Autumn populations and movements of migrant Northern Saw-whet Owls (Aegolius acadicus) at Little Suamico, Wisconsin

—Northern Saw-whet Owls (Aegolius acadicus) were once considered a “rare” bird in Wisconsin. In the 1960’s mist netting at bird-banding stations revealed Saw-whets to be a regular, uncommon migrant. Passive mist netting was initiated at the Little Suamico Ornithological Station in 1971. Our Saw-whet Owl experiences were similar to other banding stations until 1986, when we developed an “audiolure”. This technique, utilizing an amplified Saw-whet “solicitation” courtship call increased our annual catch more than tenfold. Now in use at major banding stations in the western Great Lakes area, this technique has resulted in over 2,000 Saw-whets being netted each autumn. Currently at Little Suamico 5 percent of adult owls netted have been previously banded. Approximately 40 percent of owls netted are northbound. Direct interstation recoveries reveal that Saw-whets migrate slowly at our latitude and often not in the expected southbound direction. Adults move greater distances per night than immatures. More than 200 recoveries and recaptures in subsequent years have been generated at Little Suamico since 1986. Migration dates and nightly travel distances suggest that many of the Saw-whet Owls that we encounter spend the winter in Wisconsin. Today more Northern Saw-whet Owls (Aegolius acadicus) are banded each year in North America than any other owl species. Birdbanding stations in the western Great Lakes region capture several thousand annually, mainly during autumn migration. Two major “breakthroughs” in capturing these owls have greatly facilitated these efforts. The first came from the Cedar Grove Ornithological Station located in southern Wisconsin along Lake Michigan. Mueller and Berger (1967) were the first to report that “numbers” of these owls could be captured in autumn migration with the use of mist nets left open at night. Passive mist netting has been in use at numerous birdbanding stations since the 1960’s with varying degrees of success depending upon location. The second significant breakthrough was developed at the Little Suamico Ornithological Station (LSOS) in northeastern Wisconsin. In 1986 we developed an audiolure technique, utilizing an amplified Saw-whet “solicitation” courtship call. This technique allowed us to reduce the number of nets in use by over 50 percent while increasing captures tenfold. Since 1986, over 6,000 Saw-whet Owls have been captured at LSOS and two nearby substations. Subsequently, we obtained additional encounter data on close to 400 owls. In this paper we examine some direct interstation recoveries in terms of nightly movements and habitat. STUDY AREA AND METHODS The Little Suamico Ornithological Station is located along the west shore of Green Bay, approximately 17 kilometers north of the city of Green Bay at 44 40’ N and 87 50’ W. The bayshore topography is flat, gently sloping upward to the west. The difference in elevation is so slight between the station and the bay, that during periodic episodes of high water levels, wind driven seiches have flooded the station area. Extensive marsh, shrub swamp, and deciduous swamp forest lie to the northeast, east, and 1 Associate Curator, Richter Museum of Natural History, University of Wisconsin Green Bay, 2420 Nicolet Drive, Green Bay, WI; 4815 N. Lynndale Drive, Appleton, WI; 6865 Fredrickson Road, Lena, WI; and 4093 County S, Oconto, WI, respectively.