Different Audio-lures Lead to Different Sex-biases in Capture of Northern Saw-whet Owls (Aegolius acadicus)

Abstract

Northern Saw-whet Owls (Aegolius acadicus) are banded across the United States, yet neither migration nor dispersal of Northern Saw-whet Owls are fully understood. In spring of 2007 and 2008, the banding station at Whitefish Point Bird Observatory in Michigan's Upper Peninsula began broadcasting male and female (respectively) audio lures at dedicated net arrays to complement the passive mist netting performed since 1994. We analyzed data from spring owl captures (1994–2016) that used either male “advertising call” audio lures, female “tssst” audio lures, or no audio lures. From the analysis of 6659 owls captured during that period, we found that (1) there was a substantial female-bias in the owls caught at the passive and male audio lure nets, (2) a more extreme female-bias in the owls caught at the male audio-lure nets than the passive nets, and (3) a nearly equal proportion of male and female owls caught at the female audio-lure nets. These results corroborate a model of differential migration, in which females compose a larger portion of migrating Northern Saw-whet Owls than males. We also found that audio lures that use female calls can increase relative capture rates of male Northern Saw-whet Owls during spring migration.