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Flying slowly and silently through the night sky, barn owls are majestic night hunters, successfully hunting rodents that scurry across the ground.
Scientists have long puzzled over a feature that appears to hinder the bird’s midnight hunts. The plumage on the underside and wings is bright white, an unusual characteristic that should, in theory, make it difficult for the owl to approach its prey undetected.
In contrast to the snowy owl, the barn owl lives in latitudes where it does not snow regularly. The barn owl’s paradoxically bright plumage puzzled biologist Juanjo Negro and his colleagues, who felt the fascinating biological phenomenon warranted further study.
“We started from the intuitive feeling that the barn owl’s extraordinary whiteness is so shocking that it should confer some adaptive advantage,” Negro, a research professor at the Doñana Biological Station-Spanish National Research Council in Seville, Spain, said by email .
Previous research from September 2019 had shown that the bright white coloration was essentially a form of shock and awe, exploiting the rodents’ aversion to bright light. The earlier study found that the owls’ whiteness makes the predator more visible to rodent prey, which freezes in fear, and makes it easier to kill voles.
The latest research by Negro and his team instead suggests that the barn owl’s brilliant white plumage is actually a form of nocturnal camouflage, or counter-illumination, that surprises the hunter.
The barn owl’s feathers match the moonlight
According to the new study describing the team’s findings, published Monday in the journal PNAS, the bird’s white, reflective underside effectively mimics moonlight. The bright plumage allows its silhouette to blend into the night sky and makes it difficult for mice or other rodents to spot the owl.
“When the moon is visible, the sky has a certain brightness that varies depending on the direction in which it is observed. Under these conditions, a dark bird would be seen by its potential prey as a black silhouette in the sky,” said Negro, the study’s lead author. “But if the ventral part of the bird was sufficiently reflective, i.e. ‘white’ enough, it would reflect a good portion of the moon.”
According to the researchers’ calculations, the contrast between the barn owls’ reflective, white undersides and wide areas of the illuminated night sky is below the detection threshold of rodent visual acuity, allowing the barn owls to approach prey within a few meters in different directions without being detected to become.
“This explanation has never been proposed to justify the whiteness of barn owls,” Negro added.
A similar phenomenon occurs in the ocean, where fish often have light-colored undersides that match the sunlit water and make them less visible to predators lurking in the depths.
However, not all barn owls have a white underbelly; some have reddish-brown plumage instead.
Negro said the research did not examine hunting success in relation to plumage color, but previous research has shown that white barn owls are more efficient hunters during a full moon than their darker counterparts.
Bright white shock value vs. obfuscation
Barn owl expert Alexandre Roulin, a professor of biology at the University of Lausanne’s Department of Ecology and Evolution, who had conducted previous research suggesting that the species’ white coloring shocked and immobilized its prey, said he was impressed by the new explanation not entirely convinced. However, he said he couldn’t rule it out.
“We don’t want to completely reject the authors’ hypothesis. It could actually be a complement to our own. Perhaps the white plumage serves as camouflage at long distances, while at close range it may serve to make the owl more visible,” he said via email.
An important aspect that needed further explanation, according to Roulin, was the behavior of voles in moonlight.
“We observed that voles remained stationary for longer when the approaching owl was white instead of reddish. Previous research has shown that this immobilization is an anti-predator response,” Roulin said. “On this basis, we would argue that the voles recognized the white owl as a predator, suggesting that the owl was visible and not camouflaged.”
Negro said the owl’s ability to fly in complete silence, a trait that helps it stealthily approach its prey, is inconsistent with the idea that the predator wants to make itself more visible. However, he agreed that the new hypothesis does not completely disprove the previous theory.
Camouflage is often viewed as a diurnal phenomenon based on light and shadow. Negro said there are likely other forms of animal coloration that serve as nocturnal camouflage, but these are not as well studied as daytime camouflage tactics.
“Nocturnal ecology is an emerging field, and considering that half of the animals on the planet are essentially nocturnal, adaptations to the moon’s natural cycles are certainly widespread,” he said.
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