This most recent description of the call of the mystery owl sounds more
like that of a Saw-whet Owl than any of the other candidates listed so far.
In case it is, you would usually expect a lengthy series of monotonous,
whistled notes that can go on for minutes at a time without a break. The
Screech Owl will give its series of short, whistled, descending or rapidly
accelerating notes in a burst that lasts for several seconds followed by
distinct pauses before repeating the series again. The saw-whet is often
more high pitched and less mellow than a screech owl call. The Boreal is as
described, more like a snipe winnow and it has a decidedly more evocative,
even eerie quality as opposed to the rather simple notes of the saw-whet
owl. I hope this helps but I find words often too inadequate to accurately
describe what you have heard so as to create no doubt in the readers'
minds.

Rick Howie, Kamloops, BC
rhowie at netshop.net