4th winter
Thayer's/Kumlien's Gull Larus thayeri / L. glaucoides kumlieni
February 28th 2012 Choshi Chiba Japan

Based on its size, structure, behavior and location etc, this gull seems to be the same individuals as this one in 2009 that was considered as kumlieni. But now it has grown into a darker bird than we expected, and currently looks better to be regarded as a hybrid/intergrade between thayeri and kumlieni (or possibly pale thayeri?). In a sense that somewhat disappointed us, but viewed from a different angle, it seemed very interesting and valuable, because individual identifications over a few years without any rings or tags are probably much harder in North America where numerous thayeri and kumlieni are wintering.

The overall impression almost looks like Thayer's Gull, but the primaries show a little more large amount of white, especially for a 4th winter bird. The eye looked relatively large compared to some other Thayer's Gulls on the same date (- I counted at least 16 individuals including some presumed hybrid/intergrades like this).

The dark markings are only 5 pieces, P10-P6, unlike many typical Thayer's Gulls. The dark marking of P6 is divided by a narrow white shaft, and is fading out toward inner edge with grayish tone. But the mirror on P9 does not reach the outer web, and P10 has a subterminal band. The dark parts might decrease further next year (5th winter) and possibly barely get into the range of dark kumlieni. But of course, if this is the complete adult pattern of this individual, it seems difficult to call it kunlieni, especially in Japan.

1st winter 2009 (left), 4th winter 2012 (right), presumed the same individual.
Photographed at the same place. Note the structural similarity.

For comparison: Kumlien's Iceland Gull Larus glaucoides kumleni at Niagara Canada in 2005.