In fact, for Boreal Owls and Saw-whet Owls, the average number of owls actually increased later in the month. Once we had eliminated the possibility that changes in survey timing had caused fewer owls to be recorded, we looked for a biological explanation. Some owl species wander such great distances, that biologists classify them as nomadic. Another interesting fact about owl ecology is that populations of northern owls, such as Snowy and Boreal owls tend to cycle in response to changes in availability of prey. Populations of mice, voles, and lemmings at northerly latitudes show dramatic population fluctuations, and their predators track these changes.
Most population cycles are about three to four years in duration.
Because the Ontario Nocturnal Owl Survey has only been operating for five years, it is difficult to distinguish whether the owl species monitored are exhibiting population cycles. Boreal Owl numbers were highest in 1997, and have declined every year since then. Northern Saw-whet Owls also declined dramatically in 2000, to the lowest levels since the beginning of the survey. To find out what s happening to Ontario s owls, we compared numbers on routes that have been run in multiple years. There was a large decline from 1995 to 1996, followed by a large increase in 1997, and then further declines in 1998 and 2000.