Captures of Northern Saw-whet Owls (Aegolius acadicus) correlate with the lunar cycle during fall migration

Abstract

ABSTRACT As a small nocturnal migrant, the Northern Saw-whet Owl (Aegolius acadicus) is hypothesized to reduce predation risk by avoiding migration during the full moon. However, small-scale studies have given mixed results on whether there is a relationship between capture rate of Northern Saw-whet Owls and the lunar cycle during fall migration. We used 50 years of data from the U.S. Bird Banding Laboratory to examine capture rates of Northern Saw-whet Owls throughout North America in relation to lunar illumination. A logistic regression of 156,794 fall captures compared to random dates in the fall showed that captures are significantly less likely as lunar illumination increases. Captures at high illumination were 22% less likely than captures on moonless nights during the waning half of the lunar cycle and 41% less likely during the waxing half of the lunar cycle. Modification of the lunar illumination index to correct for the duration of the night that the moon is in the sky gave similar results. Owls...