Tweeters,

As a follow-up to the message I posted a few days ago on TWEETERS,
entitled "Thayer's Gull populations underestimated", here is a
response from Don Roberson, and a further response from me on problems
of identifying Thayer's and how they can affect population estimates.
Both these messages have been posted on the "ID-Frontiers" group.

Wayne C. Weber
Kamloops, BC
contopus at home.com


----- Original Message -----
From: WAYNE WEBER <contopus at home.com>
To: DON ROBERSON <creagrus at montereybay.com>
Cc: IAN PAULSEN <ipaulsen at linknet.kitsap.lib.wa.us>; PHIL PICKERING
<philliplc at harborside.com>; RON PITTAWAY <jeaniron at sympatico.ca>
Sent: Tuesday, February 13, 2001 12:26 AM
Subject: Re: [BIRDWG01] THAYER'S GULL POPULATIONS UNDERESTIMATED (fwd)


> Don,
>
> Thanks very much for sending me a copy of your message to ID
> Frontiers. Here is a response, which you are welcome to forward to
ID
> Frontiers if you wish. I should probably join the group, but I'm
> reluctant to add more to my already horrendous volume of E-mail.
>
> You make a valid point in noting that Christmas Bird Counts of
> Thayer's Gulls include immatures (1st, 2nd, and 3rd winter birds) as
> well as adults-- and thus are not directly comparable to breeding
> populations. However, for most four-year gulls, because they tend to
> be long-lived, adults make up well over half of the midwinter
> population. (I will not try to document that statement, but I
believe
> there are published papers which would support that point of view.)
>
> In the case of Thayer's Gulls, I suspect that 80% of more of the
birds
> counted on Christmas Bird Counts are adults, and that many or most
of
> the immature age classes end up getting lumped in the "gull species"
> category. At least, I believe this is true in British Columbia,
which
> may host more than 50% of the world population of Thayer's Gulls in
> winter.
> First-winter Thayer's Gulls, which should be much more numerous than
> 2nd or 3rd winter birds, are very similar in plumage and bill color
to
> first-winter Glaucous-winged Gulls. They are most easily
distinguished
> by structural characters-- smaller size, smaller and rounder head,
and
> especially the shorter and slimmer bill.
>
> Thayer's Gulls are not usually found in pure flocks in coastal B.C.,
> but intermingle with other species. They usually make up a small
> percentage of the flocks of thousands of gulls which roost for much
of
> the day in the fields near the main Vancouver landfill in Burns Bog,
> Delta. (The wintering gull population in the Fraser River estuary
and
> vicinity is around 50,000 birds, of which at least 75% to 80% are
> Glaucous-wings.) When I scan a flock of 2000 to 5000 gulls, the
adult
> Thayer's usually are picked out easily, and an estimate can be made
of
> the number of adult Thayer's. However, the immature Thayer's are
hard
> to pick out from similar age classes of Glaucous-wings when mixed in
> with a large flock. I should probably apply a correction factor to
> estimate the total number of Thayer's in such a flock, but I have
not
> usually done so. I suspect that many other B.C. birders tend to do
the
> same thing.
> The numbers of Thayer's that I record, therefore, are usually
minimum
> counts, based mainly on adults.
>
> Taken individually, immature Thayer's of the various age classes are
> not necessarily that hard to identify. However, when mixed with
flocks
> of hundreds or thousands of other gulls, under the time pressure of
a
> Christmas Bird Count, where the emphasis tends to be on tallying
> species and making rough counts or estimates of individuals, I
believe
> that most immature Thayer's get lumped into the "gull species"
> category or the Glaucous-winged total.
>
> If this is the case, and the Thayer's gull tallies for the 10
highest
> CBC circles that I cited consist of 80% or more adult birds, the
> totals should probably be adjusted upward from about 4500 Thayer's
to
> at least 6000 or 7000, to include immature Thayer's which were
> overlooked or misidentified during the counts. And that's only for
10
> CBC circles!
>
> Because the problem of estimating global populations of Thayer's
Gulls
> is intimately intertwined with problems of identifying the species
> (especially immature plumages), I would suggest that it probably is
a
> suitable subject for discussion on ID Frontiers.
>
> Wayne C. Weber
> 114-525 Dalgleish Drive
> Kamloops, BC V2C 6E4
> contopus at home.com
>
>
>
> ----- Original Message -----
> From: Don Roberson <creagrus at montereybay.com>
> To: ian paulsen <ipaulsen at LINKNET.KITSAP.LIB.WA.US>;
> <contopus at home.com>; <birdwg01 at listserv.arizona.edu>
> Sent: Sunday, February 11, 2001 5:55 PM
> Subject: Re: [BIRDWG01] THAYER'S GULL POPULATIONS UNDERESTIMATED
(fwd)
>
>
> > This may not be the right forum to discuss concepts of estimating
> breeding
> > populations, but I want to add a supplemental thought to the Wayne
> Weber/Ian
> > Paulsen post which suggests that the breeding population of
Thayer's
> Gull is
> > grossly under-estimated in the paper recently cited here.
> >
> > Obviously, Christmas Bird Count numbers contain numerous
> non-breeders (years
> > 1-3 plus likely later age classes) and perhaps not even all "full"
> adults
> > breed every year. I do not disagree that the 8000-12000 breeding
> population
> > estimate seemed low but a direct comparison of winter numbers and
> summer
> > numbers is not possible for this reason.
> >
> > There are other dramatic examples where recent breeding estimates
in
> the
> > literature do not match non-breeding numbers, particularly in
> seabirds. The
> > just-published Birdlife International "Threatened Birds of the
> World" lists
> > the breeding population of Cook's Petrel at 50,000 pairs but the
> numbers in
> > the eastern tropical Pacific (ETP) are much greater if transect
> numbers can be
> > extrapolated over any significant space of ocean (pers. obs.).
Juan
> Fernandez
> > Petrel makes the list of the world's 1186 most-threatened birds
but
> it is the
> > commonest species in fall across vast stretches on the ETP. Most
> dramatically,
> > Pink-footed Shearwater is listed at ~30,000 breeding pairs (max
est
> for all 3
> > island groups on which it nests) but numbers off western U.S. are
> orders of
> > magnitude higher. [In contrast, Buller's Shearwater is listed at
2.5
> million
> > birds but in our area is much less common than Pink-footed.]
> >
> > What I suspect this means is that for many seabirds (and gulls?) a
> substantial
> > chunk of the world's population does not breed annually, and
> therefore one
> > cannot estimate a world population from just counting habitat and
> nesting
> > densities at breeding colonies. Therefore "breeding population
> estimates" do
> > not equal total population estimates.
> >
> > Don Roberson
> > Pacific Grove CA
>