Owls That Ventured Out In The Sun

AsianScientist (Apr. 11, 2022) – While most owls are nighttime hunters, an extinct owl species that lived over six million years ago hunted its prey during the day instead. Researchers from the Chinese Academy of Sciences examined a well-preserved fossil skeleton of the bird, reporting their findings in the Proceedings of the National Academy of Sciences.

Unlike majority of avian fellows, owls are known to be nocturnal. To prey on creatures that also wander at night, the owls have big eyes and pupils, which help them see in the dark. That pupillary characteristic is also marked by the size and shape of scleral ossicles, a ring of small bones surrounding the pupil and iris. However, a few species such as the pygmy owls are actually diurnal, favoring the hunt under the sun.

How their daytime preference came about has remained a mystery. Scientists speculate that diurnal owls evolved from nocturnal ancestors, diverging from the largely night-loving line to shift their activities to thrive in the sunlight. After all, uncovered fossils of ancient owls mostly indicated nocturnal behavior.

The newly reported fossilized owl, unearthed at the edge of the Tibetan Plateau in China’s Gansu province, may carry clues to how modern diurnal owls learned to thrive during the day. The skeleton is extremely well-preserved from the tip of the skull and pointed beak to the outstretched wings and talons. Even parts that are rarely seen in fossils were recovered, such as the trachea, kneecap, and tendons that attach muscle to bone.

But the eyes had most clues. Although, the scleral ossicles had collapsed into the eye sockets, the exquisite preservation of the fossil still allowed the research team to take precise measurements and study each bone in detail. Through a computer program, they used the measurements of these 16 bones to digitally reassemble the owl’s eye ring. The reconstruction revealed the diameter of the ring and the size of the opening where light passes through. Smaller openings tend to correlate with diurnal activity.

Further statistical analyses compared the structure of the fossilized eye bones with those of over 400 other birds and reptiles with varying times of activity. Three statistical models were built to allow for errors in the reconstruction process, yet all returned the same result: an above 60-percent likelihood that the fossil owl was active during the day.

The fossilized species’ smaller scleral ossicles and other skull features resembled the Northern Hawk Owl or Surnia ulula, medium-sized birds belonging to the diurnal owl group Surniini. As a nod to its close living relative, the researchers named the extinct owl Miosurnia diurna.

Moreover, a large-scale analysis of the bird behaviors suggested that the ancestor of the Surniini group had likely already switched to daytime hunts millions of years ago, despite the ancestor of all living owls most likely being nocturnal. When the team included the M. diurna fossil into the statistical model, the probability of a diurnal Surniini ancestor jumped to 100 percent.

The team now hopes to uncover more clues as to what conditions prompted the behavioral shift from nocturnal to diurnal activity. One hypothesis may involve the Gansu province’s cold, harsh environment where M. diurna lived. The small mammals the owl preyed on may have evolved to prefer the warmer temperatures of the day. To hunt and survive, these owl species perhaps followed suit and grew accustomed to the light over time.

“The fossil and associated analyses of the eye and behavioral evolution point to a long evolutionary history of non-nocturnal behavior among owls that has yet to be studied in detail,” the authors wrote.

The article can be found at: Li et al. (2022) Early evolution of diurnal habits in owls (Aves, Strigiformes) documented by a new and exquisitely preserved Miocene owl fossil from China.

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Source: Chinese Academy of Sciences; Photo: Li Zhiheng.
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