Annual Migration Density and Stopover Patterns of Northern Saw-Whet Owls (Aegolius acadicus)

Northern Saw-Whet Owl (Aegolius acadicus) Autumn Migration Magnitude and Demographics in South-Central Indiana

“…The lack of an irruption in 2003 did not agree with the four-year periodicity in other regions east of the Mississippi (Swengel and Swengel 1995, Brinker et al 1997, Whalen and Watts 2002, but was similar to the pattern observed in Idaho . Saw-whet owl irruptions may not always reach the lower Ohio River Valley, or irruptions along the east coast and Appalachians may be driven by different population factors, such as availability of specific prey.…”

“…Researchers at other banding stations on Lake Michigan and Lake Ontario captured similar proportions of hatch-year owls (ca. 60%) as we did at our south-central Indiana stations (Mueller andBerger, 1967, Weir et al 1980); however, Whalen and Watts (2002) reported a greater proportion (72.5%) of hatch-year owls on the Delmarva Peninsula, and Stock et al (2006) caught up to 82% hatch-year owls in Idaho. In 2004, the proportion of hatch-year owls caught in south-central Indiana was lower (30.9%), indicating either that saw-whet owls had a poor breeding year in 2004 or that many hatch-year owls died before reaching south-central Indiana that year (Table 3).…”

“…East of the Mississippi River, the annual fall movement of saw-whet owls southward from their breeding range in the boreal forest suggests partial migration punctuated by a four-year ''irruptive'' cycle when the number of captured saw-whet owls can be 10 times higher than the long-term average (Weir et al 1980, Swengel and Swengel 1995, Whalen and Watts 2002. However, saw-whet owl migration west of the Mississippi River did not show the four-year irruption cycle in Idaho from 1999 to 2004 .…”

“…Varying numbers of saw-whet owls stopover throughout these migration corridors, some staying the winter at sites along these routes and others continuing as far south as the Gulf of Mexico (Rasmussen et al 2008). The length of stopover is shorter during large irruptive years, presumably due to density-dependent competition for food (Whalen and Watts 2002). Because .95% of the diet of the saw-whet owl is composed of small mammals, local mammal populations may determine both the number of saw-whet owls migrating south and the duration of their stopovers (Swengel andSwengel 1995, Whalen andWatts 2002).…”

“…The length of stopover is shorter during large irruptive years, presumably due to density-dependent competition for food (Whalen and Watts 2002). Because .95% of the diet of the saw-whet owl is composed of small mammals, local mammal populations may determine both the number of saw-whet owls migrating south and the duration of their stopovers (Swengel andSwengel 1995, Whalen andWatts 2002).…”