Saturday, January 30, 2010

Does size matter? Part 1

The age old question.

In Larusology I think nothing can lead you further down the garden path than placing to much importance on the size of an individual bird. It strikes me that at least in the east there is very significant variation in size among the mid-sized Larus sp. (Herring, Lesser-black Backed, Thayer's, Iceland) with seemingly less variation in the larger sp. specifically Great Black-backed and Glaucous.

On checking this impression with the measurements in Olsen & Larrson, Gulls of North America, Europe and Asia I found partial validation for my theory with wide variation in all the regular large gulls. The following are the extremes in measurement using skins of various species considering both sexes (with the exclusion of wt. for Herring& Lesser Black-backed with only females included).

Great Black-backed Gull  Wing 445-520mm Wt. 1033-2272g
Glaucous Gull                   Wing 408-510mm Wt. 946- 2215g
American Herring Gull     Wing 412-468mm  Wt. 600- 1240g
Lesser Black-backed Gull  Wing 383-456mm  Wt. 620-1100g
Thayer's Gull                     Wing 384-442mm  Wt. 712- 1002
Kumlien's Gull                  Wing 380-442mm Wt. 530-970g

As you can see the largest Kumlien's Gull overlaps with the smallest Glaucous with broad overlapping of the four mid-sized species. Surprisingly (at least to me) the mean wing length for male Kumlien's was near identical to male Thayer's. The weight for a female Thayer's was however significantly higher than even the male Kumlien's attesting to its more robust structure.

The centre bird in the following picture is a great example how size of an individual bird can cause significant confusion in determining its identity. Initially I thought this was a Glaucous Gull but then I noticed the dark pigment on the primaries. I then started thinking Nelson's Gull especially as the primary projection seemed a bit short. Finally after extensive study both in the field and of the pictures and after going back and forth on a couple of occasions I came to a final definitive conclusion that it was a Kumlien's Gull

A very large adult male Kumlien's Gull

If just this one picture was available you would have a very hard time convincing me this is a Kumlien's Gull. It was as robust as most of the Herrings and larger than many. Its head is rather Herring Gull like in this posture.


Large adult male Kumlien's Gull with seemingly short primary extension

Then I looked hard at the head and bill structure in a few of the shots and I thought- this has to be a Kumlien's Gull.



Adult Kumlien's Gull head close-up




Juvenile Nelson's Gull head close-up




Adult Glaucous Gull head close-up

The top picture shows the classic dove-like head with relatively small bill so typical of Kumlien's Gull.

After reviewing the open winged shots it was clear P10 was not yet fully grown and this probably accounted for the shortish primary extension. It was also entirely clear that the primary pattern was near identical to Howell & Mactavish, 2003 -variation e wingtip pattern for adult Kumlien's Gull- Alula 9: 2-15. The illustration of these variations by Martin T. Elliot can be seen in Howell &Dunn pg. 252.




Adult Kumlien's Gull open-winged




Adult Kumlien's Gull in flight

This left no doubt in my mind that this was a Kumlien's Gull - just a real hefty one.  Even though I had noted in the past that Kumlien's Gull  was quite variable in size with the odd bird of Herring Gull size this bird through me off and it took quite a bit of study before I was convinced of its identity. The main variable that created the confusion was clearly its size

If this bird had conformed to my mental image of the classic size and structure of Kumlien's Gull (as do the following birds) I very much doubt I would have had a moment of doubt.


Second winter Kumlien's Gull


Adult winter Kumlien's Gull


Third winter Kumlien's Gull



Adult winter Kumlien's Gull




Second winter Kumlien's Gull

We need to remember that not all birds are going to be "classic" and a significant number of outliers
are waiting to lead you down the garden path. Following are a few more robust Kumlien's.

Juvenile Kumlien's gull


Second winter Kumlien's Gull


Second winter Kumlien's Gull



Juvenile Kumlien's Gull



Juvenile Kumlien's Gull       First winter Herring Gull

In comparing the first and second groups of Kumlien's Gulls there is clearly structural differences with the later group having more Herring Gull like heads and a somewhat more generous bill size. My suspicion is this is entirely based on sex the former birds being female and the later male. 

I will continue this theme in my next post by examining the variation in size and structure of Thayer's Gulls.

To be continued............

Kirk

zufelt_k@shaw.ca

2 comments:

  1. Hey Larusologist,

    Great photos. It seems if you study a photo too hard you lose perspective. Your "big" Kumlien's Gull is certainly not that species - not even close. It is without doubt a Herring x Glaucous Gull. You must be aware that Glaucous Gull bill size has considerable variation among races, as well as sex. Your gull's bill is well within normal variation for Glaucous.

    Jake.

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