With the northwards extension of the breeding and non-breeding

Transcription

1 Identification of juvenile yellow-legged Herring Gulls With the northwards extension of the breeding and non-breeding ranges of the west Mediterranean yellow-legged race of the Herring Gull Lams argentatus michahellis* in recent years (Nicolau-Guillaumet 1977; de Heer 1981; Devillers & Potvliege 1981; Devillers 1983; Grant 1983), birdwatchers along the English Channel and North Sea coasts have been confronted with a new identification problem. This paper describes juvenile michahellis in comparison with juveniles of the northwest European races of the Herring Gull L. a. argentatus and L. a. argenteus (here referred to jointly as argentatus) and of Lesser Black-backed Gull L.fuscus. Field studies of subsequent plumages are still continuing, and we do not know exactly to what extent our descriptions may apply also to the larger east Mediterranean Herring Gulls (more closely related to L. a. cachinnans) or to those of the Atlantic seaboard of Iberia ('Cantabrican' Herring Gulls; evidently a subspecies yet to be described), adults of which differ markedly from michahellis (Teyssedre 1983; personal observations). Although it has, until recently, usually been regarded as a subspecies of the Herring Gull, michahellis is in many ways more closely related to the Lesser Black-backed Gull (Teyssedre 1983a, b). This relationship is apparent in juvenile plumage, to the extent that, when michahellis is suspected, it is often easier to eliminate argentatus than Lesser Black-back. Distinctions from other juvenile gulls It is often possible to dismiss argentatus on a single feature: the pattern of the tertials (figs. 1 & 2; see also plates 170, 171, 173,200 & 202 in Grant 1982). On argentatus, this pattern is highly variable, but usually shows well-defined pale patches or notches on the sides of the feathers, and often a broad *The authors, and many birdwatchers in Europe, consider that the Herring Gulls with yellow legs should be regarded not as races of Lams argentatus, but as belonging to a separate full species, the Yellow-legged Gull Laws cachinnans. For the sake of stability, however, the current practice of British Birds is to adhere to the sequence and nomenclature of Professor Dr K. H. Voous's List ofrecent Holarctic Bird Species (1973, 1977) for species, and to Dr Charles Vaurie's The Birds of the Palearctic Fauna (1959, 1965) and BWP (1977 onwards) for subspecific names. With the authors' approval, this paper has been modified to accord with established current (rather than probable future) practice. EDS 344 IBril. Birds 77: , August 1984]

2 Identification of juvenile yellow-legged Herring Gulls 345 Fig. 1. Variations in tertial pattern of juvenile Herring Gulls Lams argentatus. Left four: typical, with and without pale subterminal bar; right two: apparently not rare variants, with restricted pale areas (Pierre Yesou) subterminal pale bar. Juveniles of michahellis and Lesser Black-backed Gull have uniformly brown tertials with only a thin pale edge. Some argentatus, however, can show darker tertials than others, with ill-defined or thin pale markings (variants in fig. 1): at long range or in worn plumage, these feamers can look very like those of michahellis or Lesser Black-back. In such cases, other 'classic' characters must be used (see Grant 1982). Fig. 2. Tertial patterns of Lesser Black-backed Gull Larusfuscus and Herring Gulls L. argentatus of the west Mediterranean yellow-legged race michahellis. Left two: typical Lesser Blackback (apparently only slight variation within the species), a pattern shared by some michahellis; middle two: michahellis, showing large pale edges (this pattern also found on some Lesser Black-backs); right two: michahellis, with subterminal indentations apparently not found on Lesser Black-back (Pierre Yesou)

3 346 Identification of juvenile yellow-legged Herring Gulls On the other hand, michahellis often differs from most Lesser Blackbacked Gulls in having a much larger pale tip on the tertials. Some show a pattern apparently not shared by the latter species: one or two indentations towards the tip of the dark area. When this is well marked, it seems that identification as michahellis can be assumed. Unfortunately, most michahellis and Lesser Black-backs cannot be separated on this character; it is best to wait until the bird flies. Identification in flight The wing pattern of a flying juvenile michahellis is intermediate between those of juvenile argentatus and juvenile Lesser Black-backed Gull (fig. 3). The greater coverts are always darker than on argentatus, and can form an 'extra' band above the secondary bar. This band is, however, usually less extensive than on Lesser Black-back, and can be very ill-defined. This is because the greater coverts are rarely as dark as those of Lesser Black-back, but caution is needed as some juveniles of the latter have paler greater coverts forming an incomplete 'extra' bar (see plate 202 in Grant 1982). Fig. 3. From top to bottom: wing patterns of juveniles of Herring Gull Lams argentatus argentatus; of yellow-legged race of Herring Gull L. a. michahellis (two types shown but others can occur, combining more or less marked 'window' and all-dark to completely absent 'extra' secondary bar); and of Lesser Black-backed Gull L.fuscus (J.-J. Blanchon) In the hand, the inner web of the inner primaries on michahellis is always paler than the outer. A pale 'window' might, therefore, be expected on flying birds, but in fact this is not always the case. Some individuals show a clear window, although never so well marked as on argentatus. More frequently, they have an ill-defined window which can recall that of a first-year Great Black-backed Gull L. marinus. Others show none, or only a poorly marked one, much depending on light. The uppertail-coverts on juvenile michahellis are always much whiter than those of juvenile argentatus, often with only a few brown markings, and the black tail-band is always well defined. These characters are, however, matched by some Lesser Black-backed Gulls. Other characters Identification is much more difficult when the bird is not flying: a

4 Identification of juvenile yellow-legged Herring Gulls 347 combination of characters may then be necessary. Although there are great individual variations in the sizes of gulls, Herring Gulls of the race michahellis are almost always larger and stouter than Lesser Black-backs; individuals from the so-called Cantabrican population, however, are often of a similar size to or smaller than the latter. Because of individual variation and the problems of light conditions and degree of wear, an average ground colour cannot be described. Some michahellis appear rather dark, but never so dark as the brownest Lesser Black-backs; others are as pale as some argentatus. With one- or two-monthold michahellis identification is helped by wear or fading, which gives some (but not all) individuals the 'distinctive rich gingery or rusty tone' described by Grant (1982); this is well marked and very distinctive mainly from mid August into September. By August, most juvenile michahellis show a whiter head, often with a dark patch behind the eye; this contrasts strongly with the dark upperparts. Their stouter, all-dark bill, combined with their bulky structure and the contrast between pale head and dark upperparts, may give them an appearance recalling a juvenile Great Blackbacked Gull. Individuals moulting to first-winter plumage The abraded tertials show variable patterns, so that, by October, this is often no longer a useful character. The bill oimichahellis remains all-dark for the whole of the first year, whereas on argentatus the base quickly becomes flesh-coloured. The distinction from Lesser Black-back is more difficult, but size, bill thickness and wing pattern remain useful characters throughout the first year. Conclusion Until recently, most birdwatchers thought it impossible to distinguish juvenile Herring Gulls of the race michahellis from those of argentatus and from juvenile Lesser Black-backed Gulls. We suggest that this distinction is often possible, and is even rather easy when good views are obtained, particularly of flying individuals. Identification in subsequent immature plumages remains problematic in many cases. We hope that further research will help to overcome this difficulty. Acknowledgments We are indebted to Peter Grant, who encouraged us to prepare this paper and kindly improved the English version of the manuscript. Jean-Pierre Cochin and Jean-Jacques Morilleau provided us with some of the wings and feathers used to draw the figures. We are happy to have been able to take advantage of Jean-Jacques Blanchon's talent for the wing sketches. Finally, our experiences were improved by stimulating exchanges of views with other birdwatchers, particularly Dr Gerard Baudoin, Dr Pierre Devillers, Alain Guillemont and Thierry Milbled. References DE HEER, P [On identification of Yellow-legged Gull and occurrence in the Netherlands.] DutchBirdingl: DEVIIAERS, P Yellow-legged Herring Gulls on southern North Sea coasts. Brit. Birds 76:

5 348 Identification of juvenile yellow-legged Herring Gulls, & POTVLIEGE, R, Le Goeland leucophee Lams cachinnans michahettis en Belgique. Gerfaui 71: GRANT, P.J Gulls: a guide to identification. Calton Yellow-legged Herring Gulls in Britain. Brit. Birds 76: NICOLAU-GUILLAUMET, P Mise au point et reflexions sur la repartition des Goelands argentes Lams argentatus en France. Alauda 45: TEYSSEDRE, A. 1983a. Etude comparee de quatre populations de Goelands argentes a pattes jaunes d'europe Occidentale. L'Oiseau 53: b. Contribution a la systematique du Groupe du Goeland argente par l'etude accoustique de Lams argentatus argenteus, L.fuscus graettsii, L. cachinnans (?) michahellis et du Goeland argente a pattes jaunes cantabrique. Behaviour in press. Dr Philippe J. Dubois, Le Cabestan, 73 avenue Robespierre, La Rochelle, France; Pierre Yesou, Reserve de Champclou, L 'lie d'olonne, France