During my first trip to Arctic Norway, I was really lucky to see over eleven Hawk Owls. This day flying owl is simply stunning.
Wikipedia states that there are three subspecies that exist across the northern holarctic. The North American subspecies Surnia ulula caparoch spans from eastern Alaska through Canada to Newfoundland and in some areas extends south into northern United States. The other two subspecies are found in Eurasia: S. u. tianschanica breeds in central Asia reaching Xinjiang (China) and S. u. ulula resides across Eurasia reaching Siberia at its most eastern range.
Occasionally, S. u. caparoch can extend its territory as far south as northern Minnesota and many other states in the northern United States including more central states such as West Virginia, New York, and South Dakota. These southern forays into the northern United States are rare and generally occur during winter, or following an explosion in a population of prey.
Due to its low density occurrence, sporadic fluctuations, and remote breeding locations, the northern hawk-owl is one of the least studied and poorly understood birds in North America. As a result, it is almost impossible to properly estimate its numbers.
In Yukon, northern hawk-owl densities were estimated to be between zero and six pairs per 100 km2 (39 sq mi). Despite these low densities, the North American population is thought to be fairly large given that they occur throughout the boreal forest. Duncan and Harris (1997) estimated that this population contains between 10 000 and 50 000 pairs.
Populations are known to fluctuate with cycles of small rodents and irruptions are known to occur in sub-boreal regions throughout the world. In Scandinavia, populations have been reported to vary from a few hundred birds in certain years to over 4000 birds in others and even up to 10 000 breeding pairs in optimal years. Irruptions can be used as indicators of small mammal abundance and in eastern North America, southern irruptions have been linked with low densities of red-backed voles in the high boreal forest.
In North America, over 50% of the northern hawk-owls’ breeding territory occurs in non-commercial boreal forests and as long as nothing threatens their northern habitats, no known factors challenge their existence. However, it is unknown what effects modern forestry would have on population levels because although it would decrease nesting localities, it would simultaneously create ideal habitat for Microtus prey. Fire suppression by humans is believed to negatively affect northern hawk-owl populations by reducing open areas for hunting and dead wood to nest in.
The status and conservation of this species is uncertain. A report by the Committee On the Status of Endangered Wildlife In Canada (COSEWIC) recommended that no designation be assigned for the northern hawk-owl. Compared to the nineteenth century, southern irruptions in the New and Old World appear to have declined. Also, North American populations seem to be declining, although no proper documentation exists to confirm this trend. In Canada, it was ranked 85th overall to set conservation, research, or monitoring priorities. Downes et al. (2000) considered the hawk-owl to be of medium concern, but with a high priority to improve monitoring
Lots more information and references here: http://www.inkednaturalist.co.uk/wp-admin/post.php?post=551&action=edit