This Ancient Owl Hunted in the Daytime

The fossil evidence fills a gap in these birds’ evolutionary history

Fossil skeleton of a bird
Fossil skeleton of the owl Miosurnia diurna, which was active during the day. Zhiheng Li et. al. via PNAS

Researchers in China have unearthed the first fossil evidence for daytime behavior in owls, filling in a gap in the species’ evolutionary history, per a study published in PNAS

Most owls are nocturnal creatures. Large eyes with more rod than cone cells allow the night-hunting birds to spot prey rustling in the underbrush, even in dim light. But a few owls are diurnal—or active during the day. Scientists suspect these birds evolved from nocturnal ancestors, but it’s unclear why this shift in activity occurred, per the New York Times’ Veronique Greenwood. 

"It is the amazing preservation of the bones of the eye in this fossil skull that allows us to see that this owl preferred the day and not the night," study author Li Zhiheng, a Chinese Academy of Sciences (CAS) researcher at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), says in a CAS statement

Though the fossil owl’s soft eye parts had long deteriorated, small bones called scleral ossicles that form a ring around the eye remained. In this fossil specimen, the scleral ossicles had collapsed into the eye socket, so scientists measured each bone and rebuilt the size and shape of the ring, per CAS. 

"It was a bit like playing with Lego blocks, just digitally," says study author Thomas Stidham, also an IVPP researcher, says in the statement. 

Scientists saw that the eye size and shape were similar to those of modern diurnal owls. Some of its features show that the owl is part of the group Surniini, meaning these owls moved away from the night millions of years ago, per the study. Researchers analyzed the bird's cranial and lower leg bones and compared them to modern relatives, per Popular Scientist’s Purbita Saha. 

The fossil also contained an unregergitated pellet with small mammals from the bird's last meal. 

Two owls in a tree, one holding a small mammal in its beak
Reconstruction of the extinct owl Miosurnia diurna perched in a tree with its last meal.  IVPP

Researchers named the species Miosurnia diurna. The fossil was found in a geological region of China called the Liushu formation. The paleohabitat of this area would have been that of an arid savannah; most diurnal owls live in open habitats today, the authors note in the study. 

“It’s neat that the research shows that there were probably owls in savannahs,” Jonathan Slaght, a biologist with the Wildlife Conservation Society, tells Popular Science. Slaght studies the Blakiston’s fish-owl, which doesn’t have features that nocturnal owls do, such as wide facial discs and silent flight.

“By modern standards this is a weird owl, and Blakiston’s fall into that category,” he tells the publication.

As for why these birds switched from night to daytime, scientists point to climatic conditions as a possible cause, per the Times. During the time period that Miosurnia diurna lived—more than six million years ago—the area where the fossil was found was likely cold and harsh, Li tells the publication. It’s possible that the small mammals these owls preyed on evolved to be active during warmer hours, and the owls evolved to prey on them over the years. 

Diurnal owls may have a “longer and more significant history than currently recognized,” write the authors. 

“[I]t is always a great feeling when you make such a discovery that has the ability to change what people, including scientists, think about a topic,” Li and Stidham write to Forbes’ GrrlScientist, a blogger and ornithologist, in an email. “We now have many new questions to study about how owls re-evolved to be active during the day by studying things like their retinas, feathers, and other features impacted by the evolutionary change in their behaviors."