Skuas to Gulls

Long-tailed Skua Stercorarius longicaudus

Long-tailed Skua is generally regarded as comprising two subspecies. Nominate longicaudus breeds in Scandinavia and western Arctic Russia but is replaced in eastern Siberia, North America and Greenland by pallescens (‘Eastern Long-tailed Skua’). This arrangement is recognised by BOURC (BOU 2004) but the validity of pallescens has also been contested (Olsen & Larsson 1997).

Nominate longicaudus is a regular migrant in Britain whilst pallescens was formerly on the British List but has recently been removed as it was potentially the product of a Meinertzhagen fraud (Kehoe 2006).

Identification is not straightforward. Current evidence suggests that there are only average differences in the appearance of adults and that birds from within the range of longicaudus can match the pale-bellied appearance of pallescens and vice versa. Furthermore, there are no published differences in non-adult plumages and there is a complete overlap in measurements (Olsen & Larsson 1997).

The occurrence of pallescens in Britain has been suspected but BBRC’s research has not established criteria by which a vagrant could be identified. Therefore claims of pallescens should contain details of a ringed or marked bird. (updated January 2015 AMS).

References

BOU. 2004. Taxonomic recommendations for British birds: second report. Ibis 146: 153-157.

Kehoe, C. 2006. Racial identification and assessment in Britain: a report from the RIACT subcommittee. British Birds 99: 619-645.

Olsen, K. M. & Larsson, H. 1997. Skuas and Jaegers: A Guide to the Skuas and Jaegers of the World. Pica Press.

 

Black Guillemot Cepphus grylle

Black Guillemot is generally regarded as comprising five subspecies. The subspecies mandtii (‘Northern Black Guillemot’) occupies the circumpolar High Arctic, arcticus breeds further south in North America, southern Greenland, Britain and Ireland, Scandinavia and north-west Russia, islandicus (‘Icelandic Black Guillemot’) in Iceland, faeroeensis (‘Faeroes Black Guillemot’) in the Faeroes and nominate grylle (‘Baltic Black Guillemot’) in the Baltic. Variation in both structure and plumage is complex, however, not always corresponding neatly with described subspecies limits, and some variation may be clinal (Cramp et al. 1985, Parkin & Knox 2010).

Only arcticus is on the British List though Cramp et al. (1985) note two ringing recoveries of Swedish birds from England. All populations undertake short movements only but mandtii has reached Iceland and the Netherlands and islandicus has reached Greenland (Cramp et al. 1985). Any of the remaining subspecies is therefore a potential vagrant.

The identification of vagrant subspecies is not straightforward but mandtii could be identifiable in all plumages by its upperwing and underwing pattern and in winter by its strikingly white head and body (Garner 2015). The subspecies islandicus and faeroeensis share a short wing.

Claims of mandtii might be acceptable based on notes and good photographs with grylle and the subspecies pair islandicus/faeroeensis potentially acceptable based on biometrics. Acceptance to islandicus or faeroeensis specifically would require a ringing recovery. (updated Sept 2015 AMS).

References

Cramp, S. et al. 1985. The Birds of the Western Palearctic Vol 4. Oxford University Press, Oxford.

Garner, M. 2015. Challenge Series: Winter. Birding Frontiers

Parkin, D. T. & Knox, A. G. 2010. The Status of Birds in Britain and Ireland. Christopher Helm, London.

 

Little Tern Sternula albifrons

Little Tern is treated as comprising six subspecies. In the Old World, nominate albifrons breeds in Europe, North Africa, the Middle East and Southwest Asia, guineae in West Africa and sinensis in India and Southeast Asia. In North America three further subspecies occur: antillarum in the east, athalassos in the interior and browni in Mexico and the Pacific West. The Nearctic subspecies are generally known collectively as ‘Least Tern’. Many authors (e.g. Olsen & Larsson 1995) award species status to ‘Least Tern’, a stance long adopted by AOU (Banks et al. 2006).

Nominate albifrons is the subspecies breeding in Britain and there is one accepted British record of a bird from the North American ‘Least Tern’ group – a bird in East Sussex in June 1983, returning annually to 1992 and also seen in Essex in June to July 1991 (Rogers et al. 2005, Yates 2010).

A combination of voice and plumage detail, in particular rump and tail colour, is required to confirm the identification of ‘Least Tern’. Detailed notes of candidates are welcomed, preferably supported by good photographs and a call transcription or preferably sound recordings. Details of a ringed or marked bird would of course also provide solid evidence. Since there appears to be no way to differentiate the North American races in a vagrant context, records not involving a ringing recovery will continue to be published as referring to the subspecies group antillarum/athalassos/browni. (updated Sept 2015 AMS).

References

Banks, R. C., Cicero, C., Dunn, J. L., Kratter, A. W., Rasmussen, P. C., Remsen, J. V. Jr., Rising, J. D. & Stotz, D. F. 2006. Forty-seventh supplement to the American Ornithologists’ Union check-list of American birds. The Auk 123: 926-936.

Olsen, K. M. & Larsson, H. 1995. Terns of Europe and North America. Christopher Helm, London.

Rogers, M. J. and the Rarities Committee. 2005. Report on rare birds in Great Britain in 2004. British Birds 98: 628-694.

Yates, B. 2010. Least Tern in East Sussex: new to Britain and the Western Palearctic. British Birds 103: 339-347.

 

Black Tern Chlidonias niger

Black Tern comprises two subspecies. Nominate niger breeds in Europe and western Asia. In North America it is replaced by surinamensis (‘American Black Tern’).

Nominate niger is a common migrant in Britain. The subspecies surinamensis is also on the British List, with five records, all of juveniles, now accepted (Andrews et al. 2006, Hudson et al. 2013).

The identification of surinamensis, in both adult and juvenile plumage, can be relatively straightforward. Observers of juveniles should pay particular attention to details of the head pattern and the colour of the flanks, rump and uppertail. There are biometric differences too, surinamensis being on average shorter-winged and longer-legged than niger (Olsen & Larsson 1995, McGeehan 2000).

Submissions of surinamensis are welcomed if supported by detailed notes and preferably good photographs. Biometrics or details of a ringed or marked bird would of course also provide additional evidence. (updated January 2015 AMS).

References

Andrews, R. M., Higgins, R. J. & Martin, J. P. 2006. American Black Tern at Weston-Super-Mare: new to Britain. British Birds 99: 450-459.

Hudson, N. & the Rarities Committee. 2013. Report on rare birds in great Britain in 2012. British Birds 106: 570-641.

McGeehan, A. 2000. Identification of American Black Tern. Birding World 1: 37.

Olsen, K. M. & Larsson, H. 1995. Terns of Europe and North America. Christopher Helm, London.

 

Common Tern Sterna hirundo

Common Tern is generally regarded as having three subspecies. Nominate hirundo breeds in the Western Palearctic and in North America. From the Ob Valley, Siberia it is replaced by longipennis (‘Siberian Common Tern’) whose range extends to the Pacific. A third subspecies, tibetana, breeds mainly in northwest China. Birds in western Siberia are variable, exhibiting intermediate characters between nominate hirundo and longipennis and are sometimes recognised as minussensis.

Nominate hirundo is a common breeding bird in Britain but no other subspecies is on the British List. Birds with an apparently complete suite of longipennis features have been recorded, however (Darby 2011, Nixon 2011). A number of claims have been submitted but are currently ‘held’ pending further research into the diagnosability of this subspecies.

‘Classic’ adult summer longipennis show ‘smoky’ grey underparts contrasting with an obviously white ‘cheek’, a wholly blackish bill and dark legs. There are biometric differences too, longipennis being on average shorter-billed and longer-winged than hirundo (Olsen & Larsson 1995). Birds with intermediate characters are an obvious source of potential difficulty, however, with immature, winter-plumaged, variant or otherwise dark-billed nominate hirundo a further potential problem, although whether such birds could exhibit a complete suite of longipennis plumage and bare part characters remains to be demonstrated.

Claims of summer plumaged adult longipennis are welcomed if accompanied by detailed notes and good photographs. Pending ongoing research, however, it remains to be established whether issues of intergradation or variability within nominate hirundo render confident diagnosis of longipennis problematic. Biometrics or details of a ringed or marked bird could provide more solid evidence. (updated November 2015 AMS)

References

Cramp, S. et al. 1985. The Birds of the Western Palearctic Vol 4. Oxford University Press, Oxford.

Darby, C. 2011. Eastern Common Terns in Suffolk and Belgium. Birding World 24: 511-512.

Nixon, S. 2011. The Eastern Common Tern in Suffolk. Birding World 24: 211-215.

Olsen, K. M. & Larsson, H. 1995. Terns of Europe and North America. Christopher Helm, London.

 

Common Gull Larus canus

Common Gull comprises four subspecies. Nominate canus breeds across northern Europe and north-west Russia where it intergrades with heinei (‘Russian Common Gull’) east of the White Sea and in the Moscow region. The latter then breeds widely across northern Asia, intergrading with kamtschatschensis (‘Kamchatka Gull’) in the Russian Far East. A further subspecies, brachyrhynchus (‘Mew Gull’), occurs in western North America. The taxonomy of this group is controversial, however, some authorities (e.g. Olsen & Larsson 2003) splitting brachyrhynchus and some (e.g. Johansen 1961) splitting kamtschatschensis.

Nominate canus is common (and breeds) in Britain. No records of heinei have ever been assessed or published by BBRC but the subspecies is on the British List, at least three birds from a sample of 250 trapped in south and east England (i.e. more than 1%) proving to be heinei on wing length measurements (BOU 1994). Given that these three must have been males at the extreme long-winged end of the range and that any shorter-winged males and all females would not have been detected, it is likely that the real proportion of heinei in this sample was even higher. In some winters this subspecies is reported to be present in good numbers as near as southern Scandinavia and northern Germany (reportedly as many as 50% of all Common Gulls at Kiel, northern Germany in winter 2000/2001) (Olsen & Larsson 2003). Ringing evidence also suggests its regular occurrence in Britain, with a number of British-ringed birds recovered from within the published breeding range of heinei as far east as east Vologda at more than 46 degrees East (BOU 1994). Birds ringed in the Netherlands, Germany, Denmark and Sweden have also been recovered within the range of heinei and a Danish-ringed bird has reached as far east as Kazakhstan (Olsen & Larsson 2003). All the evidence therefore suggests that heinei occurs regularly around the southern North Sea (including in Britain) and that its status here is probably at least that of a scarce migrant. Neither of the other two subspecies has been proven to occur in Britain although both have been suspected and both are potential vagrants.

Some male heinei are separable by wing, bill and leg measurements but field identification is highly problematic. A number of field characters have been proposed but the picture is clouded by extensive intergradation. Birds with heinei measurements and nominate canus plumage are common amongst Estonian breeders whilst many Baltic winterers have nominate canus measurements and heinei plumage (Olsen & Larsson 2003). The subspecies kamtschatschensis is large and dark but due to intergradation with eastern heinei reliable features of this subspecies are also lacking. In some cases, though, a combination of plumage and biometrics may be useful. Adult brachyrhynchus has a distinctive wing pattern but this can, on occasion, be suggested by rare, variant canus (Shepherd & Votier 1993).

Submissions of heinei are not sought but claims of kamtschatschensis and brachyrhynchus are welcomed if supported by detailed notes and good photographs. Pending further research, however, biometrics (in the case of kamtschatschensis) or details of a ringed or marked bird are probably necessary to secure acceptance to the British List. (updated Sept 2015 AMS).

References

British Ornithologists’ Union. 1994. 20th Report. Ibis 136: 253-255.

Carey, G. J. & Kennerley, P. R. 1996. “Mew” Gull: the first record for Hong Kong and the identification and systematics of Common Gull forms in East Asia. Hong Kong Bird Report 1995: 134-149.

Johansen, H. 1961. Die Superspecies Larus canus. Vogelwarte 21: 152-156.

Kompanje, E. J. O. & Post, J. N. J. 1993. Nieuwe vondsten van Russische Stormmeeuw in Nederland. Dutch Birding 15: 254-258.

Olsen, K. M. & Larsson, H. 2003. Gulls of Europe, Asia and North America. Helm.

Shepherd, K. B. & Votier, S. C. 1993. Common Gull showing characters apparently consistent with North American race. British Birds 86: 220-223.

 

Lesser Black-backed Gull Larus fuscus

This species comprises at least three and possibly as many as six subspecies. Traditionally included within Lesser Black-backed Gull are three western forms: graellsii (breeding in Greenland, Iceland, the Faeroes, Ireland, Britain and along the coasts of western Europe from the Netherlands to Iberia), intermedius (breeding in southern Scandinavia and Germany) and nominate fuscus (‘Baltic Gull’, breeding around the White Sea, the Baltic and in northern Norway).

Other, more eastern, forms – heuglini, (‘Siberian Gull’, ‘Heuglin’s Gull’or ‘Tundra Gull’, breeding in Arctic northwest Russia), taimyrensis (‘Taimyr Gull’, breeding east of the Taimyr Peninsula to the Khatanga River) and barabensis (‘Steppe Gull’, breeding in northern Kazakhstan and southwest Siberia) – are sometimes included also.

The species’ taxonomy is controversial, however. Because of differences in plumage, moult and migration strategies, Sangster et al. (1998) afforded species status to fuscus though subsequent studies demonstrated significant gene flow with intermedius (Liebers & Helbig 2002) and this taxonomic judgment was later reversed (Sangster et al. 2003). There is, however, more limited gene flow in the other direction, between fuscus and heuglini (Liebers & Helbig 2002).

The taxonomic position of the three eastern forms is also debated, some authors (e.g. Olsen & Larsson 2003) affording them collective species status as ‘Heuglin’s Gull’ whilst others (e.g. Liebers & Helbig 2002 and Collinson et al. 2008) retain them within Lesser Black-backed Gull. Further controversy surrounds the validity of taimyrensis (potentially a population of heuglini x Vega Gull hybrids) and the position of barabensis (genetically close to heuglini but sharing morphological similarities with Caspian Gull).

The subspecies graellsii breeds in Britain and intermedius occurs commonly. The subspecies fuscus is on the British List on the basis of a ringing recovery from the breeding range, and a further ringed bird in Gloucestershire in April 2007 has also been accepted (Hudson et al. 2008). A number of other claims are in circulation. Older published records pre-date the full recognition of variability within intermedius and are no longer acceptable. Identification problems prevent a full understanding of the current British status of fuscus, however. With continuing debate over what might constitute an ‘acceptable’ out of range bird, BBRC policy has until recently been to accept only ringed birds of known provenance though it is of course recognised that, as a consequence, the British status of fuscus is being under-stated. In reality, fuscus may be of regular occurrence here.

None of the eastern forms is on the British List but heuglini is a long distance migrant occurring regularly as close as Finland and is clearly a potential vagrant. The other forms may be potential vagrants also.

In terms of identification, quite marked average differences exist between fuscus and graellsii, and fuscus within its home range presents a relatively distinctive appearance. However, its separation from intermedius is problematic, especially in a vagrant context. Jonsson (1998) and Rauste (1999) proposed a set of identification criteria for fuscus, based mainly on its different moult strategy, the former identifying three age-classes – second calendar-year birds in spring, third calendar-year birds in spring and adults in autumn – which might yield an identifiable out of range bird. Subsequently, however, the moult strategy of both graellsii and intermedius (particularly that of third calendar year birds and adults) was found to be more variable than previously thought, overlapping with that described for fuscus (Gibbins 2004, Muusse et al. 2005, Winters 2006). Winters (2006) went on to propose a tighter definition of which second and third calendar year birds might be acceptable but this position was subsequently reviewed by Altenburg et al. (2011) who concluded that only second calendar year birds in April to June (and perhaps  into the summer) can be safely identified. Research is ongoing into the diagnosability of juvenile fuscus.

Further identification problems exist with the eastern forms. Much of the literature on heuglini focuses on its separation from fuscus but its much closer resemblance to graellsii/intermedius means that its identification in a vagrant context may not, on current knowledge, be possible. As with fuscus, however, moult cycle details may provide some clues. It is also worth noting that much of the identification literature on heuglini is based on the appearance of extralimital individuals in areas where graellsii/intermedius has now also been proven to occur.

Claims of any vagrant subspecies are welcomed if accompanied by details of a ringed or marked bird of known provenance. In respect of fuscus, the criteria set out by Altenburg et al. (2011) now provide an opportunity to accept claims of unringed second calendar year birds based on detailed notes (including moult analysis) and good photographs. (updated Sept 2015 AMS).

References

Altenburg, R. G. M., Meulmeester, L., Muusse, M. J. M., Muusse, T. O. V. & Wolf, P. A. 2011. Field identification criteria for second calendar-year Baltic Gull. Dutch Birding 33: 304-311.

Collinson, J. M., Parkin, D. T., Knox, A. G., Sangster, G. & Svensson, L. 2008. Species boundaries in the Herring and Lesser Black-backed Gull complex. British Birds 101: 340-363.

Gibbins, C. 2004. Is it possible to identify Baltic and Heuglin’s Gulls? Birding Scotland 7(4): 153-186.

Hudson, N. & the Rarities Committee. 2008. Report on rare birds in Great Britain in 2007. British Birds 101: 516-577.

Jonsson, L. 1998. Baltic Lesser Black-backed Gull Larus fuscus fuscus – moult, ageing and identification. Birding World 11: 295-317.

Liebers, D. & Helbig, A.J. 2002. Phylogeography and colonisation history of Lesser Black-backed Gulls as revealed by mtDNA sequences. Journal of Evolutionary Biology 15, 1021-1033.

Millington, R. & Golley, M. 2006. Identification of Baltic Lesser Black-backed Gull and two presumed individuals in Norfolk. Birding World 19: 388-390.

Muusse, T. O. V., Muusse, M. J. M., Luijendijk, B. -J. & Altenburg, R. G. M. 2005. Identification update: moult variability in 3rd calendar-year Lesser Black-backed Gulls. Birding World 18: 338-348.

Olsen, K. M. & Larsson, H. 2003. Gulls of Europe, Asia and North America. Helm.

Rauste, V. 1999. Kennzeichen und Mauser von “Baltischen Heringsmöwen” Larus [fuscus] fuscus und “Tundramöwen” L.[fuscus] heuglini. Limicola 13: 105-128 and 13: 153-188.

Sangster, G., Hazevoet, C. J., Van den Berg, A. B., Roselaar, C. S. & Sluys, R. 1998. Dutch avifaunal lists: species concepts, taxonomic instability and taxonomic changes in 1977-1998. Ardea 87: 139-165.

Sangster, G., Van den Berg, A. B., Van Loon, V. & Roselaar, C. S. 2003. Dutch avifaunal lists: taxonomic changes in 1999-2003. Ardea 91: 281-287.

Winters, R. 2006. Moult and plumage variation in immature Lesser Black-backed Gulls in the Netherlands. Dutch Birding 28: 140-157.

 

Yellow-legged Gull Larus michahellis

This species comprises two subspecies. Nominate michahellis breeds throughout the Mediterranean and around the southwestern coasts of Europe, while atlantis (variously termed ‘Atlantic Gull’, ‘Azorean Gull’, ‘Azorean Atlantic Gull’ or ‘Azorean Yellow-legged Gull’) breeds in the Azores. Birds in the Canaries and Madeira share some features of atlantis and are sometimes regarded as of that form but are also often included within michahellis (Olsen & Larsson 2003). Western Iberian birds also differ slightly from typical Mediterranean michahellis and are sometimes separated as ‘lusitanius’ or ‘cantabricans’ (‘Cantabrican Gull’). The current subspecific taxonomy of Yellow-legged Gull may therefore merit review (Collinson et al. 2008).

Nominate michahellis is a common and increasing summer visitor to southern England and occasional breeder though western Iberian birds are thought to be largely sedentary (Arizaga et al. 2010). The subspecies atlantis is not (yet) on the British List but it is a highly pelagic wanderer and regularly reaches North America (Howell et al. 2014). Nearer home, ten have now been provisionally accepted in Ireland (http://www.irbc.ie/reports/irbr/2012_IRBR.pdf). Clearly, therefore, it is likely to be reaching Britain too, indeed three claims – on the Outer Hebrides in September 2005, in Cornwall in July to November 2008 and in Oxfordshire in October to December 2009 – are now under consideration by BOURC for admission to the British List and a number of others have been suspected, not only from the west but also from eastern England. Some of these are now in circulation.

‘True’ adult atlantis have a slightly darker mantle than michahellis and, in non-breeding plumage, show a strikingly dark, heavily-streaked ‘hood’ whilst younger age classes are dark and ‘swarthy’ and may resemble Lesser Black-backed Gull or even American Herring Gull as much as Yellow-legged Gull. There are, however, potential problems in distinguishing ‘classic’ atlantis from birds from the Canaries, Madeira and western Iberia which may share, at least to some extent, some of the same features. Furthermore, hybrids involving a range of other large white-headed gulls including (at least) Herring, Lesser Black-backed, Yellow-legged and American Herring Gulls are also a potential source of difficulty.

Submissions of potential ‘true’ atlantis are welcomed if accompanied by detailed notes and good photographs. Details of a ringed or marked bird would of course provide additional evidence. (updated January 2015 AMS).

References

Arizaga, J., Herrero, A., Galarza, A., Hidalgo, J., Aldalur, A., Cuadrado, J. F. & Ocio, G.  2010. First-Year Movements of Yellow-Legged Gull (Larus michahellis lusitanius) from the Southeastern Bay of Biscay. Waterbirds 33: 444-450.

Collinson, J. M., Parkin, D. T., Knox, A. G., Sangster, G. & Svensson, L. 2008. Species boundaries in the Herring and Lesser Black-backed Gull complex. British Birds 101: 340-363.

Dubois, P. J. 2001. Atlantic Islands Yellow-legged Gulls – an identification gallery. Birding World 14: 293-304.

Elliott, M. 2008. The Azorean Atlantic Gull in Cornwall. Birding World 21: 462-466.

Howell, S. N. G., Lewington, I. & Russell, W. 2014. Rare Birds of North America. Princeton University Press. Princeton and Oxford.

http://www.irbc.ie/reports/irbr/2012_IRBR.pdf

Lewington, I. 2009. Identification of the Azorean Atlantic Gull in Oxfordshire. Birding World 22: 459-462.

Lowe, T. 2006. An Atlantic Gull in Ireland. Birding World 19: 391-392.

Olsen, K. M. & Larsson, H. 2003. Gulls of Europe, Asia and North America. Helm.

 

Iceland Gull Larus glaucoides

The taxonomic history of Iceland Gull (sensu lato) is among the most complex of any gull species and has long been the subject of debate. The current BOU treatment recognises three subspecies: nominate glaucoides (Iceland Gull), breeding in Greenland, kumlieni (‘Kumlien’s Gull’), breeding mainly on Baffin Island, and thayeri (‘Thayer’s Gull’), breeding in High Arctic Canada. There are, however, rival interpretations, some authors (e.g. Olsen & Larsson 2003) regarding thayeri as a species in its own right, a treatment long adopted by AOU (AOU 1998) and, more recently, by a number of other European rarities committees. The position of the notoriously variable ‘intermediate’ form kumlieni is the key to this problem. This form intergrades with both thayeri and glaucoides and its taxonomy remains uncertain. It is currently treated as a valid, though highly variable, subspecies though some authors (e.g. McGowan & Kitchener 2001) regard it as an invalid taxon, a ‘hybrid swarm’ between thayeri and glaucoides.

Nominate glaucoides is a regular winter visitor to Britain, generally in small numbers but with occasional larger influxes. The form kumlieni is a scarce winter visitor. The subspecies thayeri is also now on the British List with two records accepted (and eight have also now been accepted in Ireland – http://www.irbc.ie/reports/irbr/2011_IRBR.pdf). There are also accepted records from Norway and Denmark (Mjøs & Garner 2000, Klein et al. 2003).

The identification of thayeri is potentially problematic, the main difficulties being its separation from darker kumlieni, Herring Gull, American Herring Gull and hybrids (of Nearctic origin) involving a range of other large white-headed gulls including (at least) American Herring Gull and Glaucous-winged Gull. Only ‘classic’ individuals are likely to be identifiable.

Claims of thayeri are welcomed if accompanied by detailed notes and good photographs. Details of a ringed or marked bird would of course provide additional evidence. (updated Sept 2015 AMS).

References

American Ornithologists’ Union (AOU). 1998. Check-list of North American Birds, 7th edition. AOU.

http://www.irbc.ie/reports/irbr/2011_IRBR.pdf

Gantlett, S. 1991. The rise and fall of Thayer’s Gull. Birding World 4: 84-86.

Garner, M. & McGeehan, A. 1997. The Thayer’s Gull in Belfast. Birding World 10: 93-100.

Garner, M. & McGeehan, A. 1998. Identification of juvenile and first-winter Thayer’s Gull. Birding World 11: 94-101.

Howell, S. N. G. & Elliott, M. T. 2001. Identification and Variation of Winter Adult Thayer’s Gulls with Comments on Taxonomy. Alula 7: 13-144.

Howell, S. N. G. & Mactavish, B. 2003. Identification and Variation of Winter Adult Kumlien’s Gulls. Alula 9: 2-15.

Klein, S., Pedersen, K. & Thorup, K. 2003. Sjaeldne fugle I Danmark og Gronland I 2002. Dansk. Orn. Foren. Tidsskr. 97 (3).

McGowan, R. Y. & Kitchener, A. C. 2001. Historical and taxonomic review of the Iceland Gull Larus glaucoides complex. British Birds 99: 191-195.

Mjos, A. T. & Garner, M. 2000. The Thayer’s Gull in Norway. Birding World 13: 10-11.

Olsen, K. M. & Larsson, H. 2003. Gulls of Europe, Asia and North America. Helm, London.