Musings on Thayer’s, Kumlien’s, and Iceland Gull

Being a gull fanatic I thought it would be rude not to visit the Thayer’s Gull that is currently in Lincolnshire, it went missing for 3 days but turned up again yesterday in the same general area. It appears to be now showing well for the masses, if fuel prices weren't so expensive I would have gone down to see the bird again. There has been much discussion over the years on what to do with Iceland, Kumlien's and Thayer’s Gull. I have never really thought much about the later species until the arrival of the bird in Lincolnshire, of course seeing it for myself has sparked off my interest to learn more about the ‘species’. I was quite taken back at how obvious the bird I saw was, I have also sat over the past few years and watched photos of claimed birds come rolling in on BirdGuides and other websites. I can always remember some of the notable birds in Ireland and can remember the Dunbeg bird last year looking particularly striking. The difference between the Dunbeg bird and the Lincolnshire one is that the Lincolnshire bird appears to be far darker than the Dunbeg bird; I am reliably informed that these dark type Thayer’s Gulls form about 20% of the bird’s population. So it’s a mega rare gull in a rare plumage in the UK, what’s not to love?

As is expected the usual conversations started; is Thayer’s Gull a full species? Is Thayer’s Gull tickable? etc. Being a keen twitcher (I hate that word) I guess I would like to think its tickable yes, and I do believe it is a full species. However I only believe it’s a full species based on what I have found out through research. Currently most of the authorities count Thayer’s Gull as a full species, however the BOU do not. The AOU (American Ornithologists Union) do believe the bird is worthy of full species status but there is some discussion as to whether Thayer’s Gull might soon loose its status as a full species. I thought I would look at the topic in question for myself and come to my own conclusions based upon what I find out; so that is what I have done.

Thayer’s Gull has had a chequered history; it was once thought to be closely related to American Herring Gull however this changed to the view that it is now more closely related to Iceland Gull. Thayer’s Gulls breed in Arctic Canada and they typically winter on the Pacific coast of North America, whereas Iceland Gulls breed on Greenland. Between the two populations of breeding Iceland Gulls and breeding Thayer’s Gulls lies Baffin Island which is truly massive and truly a nightmare! Baffin Island is home to breeding Kumlien’s Gulls, some may have been fortunate enough to see the bird that wintered at Hartlepool Headland this year and will have seen the brownish wash to the outer primaries which distinguishes Kumlien’s Gull from Iceland Gull (but not always as some Iceland Gulls do show a brownish wash to the primaries).

Lets simplify things down a little, as far as I am aware Iceland Gull (larus glaucoides) is not know to hybridise anywhere with Thayer’s Gull (larus thayeri). So worry about the two hybridising isn’t that well founded in any evidence; the two species (bias or what!?) don’t breed closely with glaucoides breeding on Greenland and thayeri breeding in Arctic Canada. However it’s not all good news! Enter in Kumlien’s Gull (larus glaucoides kumlieni) which as I have mentioned already breeds in Baffin Island which lies between the breeding populations of glaucoides and thayeri.  Kumlieni is currently considered by most to be a subspecies of glaucoides, some believe it to be a subspecies of thayeri and others think that kumlieni is the result of hybridisation between glaucoides and thayeri. So there are 3 trains of thought, in saying all this the AOU currently treat kumlieni as a subspecies of glaucoides.

Hybrid kumlieni and thayeri gulls are known, however to what degree the two interbreed is not really all that well known; which is where it all starts to get a little bit messy. Questions are raised such as; well how do we know it’s a pure Thayer’s Gull? The Lincolnshire bird based on plumage is indeed spot on for Thayer’s Gull and there hasn’t really been much suggestion to the contrary. However is it a full species?

A lot of the authorities treat it as a full species; I feel this is a very fair assessment considering before kumlieni were even known to exist thayeri were known to exist. Which makes me wonder if kumlieni might be the result of hybridisation between glaucoides and thayeri, rather than being a subspecies of Iceland Gull? If kumlieni is a subspecies of glaucoides then it puts thayeri in an interesting category. It would seem that it would be a massive coincidence that kumlieni would have wing markings more similar to thayeri than to glaucoides if kumlieni were in fact just a subspecies of glaucoides.

I personally agree (hang out the flags egh?) with the AOU that Thayer’s Gull should indeed be a full species, but personally I am leaning toward the argument that Kumlien’s Gull is the result of hybridisation with Thayer’s and Iceland Gull. I feel that the plumage similarities between Kumlien’s Gull and Thayer’s Gull seem too close together to be just down to coincidence (assuming Thayer’s Gull is not a form of Iceland Gull). When I consider that Thayer’s Gull was known before Kumlien’s Gull even came on the scene it would seem to me that the expansion of Thayer’s Gull eastwards and Iceland Gulls westwards resulted in the disaster(?) on Baffin Island (Kumlien’s Gull). Maybe that is a bit harsh, I mean I really like Kumlien’s Gulls but to me they would seem to be the result of hybridisation in the past between Iceland and Thayer’s Gull.

But what if Thayer’s Gull isn’t a full species? Well it has been suggested that this is what will happen; no authorities will consider Thayer’s Gull to be a species and instead will consider it to be a form of Iceland Gull. However if this is the case then how do you explain the rather large ‘empty space’ between the breeding colonies of Thayer’s Gull and the colonies of Iceland Gull, then on top of this why did all of a sudden a grey winged form of Iceland Gull appear between the breeding areas of Iceland and Thayer’s Gull? Clearly the area was acceptable for breeding as Kumlien’s Gulls have found it most to their liking.

I am looking forward in the future to seeing what authorities do with Thayer’s, Kumlien’s, and Iceland Gull. I have a feeling they will be all be lumped together as ‘Iceland Gull of different forms’. But still very interesting whatever the outcome.

What does everyone else think?

Meanwhile on BirdForum there was some discussion over the length of the birds bill, some of the photos show the bird as having a fairly long but fairly fine bill. In the field I did notice this however it didn’t really concern me all that much, however others did flag up the concern that the Lincolnshire Thayer’s Gull may not be as good as hoped but the only thing that was really concerning people was the length of the bill. I started to look at this myself and fairly quickly I was looking at pictures from the USA of fairly long billed Thayer’s Gulls that looked almost like the Lincolnshire bird. As mentioned above the fact this bird is nice and dark coupled with the rest of the plumage features places this bird as a Thayer’s Gull, but the bill still bothered a few (me a little bit if I am honest). However some detective work by a few BirdForum members soon turned up some images of quite a few fairly long billed Thayer’s Gulls that really didn’t look any different from the Lincolnshire bird. A few examples were given and here are some of the best ones (in my opinion):

• Image 1
• Image 2
• Image 3
• Image 4 (very similar to Lincolnshire bird)
• Selection of Images (note the bird in the 3^rd picture!)

Fascinating bird this one and I have learnt loads already! This won’t be my last post about Thayer’s Gull.

 (Thayer's Gull - © Sam Viles)

Nice chocolate brown colour with dark centred tertials with white fringes. Upper scapulars perfect for juvenile/1st winter Thayer's Gull.

 (Thayer's Gull - © Sam Viles)

Good comparison with Herring Gull, a more delicately built bird which looks more like an Iceland Gull than a Herring Gull (in terms of build).

 (Thayer's Gull - © Sam Viles)

The large bill that was concerning some people (including myself a little bit), however Gulls are typically very variable anyway and there is nothing else about this bird that suggests anything other than a pure Thayer's Gull. 

(Thayer's Gull - © Sam Viles)

Long! Again this is unlike Herring Gull or any shots I have looked at of American Herring Gull, matches perfectly the stances I have seen pictures of American Thayer's Gulls adopt.

Until next time, Foghorn out!