[Fwd: Re: [BIRDWG01] Gavia immer/adamsii]
Tyler Bell (bell@say.acnatsci.org)
Wed, 25 Mar 1998 08:55:57 -0800
This is a multi-part message in MIME format.
--------------EF145713619
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
MDOspreyers:
Here's an informative post that came across on BirdWG01 (ID Frontiers).
Thought anyone chasing the YBLO might find it educational.
--
Good Birding! ...and all this science,
Tyler Bell I don't understand, It's
mailto:bell@say.acnatsci.org just my job five days a week.
California, MD Elton John (Rocket Man)
http://www.anserc.org/
--------------EF145713619
Content-Type: message/rfc822
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Mime-Version: 1.0
Content-type: text/plain; charset=US-ASCII
Content-transfer-encoding: 7bit
X-Mailer: AOL 3.0 16-bit for Windows sub 41
Message-ID: <5b97f65d.35184651@aol.com>
Date: Tue, 24 Mar 1998 18:48:31 EST
Reply-To: Jrhough1 <Jrhough1@AOL.COM>
Sender: NBHC ID-FRONTIERS Frontiers of Field Identification
<BIRDWG01@LISTSERV.ARIZONA.EDU>
From: Jrhough1 <Jrhough1@AOL.COM>
Subject: Re: [BIRDWG01] Gavia immer/adamsii
To: BIRDWG01@LISTSERV.ARIZONA.EDU
Dear Frontiers,
Michael Price posted a message concerning 'pale', adamsii-looking birds in the
Vancouver area. While I do not have in-depth answers to any of his queries I
would like to add a few comments based on personal experience.
Michael's queries are:
1.How likely is hybridism as the cause of these poossible cross-over
characteristics?
I have only recently relocated to the US from the UK, but I am unaware of any
hybridism between these two species from European quarters. I suspect that the
case may be similar in the US.
2.
Is this a normal stage for some immer? That a certain percentage of Basic
immer is going to resemble adamsii due to feather-wear, etc?
Here in the east, I have had the opportunity to study more immers than in the
UK and I have seen several birds matching the Vancouver individuals. As
Michael intimates, these immers are probably in their most adamsii-looking
state at this time of year due to moult and feather wear. At this time of
year, many basic immers are completing their partial, post-juvenil moult
(involving head and body feathers) and according to the literature (Appleby,
Madge & Mullarney 1986) is said to be quite protracted. Thus, I suspect that
if you spent time looking hard at immers at this time of year, 'pale' birds
may be more prevalent than we realise.
Then how diagnostic are these features? and why don't we see them more often
on the other species?
Young immers, specifically those late in their 2nd Calendar-year (Feb-March),
are likely to be the ones which are confusable for similarly-aged adamsii.
Although adamsii are typically paler on the sides of the head and neck
than most worn immers, other features to note on adamsii which separate them
from unusually pale immers are;
i) Adamsii often shows more white around the eye than immer which extends
forward over the gape rather than meeting the gape as in immer. This
difference can be likened to the difference seen in this area between the caps
of summer-plumaged Arctic and Forsters Tern for example. It gives adamsii a
paler, 'blank-faced' look whereby the 'dead' eye stands out like a Great White
Shark (!)
ii) When loons/divers frequently rise from the water to flap their wings, it
may be possible to see the pale primary shafts of adamsii, as oppsed to the
darkish ones on immer.
iii)General proportions, contour of the mantle, thickness of the neck and the
bill colour/shape are all important when confronted by tricky individuals
mentioned above.
3.
Could this be characteristic of certain populations which make it to the
Pacific Coast only rarely?
Having seen birds that come close to the birds described, I doubt it is a
charactersitic of certain populations reaching the Pacific, but perhaps just a
manifestation of plumage variation within the age-class of this species.
4.
Have there been similar problematic birds in other parts of North America?
In Europe/UK?
Of the three individuals that I have observed in the UK, all presented a
fairly straightforward identification. Other than being fooled by light and
distance, well observed birds showed all the typical features of a winter
adamsii.
5.
Is there a counterpart situation in which some adamsii display plumage
features of immer?
Not that I am aware of.
Finally, to stray to another pitfall concerning immers, I have come to realise
that here in the east, some waif-like individuals are a pitfall for arctica.
I have done a double-take on some individuals that have shown slim necks
and a rather smoothly-contoured forehead and crown. In some lights these birds
can look very black and white and often impart the impression of the 'cobra-
like' head shape of arctica. Similarly, I have seen some arctica with a
'knobbly' forehead. These small, round headed immers can be troublesome.
Julian Hough, CT
2.
Is this a normal stage for some immer? That a certain percentage of Basic
immer is going to resemble adamsii due to feather-wear, etc? Then how
diagnostic are these features? and why don't we see them more often on the
other species? In Vancouver, a *lot* of immer are present here in every
month in all plumage-states but downy--wintering birds, migrants, summering
subadults--while adamsii is a 'regular' rarity in migration and winter in a
similar range of plumages from juvenile to Definitive Alternate and Def.
Basic).
3.
Could this be characteristic of certain populations which make it to the
Pacific Coast only rarely?
4.
Have there been similar problematic birds in other parts of North America?
In Europe/UK?
5.
Is there a counterpart situation in which some adamsii display plumage
features of immer?
--------------EF145713619--