Great Black-backed Gull

Eugene and Nancy Hunn enhunn at comcast.net
Fri Jan 16 20:47:17 PST 2004


Tweets,

Seems I started something!

I've been out of town since Wednesday so am just getting caught up on the discussion.

First of all, many thanks to the photographers, especially indomitable Ruth.

I haven't been able to see all the pictures nor review the Brits critical comments but just for the record I'd like to submit the following comparative measurements which I took from my old copy of P.J. Grant's _Gulls_ book. The quite substantial size difference between the Great Black-backed and even the largest Glaucous-wingeds next to it struck me forcefully. This was particularly apparent with respect to both the standing height of the bird, the length AND depth of the bill, and especially the breadth of the chest. 

I took the average of the maximum and minimum measurements cited for wing, bill, and tarsus for Great Black-backed, Glaucous, Glaucous-winged, Slaty-backed, and Herring, then calculated the ratio of each average measurement to that for Glaucous-winged Gull, our local standard.

Great Black-backed/Glaucous-winged: wing 1.15, bill 1.14, tarsus 1.14; Glaucous/Glaucous-winged: wing 1.09, bill 1.09, tarsus 1.02; Slaty-backed/Glaucous-winged: wing 1.03, bill 0.96, tarsus 0.96; Herring/Glaucous-winged: wing 0.99, bill 0.91, tarsus 0.91.

With due allowance for the small samples and the extensive variation within each, we could say that the GBBG is ca. 15% larger in linear measurements on average than a Glaucous-winged. Glaucous almost splits the difference, except that it seems to be relatively short, while Slaty-backed is 4% smaller in bill and tarsus but slightly longer winged, while Herring is 9% smaller, except for longer wings, than Glaucous-winged. However, Glaucous-winged and Great Black-backed Gulls overlap on bill and tarsus measurements, so that a large (male?) Glaucous-winged might be nearly or quite as tall and large billed as a small (female?) Great Black-backed. However, contra Sibley, there is essentially no overlap between Great Black-backed and Herring on these measurements.

Also, with respect to Glaucous Gulls, I believe that we normally get the western race, barrovianus, which is rather smaller than the nominate race, with a more slender bill and longer wings. In my experience, our Glaucous Gulls are not noticeably larger than our Glaucous-winged and often appear less heavy about the bill.

In any case, a Herring x Glaucous or Slaty-backed x Glaucous would likely be both smaller and certainly much paler on the primaries, etc., than this bird. As for the Yellow-footed Gull from Baja, which averages larger than Glaucous-winged (Grant gives no measurements for that species). It's an intriguing suggestion that I considered first off, but it would have to have been a first winter bird of that species and such birds are not described as having such contrasting coverts. 

Also, the AOU cites a British Columbia record of Great Black-backed. What are the details of that record?

Gene Hunn.
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