Northern Saw-whet Owl: regional patterns for fall migration and demographics revealed by banding data

Abstract

We describe attributes of Northern Saw-whet Owls (Aegolius acadicus) during fall migration using 167,774 records from the U.S. Bird Banding Laboratory for central and eastern North America from 1929–2010. We describe movement among 18 geographic regions using records of 1,444 birds captured and recaptured between 1 September and 31 December of the same year. These data show little exchange between western Lake Superior and eastern North America. The direction of movement within a region was strongly influenced by large water bodies, varied greatly among regions, and showed high dispersal in the absence of shorelines of large water bodies. We used recent banding data to analyze population demographics from northwestern Minnesota to the coast of Maine in the north, and from southern Minnesota and southern Wisconsin to southern Appalachia and the mid-Atlantic coastal region in eastern United States. The weighted mean proportion of Hatch Year (HY) birds declined significantly from 63% in northwestern Minnesota to 42% in southern Minnesota/southern Wisconsin and from 70% in northern Ontario to 48% in southern Appalachia. Annual variation in the proportion of HY birds showed little correlation between eastern and central origin sites but moderate correlation within each of these regions. We define irruptions as years when the proportion of HY birds is 15% or greater than the weighted mean for each site, a level that occurs once in 4 years on average but at irregular intervals. We determined a very high correlation between the proportion of HY birds banded in northeast of Lake Ontario and the abundance of small mammals, suggesting a close relationship between food supply and reproductive success.