[CT Birds] Thayer's and Kumlien's Gull Taxonomy...some thoughts

julian hough jrhough1 at snet.net
Tue Dec 6 19:11:18 EST 2011

Peter Adriaen's, a birding acquaintance of mine from Belgium, and a very sharp 
birder posted the following on a UK forum which I thought would be of interest 
considering Nick's nice Thayer's Gull and it "tis the season".
As I have always been interested in the taxonomy of Kumlien's and Thayer's Gulls 
(well, not when I was a kid, of course, but later on ), I did a bit of 
literature research a few years ago, which I would summarise as follows:

Despite many writings in magazines and various internet fora, nobody actually 
knows the exact taxonomic position of Kumlien's Gull. The literature on the 
subject is confusing; research is hampered by the inaccessibility of both the 
(high Arctic) breeding areas and breeding habitat, i.e. tall and steep cliffs. 
Some authors treat the taxon as a subspecies of Iceland Gull (e.g. Godfrey 1986; 
AOU 1998; Olsen & Larsson 2003), a few as a valid species in its own right (such 
as Taverner 1937; Sutton 1968; Chu 1998; Browning 2002), and according to others 
it is a population of hybrids (a so-called hybrid swarm) produced by 
interbreeding between Thayer's and Iceland Gulls (Dwight 1925; Weir et al 2000). 
Although the latter theory indeed seems to offer a valid explanation for the 
enormous plumage variation seen in Kumlien's Gull, it is misleading nonetheless, 
as more seems to be going on than merely hybridisation: 

	* The breeding areas of Thayer's and Iceland Gull have been separated for well 
over a century now. The breeding area of Iceland Gull shrunk significantly from 
1860 and by 1900 the species was confined to Greenland (Weir et al 2000). 
Thayer's Gull, on the other hand, breeds in the high Arctic region of Canada and 
has disappeared as a breeding bird from Greenland since 1930 (Boertmann 2001). 
Moreover, its distribution in Greenland had always been limited to the extreme 
northwest (above 77°N), while Iceland Gull mainly breeds in the southern part of 
the island, reaching its northern limit at 74°N along the west coast (Boertmann 

	* Preliminary DNA results have shown that Thayer's Gull is more closely related 
to Glaucous-winged Gull than to either Iceland or Kumlien's Gull (AERC TAC 2003; 
Gay et al 2005). Chu (1998) had reached the same conclusion before, but on the 
basis of morphological and skeletal data. 

	* The current breeding population of Kumlien's Gull (c. 10,000 pairs) is about 
twice the size of that of Thayer's Gull (4,000 to 6,000 pairs) (Weir et al 2000) 
- a strange discrepancy for any "hybrid population". 

	* Kumlien's Gull has a separate range, both in summer and in winter. Its 
breeding area is mostly limited to the southern half of Baffin Island, Canada. 
Thayer's Gull breeds in the northern half of Baffin Island and further to the 
north and west (Howell & Dunn 2007). Therefore, it is only on Baffin Island that 
the breeding areas of both taxa meet or, perhaps, overlap to a certain extent. 
In Greenland, no more than two breeding records of Kumlien's Gull are known so 
far, so its range does not really overlap with that of Iceland Gull either 
(Boertmann 2001). Kumlien's Gull mainly winters along the east coast of Canada 
and the United States, while Thayer's Gull migrates to the west coast and 
Iceland Gull mainly winters in Greenland and Iceland (Howell & Dunn 2007; Olsen 
& Larsson 2003). 

	* There are also small ecological differences. In winter, Kumlien's and Iceland 
Gull are more of a seabird than Thayer's Gull, which appears more strongly 
dependent on coastal areas (Howell & Mactavish 2003). 

	* A general rule used by many authors to identify a certain taxon as a valid 
subspecies is that about 70-75% of the population has to differ from at least 
99% of all other populations in one or more characters. The colour of the wing 
tips as well as the iris colour separate about 98% of adult Kumlien's Gulls from 
all adult Iceland Gulls, while body size, structure, colour of the upperparts, 
and primary pattern separate them from Thayer's Gull too. Thus, there seems to 
be no reason in this respect not to consider Kumlien's Gull a valid subspecies 
(Amadon 1949; Howell & Mactavish 2003). 

	* It is true that some authors have reported extensive interbreeding between 
Kumlien's and Thayer's Gull on Baffin Island, but they appear to have 
underestimated the separation of both taxa in the field. One particular study 
from 1986, for instance, which reported many mixed pairs (Gaston et al 1986), 
simply states that all breeding pairs were censused from a small aeroplane and 
that only those birds with "wing tips as pale as in Glaucous Gull" were written 
down as Kumlien's Gulls! Whatever the case, such mixed breeding limited to a 
rather small contact zone would not necessarily affect the taxonomic position of 
Kumlien's Gull (cf. the situation of Herring and Glaucous Gulls in Iceland). 

Our current knowledge therefore indicates that there really is a stable, 
separate and self-sustaining population of Kumlien's Gull that may have sprung 
from past hybridisation, but has now evolved further. More research could show 
if such a population should be seen as a subspecies or as a full species in the 


AERC TAC 2003. AERC TAC's taxonomic recommendations. Online version: www.aerc.eu

Amadon, D. 1949. The seventy-five percent rule for subspecies. Condor 51: 

American Ornithologists' Union (AOU), 1998. Check-list of North American birds, 
7th edition. AOU, Washington DC.

Boertmann, D. 2001. The Iceland Gull complex in Greenland. British Birds 94: 

Browning, M.R., 2002. Taxonomic comments on selected species of birds from the 
Pacific Northwest. Oregon Birds 28:69-82

Chu, P. C. (1998). A phylogeny of the gulls (Aves: Larinae) inferred from 
osteological and integumentary characters.Cladistics 14: 1-43.

Dwight, J. 1925. The gulls (Laridae) of the world; their plumages, moults, 
variations, relationships and distribution. Bulletin of the American Museum of 
Natural History 52: 63-401.

Gaston, A.J., Decker, R, Cooch, F.G. & Reed, A. 1986. The Distribution of Larger 
Species of Birds Breeding on the Coasts of Foxe Basin and Northern Hudson Bay, 
Canada. Arctic 39:4, p. 285-296

Gay, Laurène, Bell, Douglas A, Crochet, Pierre-André, 2005. ADDITIONAL DATA ON 

Godfrey, W .E. 1986. The Birds of Canada. Revised Edition. National Museums of 
Canada, Ottawa.

Howell, S & Dunn, J. 2007. Gulls of the Americas. Houghton Mifflin, New York.

Howell, S & Mactavish, B. 2003. Identification and variation of winter adult 
Kumlien's Gulls. Alula 1: 2-15

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

Sutton, G.M. 1968. Review of Smith's 'Evolution of some Arctic gulls (Larus): an 
experimental study of isolating mechanisms'. The Auk 85: 142-145.

Taverner, PA. 1937. Birds of Canada. The National Museum of Canada, Ottawa.

Weir, D. N., Kitchener A.C., and McGowan R. Y. 2000. Hybridization and changes 
in the distribution of Iceland gulls (Larus glaucoides/kumlieni/thayeri). J. 
Zool., London, 252: 517-530

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