Northern Saw-whet Owls (Aegolius acadicus) captured at Cape May Point, NJ, 1980-1994: comparison of two capture techniques

Abstract

During autumn migration 1980-1994, 1,270 Northern Saw-whet Owls (Aegolius acadicus) (NSWO) were captured and banded at Cape May Point, NJ. From 1980-1988, captures were effected by passive mist-netting. From 1989-1994, an audiolure (NSWO territorial song broadcast loudly from dusk to dawn in the trapping area) was used to enhance capture rate. 638 NSWOs were captured with an audiolure during five seasons (3.08 owls/100 net-hours), while 632 NSWOs were captured in nine seasons by passive mist-netting (0.51 owls/100 net-hours). Comparisons of age, mass, weather conditions when peak capture rates occurred, seasonal timing of migration and time of night of capture revealed differences and similarities of results between the two capture techniques. For both capture techniques, the proportion of NSWOs captured that were adults varied from year to year. Without an audiolure, 38 percent were adults (range 13-88 percent). With an audiolure, 42 percent were adults (range 10-58 percent). Discriminant analysis of wing chord-mass values to assign sex showed that females were more likely to be captured irrespective of technique. With an audiolure, the mean mass of NSWOs captured increased as did the proportion of females. With either technique, most captures occurred on the nights immediately following the passage of cold fronts, when high pressure dominated the study area. With either technique, most captures occurred when the wind direction was northwest (west through northeast), although the capture rate when wind direction was southwest through east was greater with an audiolure than during passive netting. Diel timing of the majority of captures shifted from pre-dawn without an audiolure to earlier in the night when an audiolure was employed. Seasonal timing of migration was similar with both techniques.

Parent Publication

• Biology and conservation of owls of the Northern Hemisphere: 2nd International symposium

Citation

Duffy, Katharine E.; Matheny, Patrick E. 1997. Northern Saw-whet Owls (Aegolius acadicus) captured at Cape May Point, NJ, 1980-1994: comparison of two capture techniques. In: Duncan, James R.; Johnson, David H.; Nicholls, Thomas H., eds. Biology and conservation of owls of the Northern Hemisphere: 2nd International symposium. Gen. Tech. Rep. NC-190. St. Paul, MN: U.S. Dept. of Agriculture, Forest Service, North Central Forest Experiment Station. 131-137.