upperwing which are often noticeable in late summer and autumn. When a feather is lost accidentally, its replacement usually resembles that which would normally have grown at the next moult. Individuals with one or more replacement tail feathers are particularly frequent: on immatures this shows up as a white break in the otherwise complete tail band. Sick or injured birds are likely to have their moult sequence retarded. Adults which, for example, are unable to return to the breeding colony or are unable to participate in the colonial breeding activity, may lack the stimulus which produces breeding plumage. Captive birds are sometimes similarly retarded, and it is possible that vagrant gulls which have spent a long time outside their breeding range may also be affected. First-year gulls have rather pointed primaries and rounded tail feathers; at subsequent ages (second-winter onwards), the primaries are more rounded and the tail feathers squarer, although these differences are rarely discernible in the field. The effects of wear and fading are always most obvious in the summer, when the wing and tail feathers are at their oldest, just before the autumn moult. White plumage is more prone to wear than dark, and the white tips and fringes may disappear completely.

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