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Thayer's Gull / Kumlien's Gull / Iceland Gull
proposed thayeri-kumlieni-glaucoides 28-point scale
7 categories, 4 points per category, total score: 0-28
for first cycle birds
0 = all white
1 = white with very light internal markings along shafts
2 = white with thin shaft streaks and variable subterminal mark;
any solid portions are washed out and limited to near shaft
3 = solid gray-brown, perhaps paler than 4, with broad pale edges
4 = dark gray-brown, darker than body tones, with thin pale edges
0 = nearly white, largely unmarked
1 = nearly white, lightly marked
2 = same color as coverts in flight, usually with patterned tips
3 = as 4, but may be more patterned at tips and paler on inner webs
4 = forming a contrasting dark bar in flight
0 = nearly white, lightly marked
1 = mostly white with scattered markings
2 = pale, but patterned with dark bars or other markings
3 = has some solid areas, concolor with or paler than body tones, with pales notches and tips
4 = solid gray-brown (usually concolor with body tones) with variable pale notches and tips
0 = pale, faintly mottled
1 = pale with heavier mottling
2 = entirely mottled, but with a band of heavier concentration of mottling
3 = similar to 4, but may show pale “shadow band” within the dark band,
extensive pale tips, or have the dark solid areas faded pale
4 = solid an thick gray-brown band, but with extensive pale mottling to “upper corners”
(bases of outermost retrices); often with large mottled pale tips
0 = white with minimal flecking
1 = white, peppered with flecking
2 = more white than dark, appearing pale with dark patterned lines,
as if drawn with a pencil
3 = similar to 4, but with more white than dark,
especially in the greater coverts and scapulars
4 = neatly checkered, with roughly equal amounts of dark and light
Bill Color There may be significant natural variation in this feature, leading to substantial overlap. Note that bills get paler thru the winter.
0 = essentially two-toned; outer third black, inner 2/3 pale,
but with a blurred distinction between the two
1 = similar to 2 but with more pronounced paler base
2 = black outer third; inner 2/3 still dark but fades into paler tones
(usually gray or dusky pink); palest at base of lower mandible
3 = black with more obvious pale (usually pinkish) areas
4 = all black or with limited pale (usually pinkish) at base of
lower mandible and/or near nostril
Head/Bill Size and Shape Note there is a substantial difference between males and females, creating overlap in this feature. E.g., a female Thayer's can easily resemble a male Kumlien's.
0 = most rounded head and petite bill, with little gonydeal angle
4 = more sloped forehead, but still rather gentle expression similar to female Herring,
visible gonydeal angle; male and female shown
Rather than come up with my own rules (as I attempted at first), I have examined the distinctions made between these forms in various regions, focusing on the core winter ranges for glaucoides, kumlieni, and thayeri, respectively. Here is what I have found:
A birder in Iceland draws the line between glaucoides and kumlieni at a score of around 13 or 14.
Howell and Dunn's (2007) kumlieni range seems to be between 11 and 20 (using photos from Newfoundland).
Olsen and Larsson's (2003) kumlieni range seems to be between 10 and 21 (using photos from Newfoundland).
Comments from birders in Newfoundland and the US East Coast suggest their kumlieni range is between 6 and 22.
The Avian Bird Records Committee of Connecticut, when considering reports of Thayer's Gull, accepted a bird that I scored 27, but rejected one that I scored 24.
The California Bird Records Committee has accepted birds 17 and lower as "Iceland Gull". Birds 19 and higher have been rejected as Thayer's or Thayer's x Iceland.
Taken together, thus suggests that birds between 18 and 25-26 are considered in the Thayer's/Kumlien's intergrade zone. Some of these are accepted as one or the other on their expected coasts, but rejected when out of range.
Note that I have scored these birds, not them. They did not use this scoring system, but rather they used their own knowledge of the species, undoubtely using an implicit system more complicated than this.
Send comments to me at stevechampton, at gmail.
AN ALTERNATIVE APPROACH FOR SEPARATING glaucoides AND kumlieni
One possible diagnostic feature for drawing a line between glaucoides and kumlieni could be the outer primaries. On glaucoides, they get paler as you go out; on kumlieni, they get darker. This might hold regardless of how dark or light the rest of the bird is. The two examples below demonstrate this effect, showing a presumed glaucoides (left) that is, overall, darker than a presumed kumlieni (right). The kumlieni would score lower on my scale, which does not account for this effect.
Defining what total scores constitute glaucoides, kumlieni, and thayeri depends in part on your taxonomic perspective.
The traditional view is that Iceland Gull is a species composed of two subspecies: nominate glaucoides and kumlieni.
The latter birds are sometimes called Kumlien's Gull. Birds that appear intermediate between Kumlien's and Thayer's
(and they occur regularly) are called "intergrades" or, more correctly, hybrids.
An alternative perspective is that Iceland Gull has no subspecies; only nominate glaucoides is considered Iceland Gull.
The old kumlieni is merely one point on the spectrum between Thayer's and pure Iceland.
"Kumlien's Gull" is thus a term that may be applied to any of the birds within that hybrid swarm.
See Weir, D.N., A.C. Kitchener, and R.Y. McGowan. 2000. Hybridization and changes in the distribution of Iceland
gulls (Larus glaucoides/kumlieni/thayeri). J. Zool., Lond. 252: 517-530.
Special thanks to Jeff Poklen, who contributed many of these photographs. His impressive collection of gull photos can be found here.
7 categories, 4 points per category, total score: 0-28