[Tweeters] Re: Glaucous-w. X Western Gull (hybrid)

Jason Hernandez jason.hernandez74 at yahoo.com
Sat Jan 9 19:51:16 PST 2016

> Since we live in the middle of the Olympic Gull hybrid swarm, in which obvious hybrids have backcrossed over many generations with supposedly pure individuals, even those birds that look like pure Glaucous-winged Gulls may not be pure Glaucous-wingeds.

The question then arises, how much does it matter ecologically? If Western, Glaucous-winged, and Olympic Gulls do in fact occupy distinguishable ecological niches, then from an ecosystem perspective, any given gull would be best identified in terms of which niche it occupies. If an apparently-pure Glaucous-winged also occupies the niche of the Glaucous-winged, it may not make any practical difference whether or not it is truly pure. It is rather akin to the situation with humans of European descent, who have been found to carry varying amounts of Neanderthal DNA. But does that mean we should hesitate to identify such humans as definitive Homo sapiens and speak of "Homo sapiens and/or Homo sapiens X Homo neanderthalensis"? That is unnecessary splitting of hairs.
It think I have worked out a satisfactory (to me) system for sorting out Western, Olympic, and Glaucous-winged Gulls, and it is essentially the same as what Kevin Purcell recommends. My difficulty is the Thayer's (if indeed there are any in the mix). My two field guides disagree: Peterson's shows a Thayer's that looks essentially like an Olympic, i.e. intermediate gray mantle and somewhat darker gray tips. American Bird Conservancy, on the other hand, shows Thayer's with black tips, and indeed very difficult to tell apart from a Western or a Herring based on their rather incomplete descriptions. I think I have erroneously-attested Thayer's Gulls in my prior years' field notes, because I was working only from Peterson's, which does not include Olympic Gull.
Jason Hernandez
jason.hernandez74 at yahoo.com

Date: Fri, 8 Jan 2016 15:43:12 -0800
From: Kevin Purcell <kevinpurcell at pobox.com>
Subject: Re: [Tweeters] Re: Glaucous-w. X Western Gull (hybrid)
To: Tweeters <tweeters at u.washington.edu>
Cc: Kevin Purcell <kevinpurcell at pobox.com>
Message-ID: <97829B09-2A2D-4B65-AD7B-CA61DC86D5C4 at pobox.com>
Content-Type: text/plain; charset=windows-1252

I think Bob Boekelheide gives good answer but it's probably best to consider counting the obviously GW (wingtips same medium grey as mantle) and obviously WG (dark grey mantle and black tips) and then count GWxW intergrades (with non-matching dark grey to blackish tips) as all the rest.
The Condor 98: 527-546 (1996)
Genetic Differentiation, Geographic Variation and Hybridization in Gulls of the Larus Glaucescens-Occidentalis Complex

Bell finds wingtip color and mantle color to be (statistically) the best features to discriminate intergrades from the two "pure" species.

I note Sibley uses the same approach for differentiating hybrids from in 2nd edition on p222 "Identification of Hybrid Gulls". Compare his diagnostic callouts with p213 and p215 for the GW and WG to the hybrids on p222 (and I think he did this in the 1st edition too, IIRC).

I think that's what eBird (and the CBC too) have settled on too.

How well birders can do it is another matter, of course. In writing up the counts for CBC data I see people have different biases for this count e.g. I tend to count our local gulls with "too many hybrids" but other tend to count only GW gulls (and no hybrids).

It's easier to make this judgement for a standing bird. In flight it's a bit more tricky.

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