The current Iceland Gull invasion has given us a unique opportunity to study the variation of both Iceland and Kumlien’s Gulls. Admittedly we cannot know for sure where the different birds originate when they are seen outside their normal range. That does complicate things when it comes to proper identification.
I’ve wanted to write something about identification of Kumlien’s Gulls. But I have to do it in small bits – I do not have the time with all the birds just outside the window.
The question of identifying Kumlien’s Gulls starts with philosophy – or at least one must define “Kumlien’s Gull”. Some authors treat the taxon as a subspecies of Iceland Gull, a few as a valid species in its own right, and according to others it is a population of hybrids (a so-called hybrid swarm) produced by interbreeding between Thayer’s and Iceland Gulls.
1) If you go for the first position you have to define the characteristics for the subspecies in contrast to other subspecies. And this definition could be as simple as “everything that does not look like a classic glaucoides or thayeri” is a kumlieni. The weakness about this approach is that it does not take into account the possible variation of pure ssp. glaucoides and ssp. thayeri. It has for instance been stated that some pure ssp. glaucoides in 1. to 3. winter can show fain markings on the primaries – otherwise one of the best clues to identifying kumlieni.
2) If you go for the “full species” position you have to define a pure L. kumlieni. And then naturally a hybrid between a thayeri and a glaucoides would not be a Kumlien’s Gull, and a hybrid between a glaucoides and a kumlieni would not be a kumlieni. This leaves us the a species that can be most tricky to identify in the field. In practical terms it would probably be like Lesser and Mealy Redpolls – you have some classic individuals that can be identified but you will have a lot birds that cannot be identified for sure outside their normal range.
3) If you go for the hybrid swarm theory (Kumlien’s Gull is a result of interbreeding between Thayer’s and Iceland Gulls), then you can call everything that displays anything between the range of a classic glaucoides to a classic thayeri a Kumlien’s Gull. The term Kumlien’s Gull is then nothing more than a “Viking’s Gull” (Glaucous x Herring Gull) or an Olympic Gull” (Glaucous-winged x Western Gull). You could argue that the faintest markings on the primaries do indicate that there is 0,5% (or whatever) hybrid genes in the bird and thus making it a Kumlien’s Gull.
And the putative Kumlien’s Gulls that turn up in Europe typically are these very pale (sometimes almost indiscernable) birds – just have a look at the danish and swedish birds during this invasion.
I think this is enough for now… So I might just end with a song by Coldplay: Nobody said it was easy
Nobody said it was easy
No one ever said it would be this hard
Oh take me back to the start
I was just guessing at numbers and figures
Pulling the puzzles apart
Questions of science, science and progress
Do not speak as loud as my heart