Northern Saw-whet Owls begin migrating as deciduous foliage nears peak color and ends approximately 1 month after leaf fall. Thus, migration begins during mid-September north of 45 degrees north latitude, late September to early October north of 40 degrees, and mid to late October south of 40 degrees. South of 36 degrees north latitude migration probably begins during early November. In Maryland, during most years, we operate from mid October to 1 December. Moon phase is thought to have some effect on migration.
Brighter nights seem to yield fewer owls.
This may be due to the increased light of a fuller moon making the nets more visible. It might also be that the owls are avoiding predation from other owls by flying high or not migrating. Spring migration has been studied much less. Wintering birds have departed Maryland by 1 April. High winds, precipitation and fog seem to limit migration.