Newsgroups: rec.birds
Path: utzoo!snell
From: snell@utzoo.uucp (snell)
Subject: What is "Thayer's Gull" ?
Message-ID: <1988Nov11.185810.22941@utzoo.uucp>
Organization: U of Toronto Zoology
Date: Fri, 11 Nov 88 18:58:10 GMT

Recently, mjm@oliven.olivetti.com (Michael Mammoser) and
gpasq@picuxa.UUCP (Greg Pasquariello X1190) having been having an 
interesting discussion of "Thayer's Gulls", and characteristics which
might be used to identify these birds.  For the past four years I have 
been working on a PhD at utzoo which deals with hybridization and 
genetic differentiation between Herring, Glaucous, Iceland and 
"Thayers" gulls.  I have collected large numbers of specimens of 
all these "taxa", on their breeding grounds, including colonies
on Baffin Island, Iceland, Spitzbergen, Norway, southern Europe
and the Canadian maritimes.  

I have been impressed by the variability in plumage and eye-colouration
in these birds. These gulls are far more variable, morphologically, than 
one would assume from reading field-guide descriptions, and more variable than 
than one might assume even from reading P. Grants book on gull identification.  
Unfortunately, the more northern forms (especially Iceland and "Thayer's")
breed in colonies which are difficult to reach; not a great deal is known
about them.  Certainly, their systematic relationships have been hotly
debated, and are not yet fully resolved.  As well, these birds are
only a portion of a much wider complex of white-headed gulls which
frequently hybridize with each other.

A problem with published descriptions of "Thayer's Gull" is that
they are inconsistent and largely based on winter collected specimens.
Few people have ever seen "Thayer's Gull" on their breeding
colonies.  Virtually none have ever been banded on their high arctic 
colonies, either as chicks or adults.  As far as I am aware,
not a single such banded bird (if any ever existed) has ever been
recovered.  As well, not even one "Thayer's Gull" has ever been banded in  
the south and later recovered at an arctic breeding colony.  Perhaps  
those 1st and 2nd year birds sighted in the south really are "Thayer's Gull"
but there is no particular reason to believe it.  There is no evidence
that the various 1st and 2nd year "Thayer's Gulls" in museum collections
(the birds on which descriptions are based) actually were hatched in 
cliff colonies in the Canadian high arctic.  It is generally assumed 
that that was their origin, but descriptions based on such 
assumptions tend to become self-fulfilling prophecies.  It may be that
many of those small young birds are merely youthful 
variants of the more southerly Herring Gull.  I would argue that there is 
little evidence to reject that idea.

So, what is a "Thayer's Gull"?  Almost certainly it is merely a
plumage variant of a group of northern gulls (the Iceland Gull complex)
which preferentially breed together.  Call it hybridization if you
like, but "Thayer's Gull" as well as "Kumlien's Gull" are but
variants of Iceland Gulls.  This complex varies clinally from
east to west, with lighter birds more common in the east, and darker
ones more common in the west.  There are colonies where the very light
and very dark winged ones breed together (especially on Baffin Island),
and at those colonies there is considerable interbreeding between
birds of the various plumages (see E. Godfrey's discussion of this
in the revised Birds_of_Canada, 1986).  The plumages of breeding birds 
(and presumably wintering ones too) are so variable that each of these 
groups blends into the other.  There is no good evidence that 
"Thayer's Gull" is a `good' species.

When people see a gull which they might want to call "Thayer's Gull"
they compare that bird to pictures in books and to the self-perpetuating
descriptions of this supposed species which accompany them.  
The birder finds a pigeon-hole, and stuffs it.  Tick!
I am not intending this to be a diatribe against tickers, nor against
anyone who has ticked "Thayer's Gull" on their list.  However, the process
of ticking presupposes that one can assign a bird to a specific group
which is clearly defined, and biologically meaningful (the pigeon-hole).
Usually, the pigeon-holes are thought to be `species'.  Not always.
I am quite happy to tick different morphs of birds (e.g., I have ticked 
the three morphs of Parasitic Jaegers).  However, whereas the borders 
around the pigeon-holes for the plumage morphs of Parasitic Jaegers are 
clear, they are extremely fuzzy for the plumage variants of the 
Iceland Gull.  

There was a time when I too had ticked "Thayer's Gull";
I had seen a `typical' bird, so I thought, sitting at a garbage dump 
up at Churchill, Manitoba, a long way from the breeding range.
Since then, I have also seen classic "Thayer's Gulls" at their 
breeding colonies on Baffin Island.  I have also seen, at those colonies,
the range in plumage from the "Thayer" bird right through the
classic "Kumlien" to the classic "Iceland".

-- 
Name:   Richard Snell
Mail:   Dept. Zoology, Univ. Toronto, Toronto, Ontario, Canada    M5S 1A1
UUCP:   uunet!attcan!utzoo!snell 
BITNET: snell@zoo.toronto.edu