I would still be grateful if anyone had any info about how normal/unusual it is for GHOs to be seen in big cities.
No, it is not that unusual for them to be spotted in cities. Great-Horned Owls ("GHO") can be found almost anywhere (Cities, deep forest, deserts, etc.)
As for food, GHO have a menu of 250+ species, which include mice, voles, insects, fish, rabbits, cats (under 12 pounds), small dogs, rattlesnakes, ducks, geese, and even foxes. Their favorite menu item is rabbits. Last year I witnessed a GHO (south of Coxsackie NY - 250 miles south of Montreal - loaded with GHOs and other owls) attempt to take a tom turkey. That was quite a sight! GHOs are on the top of the food chain. Their only predators are humans, mature bobcats, and mountain lions.
GHO start to nest in the December/January time frame.They usually use a productive nest for 3-5 years before moving on to a new spot. If it is not productive, they will not use it again.