The owl and the Moon

A recent study shows how hunting and breeding in red and white barn owls are differently affected by the moonlight. The study also suggests that white barn owl plumage might have evolve to exploit rodent prey aversion to bright light.

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The mountainous landscape of Switzerland takes a break between the Neuchâtel and Geneva lakes. The rocky and snowy Alps and Jura mountains enclose a valley where neatly worked agricultural fields, old farms, tobacco barns, and a castle here and there offer one of the most picturesque scenes of the Helvetian country. Unfortunately for us, a group of biologists of the University of Lausanne, we are banned from such a view as we travel around the valley once the sun is down, the night takes over, and barn owls are out to hunt.

CREDIT: Jérémy Bierer

People in the area say “la nuit, tous les chat sont gris” (all cats are grey by night). And it is true, at least for us humans. When you do field work by night, the first thing you learn is to make sure you have packed your head lamp and some batteries … and an extra head lamp and some more batteries for the forgetful newbies. You don’t want to be out there, going five or more metres up a ladder to check a barn owl nest box in the pitch dark.

CREDIT: Jérémy Bierer

Only some nights, it is fine to go on the field unequipped. Those are the nights when head lamps are barely useful. When the Sun is far below the horizon and yet its light is all around. When only grey cats remain grey. Those are the nights when a full Moon is up in the sky, fabricating shadows and escorting barn owls and us in our nightly routine.

Like cats, like humans, like many animal species, barn owls come in different shades of colour. Some are dark reddish, because they have on their feathers the same pigment (pheomelanin) as red-hair humans have. Some are snowy white, because they lack pigments in their feathers, like we humans lack them on our hair as we grow older. We don’t fully understand yet why such different colours evolved but we are now sure that by night, not all the barn owls have the same colour. And again, the reason for this is the Moon.

CREDIT: Isabelle Henry

As the nights become more illuminated by the moon, dark-red owls have a worst time in hunting the little rodents with which they feed their owlets. Moonlight makes easier for the rodents to spot the owls flying around them, the same it makes fieldwork easier for us when our headlamps run out of batteries. As a consequence of this, dark-red owls bring less rodents to their owlets during these moonlit nights, and the owlets have less chances to survive.

Now, you may think that moonlit nights will be even worse for white owls. If you don’t, make this experiment yourself. Place a white and dark tissue out in a full-moon night. Get some distance and see which one is easier to spot. White objects reflect more light. White owls are no exception and should be easier for the rodents to spot them when the Moon is full. Yet, contrary to dark-red owls, white owls seem to hunt as well in full-moon nights as in nights with no Moon at all. How come!? The answer may actually reside on “white objects reflect more light”. Prey, when facing a moonlit white owl but not a red owl, freeze for longer periods of time, which may be triggered by the aversion that rodents have to bright light. Scared, immobilized rodents are easy lunch for barn owls. For white barn owls, the Moon also makes night work easier. Indeed, the white plumage of some barn owl is unique among nocturnal owls. Contrary to snowy owls, whose white plumage matches their snowed habitats, barn owls might have evolved white plumages to be seen.

It wasn’t easy to get to understand all this. We have collected data over more than 20 years now, installing and visiting nest boxes all along the year, and passing long nights awaiting quietly in the car for barn owls to come back hunting in order to equip them with precious GPS devices that sometimes owls manage to remove right after we had release them. The study has spanned 5 years of analysis, questioning each result and conducting and replicating experiments until we were convinced of the unexpected effects that the Moon has on white and red owls. All this was nevertheless worthy as we now know better the barn owl, a beautiful and amazing species that nowadays struggles to survive along-side humans despite having doing so for millennia.

CREDIT: Jérémy Bierer

COVER CREDIT: Guillaum Rapin,

Luis M. San-Jose

Researcher, University of Lausanne