A white winged barn owl flies at night holding a rodent in its claws.

White barn owls, although relatively visible in the dark of night, are highly effective hunters thanks to the way light reflects off their feathers. Credit: Rolf Nussbaumer/NPL

Evolution

Zip-lining owls reveal what really scares their prey

The light of a full moon gives white barn owls a deadly glow.

Taxidermied owls that barrelled down zip lines have helped scientists to understand how moonlight shaped the evolution of nocturnal animals.

The barn owl (Tyto alba) hunts at night, and ranges in colour from white to brownish-red. Alexandre Roulin at the University of Lausanne in Switzerland and his colleagues harnessed 20 years’ worth of observations of barn owls in Switzerland to track how plumage colouration and the phase of the Moon affect the owls’ hunting.

The team found that as the Moon brightened each month, red owls’ hunting success dipped, as did the amount of food — mostly voles — the owls supplied to their chicks. But white owls remained good providers all month long, and hunted best under a full moon.

To understand this variation, the researchers attached taxidermied owls to a zip line and flew the decoys at voles under artificial moonlight. Under ‘full moon’ conditions, the light that reflected off white owls’ plumage caused voles to freeze for an average of 5 seconds longer than did light bouncing off red owls, giving white owls a better chance of capturing their prey.