Hi All,

Twice this winter and in widely-separated locations in the Greater Vancouver
BC Canada area, I've located Basic 1 or Basic-plumaged Common Loons/Great
Northern Divers Gavia immer with head, neck and back plumage features
supposedly diagnostic of Yellow-billed Loon/White-billed Diver G. adamsii.

In each instance, the bird had a pale sandy-brown neck with a narrow darker
brown stripe running down the rear neck from a reduced dark brown crown and
a pale sandy face with a distinct crescent-shaped auricular mark. Ignoring
the bill, from the base of the neck up, these two birds had features typical
of Basic-plumaged adamsii.

In one instance, at Crescent Beach, just SE of Vancouver, the bird showed a
pale brownish back as adamsii but with the small rows of squares typical of
juv/Bsc 1 immer; this bird even carried its bill tilted up and at anything
but fairly close range resembled a First Basic adamsii so closely that it
was only seeing the bill that made me realise it wasn't that species.

In the other, just off the N side of Stanley Park in Vancouver itself a
couple of days ago (with a bona fide adamsii, perhaps two, a couple of
kilometers to the W, too far for direct comparison), the bird was in the
initial stages of pre-Alternate molt from what looked like a warm brownish
Basic plumage as adamsii rather than the typical cold dark `grey-brown Basic
plumage of immer. This bird carried its bill as immer. In each case, the
bill was completely typical of Basic or First Basic immer in shape and
color, being bluish-grey to grey-white with black culmen and tomus.

My queries are:

1.
How likely is hybridism as the cause of these poossible cross-over
characteristics?

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?


Any help and comment welcome.


Michael Price A brave world, Sir,
Vancouver BC Canada full of religion, knavery and change;
mprice at mindlink.net we shall shortly see better days.
Aphra Behn (1640-1689)