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 were also most likely to be captured on a Sunday or Monday, perhaps reflecting higher weekend capture effort. The relationship of captures to lunar illumination and wax/wane phase is consistent with an ultimate explanation of predator risk avoidance, but a proximate mechanism of altered owl movement is not demonstrated and alternative explanations such as variable human capture effort and changes in owls' ability to detect and avoid nets may also be responsible. Neither of our logistic models had much power, probably because local habitat and weather are strong modifiers of the lunar cycle effect on capture probability of Northern Saw-whet Owls. The average reductions calculated here may still underestimate the decrease in owl captures under local conditions of bright moonlight.
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