I would put my bets on it being a Great Horned Owl. All raptor's in the Calgary area are diurnal other than the Owl species which makes it less likely it was anything else based on the size you describe. GHO's are quite common in Calgary and regularly frequent suburban streets and back yards while on the hunt during nightfall. The only other raptor's that frequent backyards like this are the smaller accipiters like the Cooper's, Sharp Shinned Hawk or the Merlin, American Kestrel as they primarily feed on songbirds. Owl vocalizations also encompass way more than just 'hoots'.
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