Tuesday, 14 January 2014

Glaucous Gull (Larus hyperboreus), 1cy, 12.01.2014, Hanstholm Harbour #2: with very small size

This 2cy Glaucous Gull has caused some discussion due to its overall small size. It is significantly smaller than the average Great Black-backed Gull and even smaller than the average European Herring Gull.

I acknowledge that one can get the feeling of an Iceland Gull when watching it due to is small size. However, when I had the opportunity to see in real life my initial impression was of a Glaucous Gull; a surprisingly small Glaucous Gull, but never the less a Glaucous Gull. The overall proportions, shape and JIZZ was of a Glaucous Gull with its relatively short rear end and significant tertial step; bulbous breast; strong and broad body; relatively large head with a short and rather high and stubby bill without the narrow structure of Iceland; relatively broad and rounded hand without the more narrow and pointed impression that Iceland Gull gives when flying; and posture when swimming (short, compact and swimming high on water) or standing (long legs).

People have also argued that the plumage is wrong for Glaucous Gull and have argued that the tail and undertail coverts could cause concerns as to whether there is some Herring Gull influence. Could it be a Viking Gull (European Herring Gull x Glaucous Gull-hybrid)? I don't think so and believe that the pattern on the upper tail and its coverts are within accepted variation for Glaucous Gull even though the white spacing between the brown undertail bars are broad and the bars rather narrow.

The sexual dimorphism is often significant (see measurements below) and just to illustrate that please have a look herehere; herehere and here just for a brief impression and herehereherehereherehere for one with a round head.

The tail feathers have a pattern not that dissimilar to this 2cy Glaucous Gull.
I would argue that the pattern on the rump and undertail is within accepted variation for Glaucous Gull even though the white spacing between the brown undertail bars are broad and the bars rather narrow. The pattern is not that dissimilar to this 2cy Glaucous Gull.
Note that the spacing between primary tips and that tail tip falls between P7 and P8 as is identical to the normal sized Glaucous Gull in the photo below.
Note that this normal sized 2cy Glaucous Gull has an identical primary projection to the small sized gull in the other photos.
Note that in this 2cy Iceland Gull the tail tip falls even with the tip of P7 and not between P7 and P8 as on the two Glaucous Gulls in the two photos above.
Note that shape, proportions and micro characters in head and bill on this small Glaucous Gull is matching this normals sized individual.

Our current field guides don't give the impression that there is a high degree of dimorphism between sexes in several species of large gull unless you evaluate the measurements. So let's do that. In “Gulls of North America, Europe and Asia” by Olsen & Larsson, measurements document that there is high degree of size variation among the larger species. 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.

It is obvious that Glaucous Gull overlap with most of the larger gull species 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.

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 

From the 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).

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


Lars Norgren said...

It is sobering to compared variation of weight within species. Every species listed in this posting shows the largest individual to be at least 100% heavier than the smallest and in some cases 150% heavier.

Kent Olsen said...

Thanks Lars for highlighting that point. Sexual dimorphism can be significant