Moonlight transforms barn owls into terrifying "ghosts" | NOVA | PBS | NOVA

Being white can make fusion difficult – if you're a night bird, that's it.

Researchers have been questioning for years on the snowy plumage of some owls of the steeples (Tyto alba), which makes them stand out like sore inches against the dark background of the night. This lack of camouflage becomes even more obvious when these pearly predators invade the sky under the full moon, which can brighten the birds as they descend on their rodent prey.

But these creatures with frost feathers may have good reasons not to escape this lunar spotlight. It turns out that white owls hunt better when the moon is big, not in spite of their obvious coloring, but because of that.

The driving force of this counter-intuitive trend, described in a study published today in the journal Nature Ecology & Evolution, is a remarkable visual trick: when the light bounces on a bright plumage, it has a deer effect in the headlights on rodents, which triggers a frost reaction that makes it easier to catch.

In other words, the ghostly plumage of the white owl is so visible that it's scary – and it can mean curtains for its super-capricious prey.

"It's a really exciting document," says Monica Carlson, who studies bird plumage at Princeton University, but did not participate in the study. "Since we [humans] are diurnal species … we prefer to study the species we see most often. But this creates a new territory in the study of the evolution of animal colors. "

In a world where predators often rely on camouflage to sneak, most night hunters have created coats, scales, hairs and feathers in shades of brown, gray or black that fit the sunless landscape . The owls of the steeples, whose feathers range from dark red to bright white, defend this tendency. Until now, it was absolutely unclear why, says author of the study Alexandre Roulin, biologist of behavior at the University of Lausanne in Switzerland. (A little pause: although snow owls are also white, it's more to help them melt into their native Arctic tundra, where they often hunt during the day.)

But inspiration spikes began to emerge on Roulin during his evening getaways he took to capture the barn owls for study. The nights when the moon was full, the whitest of the birds appeared to him "like stars crossing the sky," he says. "They [were] so visible.

Roulin suspected that the ghostly pallor of white owls betrayed their presence, making these birds a dazzling animal that would betray their presence and impose a hunting handicap on sunny nights. But when he and his colleagues searched their barn owl database – which contained nearly 30 years of meticulous recordings in 360 nesting boxes in French-speaking Switzerland – and superimposed some of the measurements on the lunar cycle, they discovered exactly the same. opposite. true.

On the nights of the new moon, when the moon was least visible in the sky, white owls and red owls were equally effective hunters and brought the same number of prey to the owls in their nests. However, as the moon spun, red owls began to weaken. And by the time the full moon arrived, their chicks weighed less than those raised by white parents – who, on the other hand, seemed not to be subjected to additional light.

To discover the secrets of the relative success of white birds, the team gathered several dozen voles (Microtus arvalis), favorite food of barn owls, and placed each rodent in a lighted or dimly lit room imitating the beginning or end of the lunar cycle. Then, as the live birds would have been too hard to test, the team acquired a set of taxidermized red and white owls – all immortalized in flight stances – and tossed them on voles via a 7-foot zip line.

Of course, stuffed birds did not pose real threat. But these vulnerable voles were not the wisest. As the owls ran down the zipline, most rodents froze for fear for several seconds – a common reaction to an approaching predator, Roulin explains. In the conditions of the new moon, the voles reacted in the same way to all the owls sent by the researchers. But when the "moon" of the room was full, the paralysis of the voles lasted five more seconds if their attackers were white instead of red.

The stuffed white owls also seemed to draw the predatory power of the false moonlight: the brighter the room was, the more the birds shone – the longer the voles remained motionless.

Still prey is much easier to catch, which is great news for white owls, says Roulin. In nature, this terrifying tactic can allow the birds to gain a few more seconds, which may explain their hunter's feats on moonlit nights, when they are essentially "ghosts", jokes Roulin .

"What's good about this study is that it's not just about describing [how the owls’ color affects survival]"She also describes the mechanism behind the process," says Maria Delgado, behavioral ecologist at the University of Oviedo in Spain, who did not participate in the study.

From the point of view of colors, the rodent has a big difference between the red owl and the white: the brightness, generated by the light reflected by their feathers. On a small scale, the glow like a predator tag could compromise its stealth. To the extreme, this weakness becomes a force that exploits the natural aversion of rodents for intense light.

That's exactly what seems to be happening here, Roulin says. In a follow-up experiment, the researchers introduced another taxidermal white owl into the voles. Only this time, they coated the bird's feathers with duck-fluted wax, a substance that lessens their natural glow. Faced with this bird more muted, voles seemed much less discolored and were faster to thaw. "It's almost like [these waxed birds] were red, "he says.

However, we should not feel as well bad for the browner birds, adds Roulin. The melanin located behind their red plumage strengthens the strength of the feather and helps the birds to resist abrasion and temperature fluctuations. And although barn owls may shine when the moon is full, their visibility may be costly in other circumstances, for example, to avoid being detected by competing bird species, such as crows.

Such compromises are probably the reason why the species did not become monochromatic. In the wild, the plumage of barn owls extends from red to white, and each subtle variation may be perfectly suited to its time and place. Roulin and her team are currently surveying barn owl populations around the world to determine if certain habitats, such as those with more cloud cover or tree cover, could favor birds of some shade.

Many questions remain, with few researchers to answer, says Roulin. It is not easy to study nocturnal animals as a daytime human: when night work in the field becomes particularly intense, some Roulin students sleep just four hours a night.

Yet such studies are essential to deepen our understanding of the many factors that determine the evolution of plumage, says Carlson. From our point of view, it may seem that there is little to see after sunset. But for nocturnal predators like the barn owl, it could not be further from the truth.

"We need to conduct these studies taking into account the ecological niche of the species we are studying," she says. After all, taking a different point of view – even one that is often in the dark – can be very enlightening.