Nocturnal mysteries: Fossil holds clues to how owls evolved into daytime hunters

In the silty red soil of Gansu Province in China, a small owl has lain nestled for about 6 million years, since an era known as the Late Miocene. The fossilized bird’s talons are outstretched, one of its wings is spread wide and its sharp beak is turned back over its shoulder. You might imagine this little hunter swooping down on some unsuspecting mammal one chilly night long ago.

But an analysis of the fossil published on Monday in the Proceedings of the National Academy of Science suggests the scene should probably be set in the light of day: Judging from the size and shape of its eye sockets, the owl hunted under the sun, rather than the moon.

The fossil could offer clues into the evolutionary forces that transformed this bird and some other species into the owl equivalent of a morning person.

The fossil owl, an extinct species that the researchers have named Miosurnia diurnal, is exquisitely preserved, said Li Zhiheng, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and an author of the new paper.

This allowed the team to take precise measurements of its bones, something that had not been possible with most other fossil owls.

They then fed the bird’s dimensions into a computer program that made predictions about an organism’s lifestyle, comparing the data with the anatomies of a variety of reptile and bird species.

Owls are known best for their nighttime hunting and hoots, and indeed, many modern owls are nocturnal. They prey primarily on creatures that are also awake at night.

The eyes of night owls have many more rod cells than cone cells, allowing them to see better in dim light.

But some of the birds are crepuscular, meaning they come out at dawn and dusk, and still others, a small handful that includes burrowing owls, are diurnal, meaning they are active in the daytime.

Scientists suspect that these daywalker owls evolved from nocturnal ancestors, meaning that they shifted their period of activity at some point in the past. But there are no clear answers to explain how a limited assortment of owls came to thrive in the daytime.

The fossil in the new study has elongated eye sockets and rings of bone around the eyes. These shapes resemble those of modern diurnal owls. The researchers found that with eyes of this size and shape, it is more likely than not that the owl was seeing by daylight. Of course, Dr. Li said, without anyone having been around to observe the owl in action, researchers have to make educated guesses — no one knows for sure what this owl’s behavior was.

Still, if some owls shifted to a diurnal lifestyle as early as six million years ago, it may be possible to find clues about what caused them to make this change in what we know about their environment. The part of Gansu Province where the fossil was found is near the Tibetan Plateau, and it was likely a cold, harsh place to live, Dr. Li said.

Perhaps the small mammals that owls preyed on evolved away from nighttime activity to take advantage of warmer temperatures in the day. They would perhaps have drawn their predators, over the eons, into the light themselves.

For now, the group is looking forward to analysing another already unearthed, well-preserved owl of another species, one that they suspect was also diurnal and could offer additional clues to what made some owls take this leap. “There are more stories to tell about the eye,” said Dr. Li.