Strigidae – Owls

The Strigidae (true owls or typical owls) are one of the two generally accepted families of owls, the other being the Tytonidae (barn owls and bay owls). This large family comprises two hundred and twenty-five species in twenty-three genera. They have a global distribution being found on every continent except Antarctica.

While typical owls vary greatly in size, with the smallest species, the elf owl, being a hundredth the size of the largest, the Eurasian eagle-owl and Blakiston’s fish owl, owls generally share an extremely similar overall look. They have proportionately large heads, short tails, cryptic plumage, and round facial discs around the eyes. The family is generally arboreal (with a few exceptions like the burrowing owl) and obtain their food on the wing (with exceptions like the fishing owls). The wings are large, long, broad and rounded. As is the case with most birds of prey, in many owl species females are larger than males.

Burrowing Owl Athene cunicularia

Because of their nocturnal habits, they tend not to exhibit sexual dimorphism in their plumage. Specialized feathers and wing shape suppress the noise produced by flying, both taking off, flapping and gliding. This silent flight allows owls to hunt without being heard by their prey. Owls possess three physical attributes that are thought to contribute to their silent flight capability. Firstly, on the leading edge of the wing, there is a comb of stiff feathers; secondly, the trailing edge of the wing contains a flexible fringe and, lastly, owls have downy material distributed on the tops of their wings that creates a compliant but rough surface (similar to that of a soft carpet). All these factors result in significant aerodynamic noise reductions.

Northern Hawk-Owl Surnia ulula

Northern Pygmy Owl Glaucidium gnoma

The toes and tarsi are feathered in some species, and more so in species at higher latitudes. Numerous species of owls in the genus Glaucidium and the northern hawk-owl have eye patches on the backs of their heads, apparently to convince other birds they are being watched at all times.

Eastern Screech Owl Megascops asio

African Barred Owlet Glaucidium capense

Numerous nocturnal species have ear-tufts, feathers on the sides of the head that are thought to have a camouflage function, breaking up the outline of a roosting bird. The feathers of the facial disc are arranged in order to increase sound delivered to the ears. Hearing in owls is highly sensitive and the ears are asymmetrical allowing the owl to localise a sound in multiple directions. Owls can pinpoint the position of prey, such as a squeaking mouse, by computing when the sound from the object reaches the owl’s ears. If the sound reaches the left ear first, the mouse must be to the left of the owl. The owl’s brain will then direct the head to directly face the mouse. In addition to hearing, owls have massive eyes relative to their body size. Contrary to popular belief, however, owls cannot see well in extreme dark and are able to see well in the day.

Buffy Fish Owl Ketupa ketupu

Cebu Hawk-Owl Ninox rumseyi

Owls are also able to rotate their heads by as much as 270 degrees in either direction without damaging the blood vessels in their necks and heads, and without disrupting blood flow to their brains. Researchers have found four major biological adaptations that allow for this unique capability. First, in the neck there is a major artery, called the vertebral artery, that feeds the brain. This artery passes through bony holes in the vertebra. These bony holes are ten times larger in diameter than the artery that passes through them (extra space in the transverse foramina) which creates air pockets that allow for more movement of the artery when twisted. 12 of the 14 cervical vertebrae in the owl’s neck have this adaptation. This vertebral artery also enters the neck higher up than it does in other birds. Instead of going in at the 14th cervical vertebrae, it enters in at the 12th cervical vertebrae. Finally, the small vessel connection between the carotid and the vertebral arteries allow the exchanging of blood between two blood vessels. These cross connections allow for uninterrupted blood flow to the brain. This means that even if one route is blocked during extreme head rotations, another route can continue blood circulation to the brain.

Several owl species also have fluorescent pigments called porphyrins under their wings. A large group of pigments defined by nitrogen-containing pyrole rings, including chlorophyll and heme (in animal blood), make up the porphyrins. Other bird species will use porphyrins to pigment eggshells in the oviduct. Owl species, however, use porphyrins as a pigment in their plumage. Porphyrins are most prevalent in new feathers and are easily destroyed by sunlight. Porphyrin pigments in feathers fluoresce under UV light, allowing biologists to more accurately classify the age of owls. The relative ages of the feathers are differentiated by the intensity of fluorescence that they emit when the primaries and secondaries are exposed to black light. This method helps to detect the subtle differences between third and fourth generation feathers, whereas looking at wear and colour makes age determination difficult.

Most owls are generally nocturnal or crepuscular and spend much of the day roosting. They are often misperceived as ‘tame’ since they allow humans to approach quite closely before taking flight, but in reality, they are attempting to avoid detection through stillness. Their cryptic plumage and the inconspicuous locations they adopt are an effort to avoid predators and mobbing by small birds.

In 2002, the late Friedhelm Weick, author of Owls of the World, wrote this for Fatbirder:

Owls, my favourite birds

Owls are outstanding birds! Is it their silent hunting in the poor evening light and at night, their distinctive appearance – huge head and forward-looking eyes or maybe the echoes of ancient myths and tales of woodland goblins and bewitched owl-shaped princesses? However, since my childhood owls were my favourite birds. They even outranked my ambition for the diurnal birds of prey. As a wildlife artist, birder and ornithologist – I never lost my contact with these feather-armoured knights of the twilight. When in 1980 my book Birds of Prey of the World was printed, I had the idea for a similar project on the Strigiformes of the world. So, I began to collect every bit of information about this subject – and, remember: the Internet wasn’t in reach for the common wildlife artist then.

I filed scientific articles, photos and colour-copies of books, sketches and other descriptions back to 1758, photos from living owls in the wild or captivity. Detailed lists of measurements and colours of skins, weights and feathers grew larger and larger. Beside that, I also visited zoological gardens, ornithological institutes and a lot of private owl-keepers, birders and ornithologists. In the early nineties I wrote three different annotated checklists for Strigiformes. The first included all recent complete taxa and their distribution. A second listed all-important measurements and weights linked to an index of photos and paintings. The third list was a full description of plumages and soft parts combined with a detailed bibliography.

This situation led Claus König, head of the Natural Science Museum at Stuttgart and an ambitious ornithologist, asked me if I would be interested in a project that should result in nothing less than the standard guide to all owls of the world. The publisher would be Pica Press. Of course I was, wasn’t I?

The three checklists were the basic-stock for our further work. In the following years I examined some thousands of skins and mounted owls in all-important museums of the world, only supported by my dear wife Christel. Within nearly three years I painted all different taxa and morphs that were sometimes known only by one skin…

In 1999 the book was published… leaving me more time for birding and outdoor projects and, last but not least, for new paintings of my favourite birds, guess which?