to breed, while another also covered a similar route but ended up nesting on Banks Island. Another female migrated to the Canada–United States border, then moved back to the Gulf of Alaska, then to winter in the same border areas and then finally to both Banks and Victoria Island. Snowy owls from the Canadian Arctic were monitored to have covered an average of in one autumn then covered an average of a year later. In late winter, owls from the same area were found to have covered a mean of of ground in the tundra and spent a mean of 108 days, apparently searching for a suitable nesting situation the entire time. In no fewer than 24 winters between 1882 and 1988, large numbers have occurred in Canada and the United States. These were irruption years. Record breeding irruptive years were recorded in the winters of 2011–2012 and 2014–2015. In the 1940s, it was calculated that the mean gape in time between large irruptions was 3.9 years. Southbound movements as such are much more conspicuous after peak vole years, once thought to be separated by periods of around 3–7 years. However, more extensive research has weakened the argument that irruptions are entirely food-based and the data indicates that irruptive movements are far from predictable. This is because a statewide survey in Alaska found no statewide synchrony in lemming numbers. Therefore, rather than decline of lemmings, it is the successful productivity of several pairs that plays the role, resulting in a large number of young owls that then irrupt. However, the snowy owls cannot breed in high numbers unless lemmings are widely available on the tundra. This connection of irruptions to high years of productivity was confirmed in a study by Robillard et al. (2016). About 90% of the snowy owls seen in irruptive years from 1991 to 2016 that were ageable were identified as juveniles. Hunting Hunting techniques Snowy owls may hunt at nearly at any time of the day or night, but may not attempt to do so during particularly severe weather. During the summer solstice, the owls appear to hunt during "theoretical nightfall". Night-vision devices have allowed biologists to observe that snowy owls hunt quite often during the extended nighttime during the northern winter. Prey are both taken and eaten on the ground. Snowy owls, like other carnivorous birds, often swallow their small prey whole. Strong stomach juices