Animals need to blend into their immediate surroundings to avoid detection, whether they are prey or predators. Camouflage is considered a diurnal phenomenon, based primarily on light and shadow, but can an animal camouflage itself at night beyond the cover provided by darkness?
A new study published in the journal PNAS led by the Doñana Biological Station – CSIC, addresses this question by investigating the paradox of the coloration of the barn owl. It is an owl with a striking white color and different from the rest of the nocturnal birds of prey, which have plumage with complex designs that clearly hide them in their environment. The flight of the owl is as silent as that of any other owl, but its white ventral plumage, far from being discreet, could alert its prey. According to the new study, this is not necessarily the case.
“We worked with the hypothesis that the owl’s plumage could represent a novel type of nocturnal camouflage,” explains Juan J. Negro, specialist in behavioral ecology and researcher at the Doñana Biological Station – CSIC. “The concealment of the owl in hunting flight would be achieved when its contrast against the sky is below the threshold for detecting objects by its prey, typically rodents that look for food on the ground.”
The scientific team’s condition of practical “invisibility” for the unsuspecting mouse is generally not met on completely cloudy or moonless nights, known as isotropic skies. However, on clear nights, the isotropy of the sky’s glow is altered if the moon is present, allowing a compensation between the light from the celestial vault behind the owl and the light reflected by it towards the ground where the owl is. prey. In this way, the owl can become almost undetectable to the mouse, which, when it sees the owl, is usually too late and has no escape.
These results agree with previous observations by other authors indicating that, on full moon nights, whiter owls manage to catch more prey than darker owls. And the color of the ventral plumage of owls naturally varies from snowy white to orange.
“We examined the efficiency of owls’ white plumage as a means of camouflage when hunting, taking into account the radiometric properties of the sky, the ground, and the owls themselves, as well as the mouse’s visual system, which is different from that of humans,” he says. astrophysicist David Galadí, researcher at the University of Córdoba and co-author of the study. «Our model indicates that it is enough for the moon to be above the horizon in any of its phases for an owl in flight to approach rodents from large regions of the night sky, always remaining below the detection threshold of the particular visual system of the moon. mouse”.
Specifically, the study shows that the white plumage of owls serves as effective camouflage adapted to the moonlit sky background, providing a new basis for understanding nocturnal camouflage in other species.
Light pollution
The implications of this multidisciplinary work go beyond camouflage itself, as explained by Carlos Camacho, specialist in the study of nocturnal species and researcher at the Doñana Biological Station: “The lighter and darker owls could choose different times to hunt, coinciding with the lunar conditions that maximize the hunting effectiveness of both. “This temporary uncoupling could make pairings between owls of different colors difficult despite sharing the same space.”
The study also implies that increasing light pollution could interfere with the natural functioning of ecosystems in unexpected ways, because it increases the brightness of the night sky in a way that counteracts the camouflage provided by the owls’ white plumage and makes the predator more visible to its prey.
To carry out the study, both biologists from the Doñana Biological Station and scientific personnel specialized in physics from other institutions participated. For example, for radiometric studies of the white plumage of the owl, sophisticated measurement systems have been used in laboratories at the CSIC in Madrid and the University of Granada. The radiometry and photometry of the sky with the moon has been estimated using a calculator that provides various indicators of sky brightness depending on the height of the moon above the horizon, its phase, the aerosol content of the atmosphere and the albedo of the ground. This calculator has been developed by Eduard Masana, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC) and Salvador Bará, independent researcher, who had the collaboration of Carmen Bao -Valera, from the University of Santiago de Compostela.
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