In his book, Peter Dunne writes about distinguishing Arctic from Pacific:

"In all plumages, Arctic Loon shows a triangular or wedge-shaped white
flank patch bulging prominently above the water line just in front of
the tail.
All other considerations....are virtually insignificant. Just use the
flank patch. It works.
In winter, however, Pacific Loon often shows a shadowy 'chin strap'.
Arctic does not....
NOTE: If the demarcation between the dark upper parts and white
underparts is a straight, horizontal line, the bird is Pacific. If the
white flank swells above and behind the wing, it's Arctic. If you're
not sure, it's Pacific."


Gregory Askew wrote:
> Just an idea...not having seen the birds in question myself...but in the
> full-length Sibley there's a head shot of what seems to be a different
> variation of immature plumage in the Pacific Loon...lacking the chin strap
> of the adult winter and regular immature plumage and resembling perhaps a
> slightly smaller version of an Arctic Loon. There hasn't been any discussion
> as of yet concerning positive identification of these birds. At the very
> least, two Pacific Loons would certainly still be worth the trip.
> G
> Vergennes
>
> On 11/7/07, Taj <[log in to unmask]> wrote:
>
>> This sighting doesn't seem to be generating the excitement it
>> deserves. As far as I know, yesterday at least one, possibly two, very
>> experienced birders refound the probable Arctic loon sitting alongside
>> another loon which they ID'd as a Pacific Loon. There is potentially a
>> Pacific and an ARctic loon up on the Lake! I can't get up to the lake until
>> this weekend, but i would strongly encourage birders to look for this bird
>> and document it as best they can. As far as i know no one has taken photos
>> of this bird. There are no accepted sightings of Arctic loon anywhere on the
>> east coast.
>> This is a great find by David Hoag and let's hope it sticks around long
>> enough for more birders to see it.
>> Good birding.
>> Taj
>>
>>
>>
>>
>>
>> Taj Schottland
>> Putney, VT
>> ---------------
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