Bubo virginianus scalariventris.
Thank you to Ron Pittaway and Jean Iron for the following information:
The arctic (subarcticus) subspecies of the Great Horned Owl breeds east of the Rocky Mountains, across the boreal forest and prairies to northern Ontario. However, L.L. Snyder of the Royal Ontario Museum considered most of the northern Ontario population to be distinct from subarcticus and designated it a separate subspecies, scalariventris. Snyder's Great Horned Owl breeds in northern Ontario from near the Manitoba border eastward to the Quebec border, south to about Thunder Bay, Sault Ste. Marie and North Bay. It has distinctive gray coloration and general absence of rufous in the plumage. The facial discs are usually pale gray, sometimes with a tinge of rufous. Scalariventris is much more coldly gray with bolder darker ventral bars than subarcticus, which is whiter.
More information can be found in Ron Pittaway's article on the subspecies of Great Horned Owl: Ontario Birds Vol 11 Number 2 (64-69).