We struggle to define the threshold for confident identification of out of range occurring Kumlien’s Gulls and it is often reduced to a one-sided question: “Where can we draw the line so that we are sure the accepted records of Kumlien’s Gull are correct? Since the goal is high confidence, this leads to a narrow definition of Kumlien’s Gull – only the well pigmented which ghost the overall pattern like a subdued Thayer's Gull are named and reported as Kumlien’s. This leads to two problems. First, we only identify part of the population – some pale Kumlien’s are excluded. Second, many of those pale and faint pigmented individuals are simply lumped into our definition of Iceland Gull, and receive no extra attention. There is nothing wrong with excluding some pale Kumlien’s from being called Kumlien’s, as long as birders understand that some Kumlien’s are being excluded. This is conservative. But most people give Kumlien’s Gull and Iceland Gull very uneven treatment by demanding that Kumlien’s meet high standards, and then calling everything else Iceland Gull. This is not conservative. We should at least be fair and apply equally strict criteria to our Iceland Gull identifications. Calling it an Iceland Gull, when it is really a possible Kumlien’s Gull could be just as wrong as naming it as a definite Kumlien’s Gull.
The identification is challenging because Kumlien’s Gull seem to show an unbroken continuum of variation from being almost as dark as a Thayer’s Gull to be pale and white-winged as an Iceland Gull, and there are no fully reliable plumage differences between the latter and the extreme pale Kumlien’s Gulls. The controversy with the Kumlien’s Gull-Iceland Gull identification, however, is part fuelled by a desire to name every individual. That some observers would be happier to call a gull an Iceland Gull even though faint pigmentation is present in its outer primaries may be more through conservatism than accurate identification. Using the term ID uncertain is probably more appropriate and realistic.
