Sexual size dimorphism in Thayer's Gull is significant

Last week I spend two days at the Zoological Museum Copenhagen where they have one of the largest tissue collections of birds in the world. While working my way through different topics I came across the large gull species where a male-type Thayer’s Gull immediately attracted my attention - it is the same individual in all the photos. I was struck by its impressive head and bill, but also its apparently large size compared to the argentatus Herring Gulls in the tray next to it. This reminded me of all the people I have heard talking about how they normally consider thayeri to be a relatively small and petit species. I think the literature is to blame for this rift as our current field guides don't give the impression that there is a high degree of dimorphism between sexes in Thayer’s Gull unless you evaluate the measurements.

Another thing that called for attention was the apparent shape of the nostrils. I have looked at this character before without being able to conclude its relative importance when identifying Thayer's Gull, but again I was surprised to see that even though there were not many thayeri in the museum collection, all had the same nostril shape where the rear end is very narrow and unlike what I regularly see in Herring Gulls. I think this is worth evaluating. Does Herring Gull have a more widely open nostril compared to a narrower one in Thayer's Gull just like most Caspian Gulls and also some of the Glaucous Gulls I have assessed? Future will tell whether this micro character is of any significance...

Adult Thayer's Gull male, Danish Thule and Ellesmere Land Expedition 1939-40. Specimen found at Zoological Museum Copenhagen. Note that the rear end of the nostril is very narrow

When evaluating measurements in “Gulls of North America, Europe and Asia” by Olsen & Larsson, a high degree of size variation among the larger species is apparent. There is significant variation between species, but also within and between sexes of the same species. The measures in table 1 represent the extreme values from skinned adult birds of both male and female.

	Wing	Bill	Weight
Great Black-backed Gull	445-520 mm	53.9-66.7 mm	732-2275 g
Glaucous-winged Gull	392-480 mm	46.4-64.3 mm	820-1690 g
Glaucous Gull ssp. hyperboreus	408-501 mm	50.2-69.1 mm	964-2215 g
Glaucous Gull ssp. barrovianus	414-484 mm	49.0-65.5 mm	-
Glaucous Gull ssp. pallidissimus	436-472 mm	56.3-70.0 mm	-
Iceland Gull	379-443 mm	36.3-53.5 mm	480-1039 g
Kumlien's Gull	380-442 mm	38.8-50.6 mm	530-970 g
Thayer’s Gull	384-442 mm	44.0-55.5 mm	712-1002 g
American Herring Gull	412-468 mm	44.2-62.1 mm	600-1650 g
European Herring Gull ssp. argentatus	385-480 mm	43.9-65.2 mm	717-1525 g
European Herring Gull ssp. argenteus	381-460 mm	44.4-63.9 mm	600-1150 g
Slaty-backed Gull	406-479 mm	48.4-64.6 mm	-
Lesser Black-backed Gull ssp. graellsii	383-456 mm	45.5-57.2 mm	620-1100 g

Table 1. Source: “Gulls of North America, Europe and Asia” by Olsen & Larsson 

It is obvious that Thayer’s Gull overlap with most of the larger gull species in the list and even significantly so with the five medium sized species such as American Herring Gull, European Herring Gull, Lesser Black-backed Gull, Kumlien's Gull and Iceland Gull. The weight of female Thayer’s is higher than that of male Kumlien’s which supports the more robust structure in Thayer’s

Adult Thayer's Gull in the middle between two apparent argentatus Herring Gulls. All specimens found at Zoological Museum Copenhagen

From the standard measures for adults (again with sexes combined) in “Gulls of the Americas” by Howell & Dunn an obvious overlap (often extensive) in also apparent in most characters among similar species (table 2):

	Wing	Bill	Tarsus
Great Black-backed Gull	442-511 mm	54.0-71.7 mm	67-84 mm
Glaucous-winged Gull	390-455 mm	48.7-63.8 mm	60-75 mm
Glaucous Gull (all subspecies combined)	417-494 mm	48.6-70.0 mm	61-80 mm
Iceland Gull	379-435 mm	36.7-49.6 mm	50-65 mm
Kumlien's Gull	382-431 mm	38.5-50.1 mm	51-65 mm
Thayer’s Gull	380-439 mm	42.6-55.2 mm	53-68 mm
American Herring Gull	396-458 mm	46.1-61.5 mm	56-73 mm
Slaty-backed Gull	402-465 mm	49.4-61.3 mm	60-75 mm
Lesser Black-backed Gull ssp. graellsii	380-446 mm	45.0-57.9 mm	54-69 mm

Table 2. Source: “Gulls of the Americas” by Howell & Dunn

Zimmer wrote in 1990 (in ‘The Thayer's Gull Complex’, Pp. 114-130 in ‘A Field Guide to Advanced Birding’ by Kenn Kaufman): ‘I believe that our ideas regarding what a 'classic' Thayer's Gull should look like (structurally) are based primarily on the appearance of female Thayer's. (...) With Thayer's, intersexual differences can be dramatic’. This bias can temper our analysis of birds in the field and on photos

Note that even though the Thayer’s Gull (left) has a huge bill, the gonys angle is very insignificant and flat compared to the Herring Gull (right). Note also that the Herring Gull has a more widely open nostril whereas Thayer's Gull has a narrower one just like most Caspian Gulls and also some of the Glaucous Gulls I have assessed.

Adult Thayer's Gull male, Danish Thule and Ellesmere Land Expedition 1939-40. Specimen found at Zoological Museum Copenhagen

Adult Thayer's Gull male, Danish Thule and Ellesmere Land Expedition 1939-40. Specimen found at Zoological Museum Copenhagen

Adult Thayer's Gull male, Danish Thule and Ellesmere Land Expedition 1939-40. Specimen found at Zoological Museum Copenhagen