Have you ever noticed we tend to classify people using animal metaphors? A ‘night owl' for instance is usually used to describe someone who prefers to stay up late at night and of course refers to the nocturnality of owls. But in fact, not all owls are active at night and some species are considered as crepuscular (active at dawn and dusk) or even diurnal…
To get a better understanding of this diversity of owl’s ecology, let’s rewind the clock 6 million years back to the late Miocene Epoch. Then the bird skeleton of an extinct member from the typical owl family (Strigidae family), is being perfectly preserved underground in the Tibetan Plateau, China. This recently unearthed species (𝘔𝘪𝘰𝘴𝘶𝘳𝘯𝘪𝘢 𝘥𝘪𝘶𝘳𝘯𝘢 Li, Stidham, and Zhou, 2022) provides evidence of the diurnal origin of the owls belonging to the Surniini clade (a group of related species of owls). Scientists determined that this ancient member of the Strigidae family was active in daytime on the basis of the relatively large exterior scleral ossicle ring diameter measured on the bird skull, which indicates a smaller pupil size. The smaller the pupils are, the less the light enters the eyes, which protect the diurnal hunters from excessive light exposure.
Modern owls, however, are mostly nocturnal. They are birds of prey hunting at night. One of the largest extant night raptors, 𝘉𝘶𝘣𝘰 𝘣𝘶𝘣𝘰 (Linnaeus, 1758), the Eurasian Eagle-owl inhabits much of Europe, Middle East and Asia, including Hong Kong. It is possible to hear or witness the evening activities of an Eurasian Eagle-owl in Lantau Island, where most of the rocky hillsides are located. Offshore islands with rocky landscapes often become their nesting habitats as they tend to build nests on the ledges of escarpments and cliffs. Despite the fact that Eurasian Eagle-owls also belong to the family Strigidae, the following species traits prove that they are adapted both morphologically and genetically to a night-active behavior pattern.
To begin with, nocturnal owls have larger eyes in proportion to their body weight than other birds. If the body structure of a Eurasian Eagle-owl is scaled up to that of a human adult, its eyes would be the size of oranges (try this comparison with your fingers or with a real orange!). Nocturnal owls have greater pupils and corneas that enable them to collect more ambient light at night. Thus, the sensitivity and acuity of their night vision are improved. One other interesting fact about the Eurasian Eagle-owls is their unusual iris color. Researchers suggest that night raptors usually have dark eyes, however, a pair of bright orange-red or golden-yellow eyes is a distinctive feature of 𝘉. 𝘣𝘶𝘣𝘰.
On the other hand, Eurasian Eagle-owls have a pair of asymmetrical ears, unlike humans, their ears are concealed by the hair-like feathers of their facial discs. A facial disc is a wall of feathers rims and covers the face of an owl. The feathers are serving as sound receivers and transmitters to form a bigger surface area for audio collection. Therefore, this unique feature contributes to the excellent hunting ability of the owls. They are able to determine the location of prey through directional hearing achieved by asymmetrical audio signals. Meanwhile, owls can rotate their heads at a maximum of 270 degrees searching for the source of sound. Excellent vision and hearing even in the darkest environment allowed 𝘉. b𝘶𝘣𝘰 to become one of the strongest predators in the forest.
In spite of the fact that most of the owls have similar facial structure, there are approximately 50 owl species that have ear tufts. Even though the function of these furry structures has not been proved, these exquisite morphological traits in different species is likely to have contributed to the biodiversity of the owl family. Witnessing an elusive Eurasian Eagle-owl has never been easy, so come to the HKBM and have a closer look at our specimens. Be careful not to get caught in their big dazzling eyes!
Text: Millian Hui
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