Identification of Juvenile and Immature Baltic Gulls Detlef Gruber

Identification of juvenile and immature Baltic Gulls Detlef Gruber n recent times, knowledge of the identification and immature Baltic Gulls. It should, however, Iand systematics of large white-headed gulls be pointed that it is written from a western (hereafter ‘large gulls’) has been progressed by European perspective, ie, concentrating on the the work of many gull enthusiasts. However, al- problem of separating them from juvenile and though we now know much more about the immature Lesser Black-backed Gulls. identification of ‘yellow-legged gulls’, ie, Pontic Larus cachinnans and Yellow-legged Gulls Taxonomy and geographical variation Lmichahellis (hereafter cachinnans and micha- Until recently, it was common practice to sub- hellis, respectively), the identification of ‘lesser divide ‘lesser black-backed gulls’ into the sub- black-backed gulls’, ie, Baltic L fuscus, Lesser species ‘Lfuscus fuscus’ (northern and central Black-backed L graellsii and Heuglin’s Gulls Norway and Sweden, Finland, western Kola pen- L heuglini, is less well understood. This is reflect- insula and western White Sea), ‘Lf graellsii’ ed by the fact that there are only a few relevant (north-western Spain, France, Belgium, the studies on their identification (eg, Post & Lewis Netherlands, Britain, Ireland and Iceland), ‘Lf 1995, Nikander 1996, Eskelin & Pursiainen intermedius’ (the Netherlands, Germany, Den- 1998, Jonsson 1998, Rauste 1999). The present mark, south-western Sweden, and southern and article is intended as a contribution to a better central Norway), ‘Lf heuglini’ (southern Kola understanding of the identification of juvenile peninsula to Yenisey) and ‘Lf taimyrensis’ 103 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Turku, Finland, 7 September 1997 (Henry Lehto). Pale individual; note several fresh first-winter scapulars [Dutch Birding 21: 129-147, 1999] 129 Identification of juvenile and immature Baltic Gulls 104 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Lahto, Finland, 24 September 1997 (Henry Lehto). Note slender and long-winged appearance 105 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Turku, Finland, 7 September 1997 (Henry Lehto) 106 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Turku, Finland, 20 August 1997 (Henry Lehto) 107 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Lahti, Finland, 27 August 1997 (Visa Rauste). Note slender bill, Mew Gull L canus-like rounded head, long wings, whitish fringes of wing-coverts and tertials, and contrastingly pale underparts 108 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Altwarmbüchener See, Niedersachsen, Germany, October 1997 (Detlef Gruber). Dark individual; colour-ringed as pullus (C9C7) at Sahalahti (61:31N, 24:21E), Finland, on 15 July 1997 109 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Lahti, Finland, 22 September 1997 (Visa Rauste) 130 Identification of juvenile and immature Baltic Gulls 110 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Helgoland, Schleswig-Holstein, Germany, 6 October 1993 (Arnoud B van den Berg). Note pattern of several fresh first-winter scapulars 111 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, IJmuiden, Noord-Holland, Netherlands, 12 October 1982 (Arnoud B van den Berg) 112 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Newfoundland, Canada, November 1991 (Bruce Mactavish). Only little contrast between upper- and underparts, unlike fuscus 113 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Scheveningen, Zuid-Holland, Netherlands, 17 August 1998 (Peter de Knijff). Rather dark individual; note brownish fringes of wing-coverts 114 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Cork, Ireland, July 1996 (Anthony McGeehan). Tail-pattern like fuscus but uppertail-coverts slightly more marked than in most fuscus 115 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Belfast, Northern Ireland, August 1997 (Anthony McGeehan) 131 Identification of juvenile and immature Baltic Gulls (Yenisey and Taimyr peninsula) (eg, Cramp & michahellis and most graellsii. Large fuscus over- Simmons 1983). In this article, Baltic L fuscus lap in size with graellsii, even with small heugli- (hereafter fuscus), Lesser Black-backed L graellsii ni and michahellis. (including the forms ‘graellsii’ and ‘intermedius’) The wings of fuscus are distinctly long (a fea- (hereafter graellsii) and Heuglin’s Gulls L heugli- ture of a long-distance migrant). The primary ni (including the subspecies Lh heuglini and Lh projection is more than 150% of the length of the taimyrensis) (hereafter heuglini) are treated as exposed tertials. Given good views, up to six separate species. This taxonomic treatment is in exposed primaries are visible beyond the tertials accordance with recent decisions by the Dutch in the folded wing. Because of the almost all- committee for avian systematics (CSNA) (Sang- black primaries of juveniles and the lack of nar- ster et al 1998). row pale fringes of the primaries (contrary to In graellsii, adults become darker from north- argentatus), it is often difficult to count the num- western Spain (the form ‘graellsii’), the Nether- ber of visible primaries in the field. In addition, lands (the ‘intermediate’ form ‘graellsii x interme- the two outermost primaries (p9-10, primaries dius’, the so-called ‘Dutch intergrades’; cf Post & numbered from inside) are of nearly equal length Lewis 1995), toward southern and central Nor- and, therefore, appear to fall together. Further- way (the form ‘intermedius’). Interestingly, the more, depending on the position of the folded recently (1985) founded Belgian population wing, p5 may be entirely concealed by the mainly shows characteristics of the form ‘graell- overlaying tertials. The wings project noticeably sii’; this population is of British and Dutch origin beyond the tail; often, the wing projection cor- as ringing evidence suggests (Gunter De Smet in responds to the distance from the bill-tip to the litt). In heuglini, a cline of increasing darkness rear corner of the eye. runs from east to west along the Arctic Ocean (ie, Fuscus have short legs. In particular, the tibia from Taimyr peninsula to southern Kola pen- are short; sometimes, they seem to protrude insula). There is no evidence for intergradation of hardly beyond the belly-feathers, even when fuscus and graellsii at the contact zone in these are compressed. The long wings accen- Trøndelag, central Norway (cf Strann & Vader tuate the short-legged appearance of fuscus. 1992, Wim Vader in litt), supporting the view that However, it is not possible to identify fuscus on fuscus and graellsii are specifically distinct. this character alone. Fuscus have a delicate bill, even when Identification compared with graellsii. Despite being short, the Adult ‘lesser black-backed gulls’ can easily be bill may appear long because of its slenderness, distinguished from adult Herring Gulls Largenta- shallow gonydeal angle and pointed tip (often tus (hereafter argentatus), cachinnans and micha- reminiscent of cachinnans). hellis by the obviously darker upperparts; often Slight differences in head shape between fus- juveniles and immatures have darker upperparts cus and graellsii can be detected, especially as well. On average, ‘lesser black-backed gulls’ between juveniles (less so between adults). are slightly smaller, more elegant and slender, Juvenile fuscus have, on average, a more round- with longer wings projecting noticeably beyond ed and smaller head. Both the front and rear of the tail. More information on separating ‘lesser the head are rounded, resulting in a more sym- black-backed gulls’ from argentatus, cachinnans metric shape which can be rather pigeon-like. and michahellis can be found in Gruber (1995), Juvenile graellsii often have a slightly flatter fore- Harris et al (1996), Garner & Quinn (1997), head that is both less rounded and less steep Garner et al (1997) and Klein & Gruber (1997). than in some fuscus. Overall, compared with juvenile graellsii, the head shape of juvenile fus- Size and structure cus is more reminiscent of smaller species such When trying to establish differences in size and as Mew Gull. structure between fuscus and graellsii, one In flight, structural characters of fuscus are should always keep in mind that there is con- even more obvious. The narrow tern-like wings siderable variation which is largely due to indi- are pointed and the hand is longer than in any vidual variation and sexual dimorphism. other large gull (including graellsii). Fuscus are dainty, sometimes resembling smaller gulls, like Ring-billed L delawarensis and Juvenile birds Mew Gulls L canus, rather than larger and more Separating juvenile fuscus from juvenile graellsii powerful ones, like argentatus, cachinnans and can be very difficult. Since no single character is 132 Identification of juvenile and immature Baltic Gulls 116 Lesser Black-backed Gull / Kleine Mantelmeeuw Larus graellsii, juvenile, Maasvlakte, Zuid-Holland, Netherlands, August 1990 (Arie de Knijff). Rather pale-headed individual 117 Baltic Gull / Baltische Mantelmeeuw Larus fuscus, juvenile, Turku, Finland, 3 September 1997 (Harry J Lehto). Note pattern of fresh first-winter scapulars, whitish fringes of wing-coverts and tertials, and pale ground colour of head and underparts 133 Identification of juvenile and immature Baltic Gulls 118 Baltic Gull / Baltische Mantelmeeuw

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