Here in the Western Great Lakes region, many Kumlien's & Thayer's could be suspected of being one or the other. You see, the two have a suite of characteristics that can - and do - overlap in every possible respect. Of course this is not to say that there aren't any clear-cut Kumlien's or clear-cut Thayer's here. There are.
But we also have many birds that take on a "range bias" when they're looked at by outsiders. Thus, the majority of gull enthusiasts on Lake Superior and Lake Michigan are left having to make arbitrary identifications with some of these birds. Fortunately, 2nd cycles are not as difficult as 1st cycles and adults.
Consider this 2nd cycle:
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| Dark Kumlien's or pale Thayer's? Lake County, IL. 28 November 2014. |
I circulated the photos of this bird to several trusted gull aficionados from both regions of the continent (West and East coasts), and as expected, some birders from Colorado and areas to the west, identified it as a pale Thayer's, while birders from New York and east called it a Kumlien's. I favor the latter identification.
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| Rather plain outer primaries and overall pallid upperparts seem fine for Kumlien's Gull. |
One of the comments made by a birder from the western United States was that the tertials show solid pigmentation throughout and that this ought to support a Thayer's ID. I think that's generally true, but the caveat is that the tertials need to be considered in conjunction with the primaries and upperwing coverts. That is, the tertials on Thayer's tend to be a bit paler than the primaries, which are ideally a darker brown than is seen on this bird. Here's my idea of a "no gimmick" 2nd cycle Thayer's:
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| Thayer's Gull (2nd cycle). Santa Cruz, California. 17 January 2014. |
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| Thayer's Gull (2nd cycle). Lake County, Illinois. 31 January 2014. |
Notice how the tertials act as a color-bridge between the noticeably darker primaries and somewhat paler upperwing coverts.
It would be nice if all of our 2nd cycle Kumlien's were as obvious as the bird pictured below:
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| Kumlien's Gull (2nd cycle). Lake County, Illinois. 31 January 2014. |
Or even this one:
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| Kumlien's Gull (2nd cycle). Niagara River. New York. 29 December 2012. |
But some do have dark, solid-colored, tertials. Here are a couple from the Eastern Great Lakes that would never be second guessed as Kumlien's:
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| Kumlien's Gull (2nd cycle). Toronto, Ontario. 27 December 2012. Overall solid dark tertails centers, but with lots of internal markings and stippled upperwing coverts. The primaries are not as dark as I'd like to see on a textbook Thayer's. |
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| Kumlien's Gull (2nd cycle). Toronto, Ontario. 27 December 2012. Very similar to the subject bird. The lack of contrast between the solid tertial centers and pale primaries is standard Kumlien's. |
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| Same bird pictured above. The outer primaries don't contrast much with the rest of the upperparts. |
Consider the difference when a classic, 2nd cycle Thayer's opens its wing:
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| Thayer's Gull (2nd cycle). Michigan City, Indiana. 27 November 2014. The outer 5-6 primaries are markedly darker than the rest of the flight feathers and upperwing. |
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| Thayer's Gull (2nd cycle with adult Lesser). Michigan City, Indiana. 28 November 2014. Although the tertials show pale outer halves, the bases have a solid brown color that's complemented by considerably dark brown primaries. |
The take-away from this post should be that 2nd cycle Kumlien's with solid tertial centers are not uncommon, and the key is not to look at this field mark in isolation (as is the case with any field mark). Always compare the tertials to the color intensity of the primaries, along with the overall color of the upperparts.