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Mute Swan is on the Missouri checklist as a hypothetical species, along with 
Barnacle Goose, American Flamingo, Mountain Plover, Gray Jay, Bullock's 
Oriole and several others.

Birds can get on the hypothetical list if enough of the 7 MBRC members vote 
for it (I believe the number is 5). It takes 6 votes to give a bird full 
status. I believe that a member can vote  to give a bird hypothetical status 
only. Thus, hypothetical status does not mean it got 5 votes for full 
status.

There are different reasons why a bird is given only hypothetical status. In 
some cases, only one observer reported the bird, and there is no specimen, 
photo or sound recording. Such is the case of a Cassin's Sparrow that I 
reported after making meticulous notes at close range during a 20 minute 
observation. Conservative committee members usually want at least two 
independent observations and/or objective evidence to accord a species full 
status. Bullock's Oriole, Gray Jay, Black-backed Woodpecker and Mountain 
Plover, all on the hypothetical list,  probably also fall into this 
category. In some cases where the ID is difficult, a committee may believe 
two or more independent sightings are not enough.  Such is the case with 
Baird's Sparrow, which has been reported several times.  The one observer 
rule is probably a good one. I remember one case when one of Missouri's very 
best and most reliable birders reported a Red-necked Stint, only to decide 
later on that it might have been a Sanderling changing from alternate to 
basic plumage.

In the case of Mute Swan, there is no question that birds of that species 
have been correctly identified in Missouri many times. However, there is 
question whether the birds were wild or escaped captive birds. See also 
Barnacle Goose and American Flamingo, both on the hypothetical list.

Still a third category is that of Hoary Redpoll. The bird was trapped, 
banded, measured and photographed from several angles and the data sent to 
experts around the continent. As I understand it, one expert said it was 
definitely a Hoary Redpoll. Another opined that it might be a hybrid. To 
complicate matters further, Hoaries and Commons are said to hybridize in one 
area of sympatry but not in another.

Can you count a bird on the hypothetical list? I count the Cassin's Sparrow 
because I'm convinced that is what I saw, and there was no reason to think 
it was an escape or a hybrid. I would not count a Barnacle Goose because the 
odds are almost 100% that a Barnacle Goose in this part of the country is a 
not a stray from Eurasia. What about Mute Swan? The odds are pretty strong 
that the two I saw at Smithville Lake last week are not from an established 
feral population, but there is a tiny chance that they are. The odds are 
better for Mute Swans seen closer to Illinois.

It's your list.

Bob Fisher
Independence, Missouri
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The Audubon Society of Missouri's Wild Bird Discussion Forum
ASM Spring Meeting: May 1-3, 2009 in Columbia, MO.
http://www.mobirds.org