Increasing mist net captures of migrant Northern Saw-whet Owls ( Aegolius acadicus ) with an audiolure

Abstract

—In 1986, an “audiolur e” was developed and tested at the Little Suamico Or nithological Station near Gr een Bay, Wisconsin. The audiolur e consisted of br oadcast amplified “solicitation” calls of Norther n Saw-whet Owls (Aegolius acadicus) nor mally associated with courtship activities. Fr om 1971 thr ough 1985, using passive mist netting at Little Suamico, the mean number of owls captur ed each autumn was 57. Using an audiolur e, from 1987 thr ough 1995, the mean annual catch was 636 owls. During 1989, an audiolur e was also tested at Finzel Swamp in western Maryland by comparing captur es during adjacent 3-hour time blocks. Use of an audiolur e significantly incr eased capture rates over passive mist netting. Audiolur es are now being used by all major Norther n Saw-whet Owl banding stations in the wester n Gr eat Lakes area to net 2,000-3,000 owls each autumn. Audiolur es have also proven effective for netting saw-whet owls during both the br eeding and wintering seasons. Descriptions of the audiolur e and basic characteristics of captur es at simple autumn migration netting operations ar e also presented. Each year mor e Norther n Saw-whet Owls (Aegolius acadicus) are banded in North America than any other owl species. In the Great Lakes region 2,000-3,000 ar e banded during each autumn migration. It has been assumed that these small owls, like most diurnal raptors, ar e reluctant to cr oss large bodies of water and that they concentrate along certain shor elines. Most bir d watchers, unfamiliar with the r esults at banding stations, still consider saw-whet owls rare and a “great find.” Taverner and Swales (1911) suggested that Norther n Saw-whet Owls were migratory during the early 1900s. The first published attempt at capturing and studying numbers of migrant Norther n Saw-whet Owls came from the Cedar Gr ove Ornithological Station located along the Wisconsin shor eline of Lake Michigan. Mueller and Ber ger (1965) reported that “numbers” of these then “rar e” owls could be captur ed with mist nets left open at night. Using that technique they documented a pronounced autumn migration during October and November, confir ming T averner’s (1911) suggestions. Since the 1960s many other banding stations have tried mist netting migrant owls, with varying degrees of success. The use of mist nets to captur e migrant owls has been a passive technique. A general rule of passive mist netting is that to captur e more individuals, one must operate mor e nets and cover mor e area. This paper details our development and use of audiolur es to incr ease mist net captur es of migrant Norther n Saw-whet Owls. Use of audiolur es significantly incr eases captures of migrant saw-whet owls while simultaneously allowing a reduction in the number of nets that must be maintained and operated. STUDY AREAS AND METHODS The breakthrough was developed in 1986 at the Little Suamico Or nithological Station (LSOS) along the western shore of Green Bay, 17 km north of Gr een Bay, Wisconsin. Additional testing of the audiolur e was conducted in 1989 at Finzel Swamp in the mountains of the Allegheny Plateau, 16 km west of Cumberland, Maryland. Data are also included fr om banding stations at Assateague Island along the Atlantic Coast, 22 km south of Ocean City, Maryland and along the Casselman River , 40 1 Associate Curator , Richter Museum of Natural History, University of W isconsin-Gr een Bay, Green Bay, WI; and Central Regional Manager , Heritage and Biodiversity Conservation Pr ograms, Maryland Department of Natural Resources, Annapolis, MD, r espectively.