Researchers in China have unearthed the first fossil evidence of diurnal behavior in owls, filling a gap in the species' evolutionary history. The ancient owl, unlike other species, had somewhat unusual habits by the standards we find today.

Most owls are nocturnal creatures. Their large eyes have more rod cells than cone cells, allowing them to find prey in the undergrowth even in low light. However, there are some diurnal owls, which are active during the day.

Scientists suspect that these birds evolved from nocturnal ancestors, but an understanding of why this change in activity occurred is uncertain. However, the discovery sheds some light on the topic.

The fossil of the ancient diurnal owl

"It is the incredible preservation of the eye bones in this cranial fossil that allows us to realize that this owl preferred the day, not the night," said Li Zhiheng, a Chinese researcher at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP).

Even though the soft parts of this ancient owl's eyes had long since deteriorated, small bones called "scleral ossicles," which form a ring around the eye, remained. In the specimen found, the ossicles had collapsed into the eye cavity, so scientists measured each bone and reconstructed the size and shape of the ring.

"It was like playing with Lego blocks, but digitally," said Thomar Stidham, author of the study and a researcher at IVPP.

Fossil of the ancient owl found. Image: Zhiheng Li et. al. via PNAS

The scientists noticed that the shape and size of the eye were similar to those of modern diurnal owls. Some of its components showed that the owl is part of the Surniini group, which means that these owls abandoned their nocturnal habits millions of years ago, according to the study.

The researchers analyzed the bones of the skull and lower limbs, comparing them to those of their modern relatives.

The fossil still contained a remnant pellet with mammalian remains from the bird's last meal.

They named the species Diurnal myosurnia The fossil was found in a geological region of China called the Liushi Formation. The area's habitat at the time would have been an arid savanna; most diurnal owls live in open habitats, the authors note in the study.

"It's cool that the researchers showed that there were probably owls in savannas," said Jonathan Slaght, a biologist with the Wildlife Conservation Society. Slaght studies the Blaskoni's barn owl, which lacks the characteristics of nocturnal owls, such as silent flight and broad facial discs.

"By modern standards, that owl is weird, and Blaskoni's owl falls into that category," he recounted.

Regarding the reason these birds have changed their nocturnal habits to diurnal ones, scientists point to climatic conditions as a possible reason. During the period when the Diurnal myosurnia lived - more than six million years ago - the area where the fossil was found was likely severe and cold, Li recounted.

It is possible that the small mammals that these owls hunted evolved until they became active during warmer times, and then the owls evolved to hunt them at those times over the years.

The authors also write that diurnal owls have a "longer and more significant history than is currently recognized."

"It's always a great feeling when you make a discovery that has the ability to change what people, including scientists, think about a subject," Li and Stidham wrote in an email.

"We now have many questions to study about how owls 're-evolved' to become active during the day, studying things like their retinas, feathers, and other components impacted by evolutionary change in their behaviors."

The study was published in PNAS.