Sunday 27 January 2013

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

1 comment:

Mars Muusse said...

Hi Kent,
Well done, you really put a lot of effort and energy in this bird. One day I think we will all agree you made a fine set of images of a beautiful bird, apparently hard to proof in Europe.

Mars