I contacted Ontario Field Ornithologists, the folks who maintain the
officialOntario List.  The short answer was, Trumpeter in Ontario is NOT
countable.

Below the answer I got from the kind folks up north:

Dear Kurt,
 
I am afraid that your Trumpeter Swan is not countable according to the
Ontario Bird Records Committee. The  T. Swans here are still being fed over
the winter by some dedicated people. They are not yet considered
self-sustaining, though there is debate about that.
 
The T Swan is on the Ontario list because there are some records from the
1800s. Indeed there is even more hot debate in some quarters about whether
they were ever really here in the numbers that would justify the
reintroduction program.

Also, the bird did NOT orginate in Wye Marsh.  Origin uncertain, but it was
tagged in Burlington, Ontario, last winter.

Woulda been #300 in MD for me.  So it goes.

Kurt Schwarz
HowCo
goawaybird at comcast.net