Barred Owl

Barred Owl Soldier's Delight 151128
The pronounced binocular  aspect is a key feature of  owls adapted for night vision

Common Name: Barred Owl, Hoot Owl, Eight Hooter, Bard Owl – The distinctive white and brown feathers alternate in colored bars (bard is a malapropism). The word ‘owl’ is of ancient provenance and likely originated as onomatopoeia (word taken from a sound – like hoot) from either the Old German uwwalo or the Old Norse ugla. This latter application gave rise to the word ugly in English but which meant ‘dreadful’ in the original usage – a possible traditional association of the owl as a bad omen.

Scientific Name: Strix varia – The generic name is Latin for Screech Owl and derives from the mythological Strix that was described in the Metamorphoses of the Greek Philosopher Liberalis as “a harbinger of war and civil strife to men;” the term persisted into medieval times referring to an owl-like vampire. Varia is a Latin form of the word for diverse (or various) and also refers to the contrasting hued bars of its alternating feathers.

Potpourri: As vestigial dinosaurs, the birds of the class Aves passed through the evolutionary keyhole of the Chicxulub asteroid impact extinction event at the end of the Mesozoic Era; their wildly diverse forms and features necessary and sufficient for survival. Among the birds, the owls of the order Strigiformes are among the more notably evolved in both form and function. Owls are quintessential night stalkers, an avocation that mandates low light vision, auricular homing, silent stealth and a lethal first strike capability; attributes comparable to those of a nuclear attack submarine. There are about 200 species of owl divided into two families, the Tytonidae or Barn Owls noted for their heart-shaped faces, and the more prevalent Strigidae, the “Typical Owl” family to which Barred Owls are assigned.

The unblinking, binocular stare of an owl is disquieting, seeming to convey the prescient powers of a soothsayer’s wisdom; the Arthurian Merlin is frequently depicted with an owl on his shoulder to suggest an advisory role like a pirate’s parrot. Owl eye physiology is predicated on nocturnal detection of prey, it has been demonstrated experimentally that owls can capture dark mice on a light background or light-colored mice on a dark background at an illuminance level of .0000001 foot-candle. This is essentially ebon black; the light at full moon is about .01 foot-candle and the light on an overcast night is .00001 foot-candle. Owl’s eyes are not eyeballs but rather elongated tubes that are held rigidly in the skull by bony sclerotic rings that can comprise up to five percent of its body weight; a tradeoff of this magnitude is extraordinary for an animal whose critical flight appetency mandates weight reduction. The large diameter cornea admits light through a comparably-sized pupil to a sensory field of photo receptors consisting primarily of light sensitive rod-shaped cells or “rods” and very few color sensitive cone-shaped cells or “cones.” The eyes are protected by three lids: the upper lid closes when the owl blinks; the lower eyelid closes when the owl sleeps; and the third, called a nictitating membrane, sweeps across the eye from one side to the other to clean and protect it.

Binocular vision occurs when both eyes see the same object simultaneously to provide the slight angular difference of parallax affording three dimensional height, width and depth perception; an important attribute for prey localization. The large, forward looking owl eye-tubes are widely set on a flat facial plane to allow for a binocular field of 70 degrees. Although less than human eyes which have 140 degrees subtended binocular vision, this is substantively improved over the approximate 10 degree arc of binocular vision for most birds. To maintain the binocular configuration while scanning across a wide vista, the head of an owl is capable of moving 270 degrees in either direction, a contortion of near Exorcist proportions. While magical and ominous from the perspective of human anatomy, rotation of this magnitude is only a matter of physiology. Owls have twice as many neck vertebrae (14) as do humans (7), and, more importantly, they have a single articulation at the cervical connection to the backbone (humans have two) so that it is in essence a pivot. The flow of blood through this critical juncture that connects the heart and lungs to the brain is ensured by allowing ample leeway for the arteries and veins of the jugular plumbing system. The result is that, like a homing radar beacon, the owl can rotate, locate and lock on to their target to prepare for an attack. However, the eyes are secondary to the ears in efficacy.

Owls have a sound navigation and ranging (airborne SONAR) capability with exceptional acoustic properties perfected by survival contingent on the success of the hunt. The ears are the detectors for noise that the eyes can then pinpoint. Over the low frequency sound ranges of rustling leaves that would result from mobile rodents, owls have hearing sensitivity similar to cats, distinguishing noises as low as -15 decibels (dB) in the 1-7 kilohertz range. By comparison, +10 dB is the sound of a pin drop which means that owls and cats can hear noise more than two orders of magnitude lower (decibels are logarithmic in scale) than the quietest sound for which there is an analogy. Domestic cats evolved from wild cats with the advent of human agriculture and the concomitant opportunity for pillaging rodent populations so it is probable that their similar hearing sensitivities is an example of convergent evolution; both cats and owls were subject to the same rodent hunt and kill needs yielding the same evolutionary result.

The owl’s iconic round face bifurcated by a narrow vertical beak is in stark contrast to the elongated skulls, lateral eyes, and a stout pointed beaks of most other birds. The reason is that the head is in essence a disk-shaped audio antenna without the interference of a large beak to concentrate the night’s merest whispers for detectability in amplifying signal over noise. Like the parallax eyes, the ears are separated to maximize the arrival time difference of emanated sounds. Turning the head to reduce the difference to less than 30 millionths of a second between left and right yields a vector to the noise with a now rotated head to apply the transfixed owl-stare. There is no good analogy for what this time means in human terms, but it is evidently sufficient to catch mice. That there is a lot of wiring needed to achieve this level of sensitivity is evident in that the owl’s medulla hearing center brain region which has about 100,000 neurons, three times more than that of a crow. The sensory suite of ears and eyes operate continuously through the night, a detection resulting in a furious yet silent assault of feathers and talons.

A Barred Owl on its night hunting perch

Owl attacks are stealthy, their silence the result of specialized feathers that squelch the flow noise of the air passing through them. The physics of aerodynamics yields a vortexat the trailing edge of an airfoil, a necessary result of the circulating air flow that generates lift; a similar effect results from a ship propeller when the fluid is water instead of air. The vortices cause air flow differentials called turbulence which in turn causes the air pressure surges of noise. Owls have a unique feather physiology that minimizes noise; their primary or contour flight feathers have fringed leading edges called flutings that channel the airflow into smaller vortices to muffle the sound. It is not clear scientifically whether the sound reduction is due to lowering the amplitude of the sound waves or due to an increase in the frequency to a level above the threshold of prey audibility. But one thing is clear – an owl strike is silent, swift and sure.

Barred owls are among the most successful and therefore most common of the Strigiformes; they are consummate hunters, extending the hours of darkness by exploiting the crepuscular periods of dim light in the late afternoon. They spend diurnal hours in the home nesting area, concealed by the apatetic coloring of the barred plumage against predation, traveling to perch in an established hunting area, where they can be seen on occasion skulking above hiking trails, evidently a place where meadow voles and mice noisily scamper. The night’s watch normally begins with a song followed by foraging in repetition throughout the night. The barred owl’s primary song is distinctive; the alternative common name ‘eight-hooter’ is mnemonic. The eight tones are “hoo, hoo, too – HOO – hoo, hoo too – HOOooo” the latter extension to signify an ending with a falling syllable. This has been anthropomorphized to “who cooks for you, who cooks for you – all.” That having been said, the barred owl reverts to the silence of a lurking predator poised to pounce at first incitement.

Like most birds, owls are monogamous during a single mating season and frequently for life. This arrangement is necessitated by avian reproductive strategy; eggs must be laid and guarded until the subsequent hatching and rearing of chicks by one of the two partners in attendance at almost all times. Their union in a proposed breeding transaction is normally a cavorting male displaying derring-do coupled with an offer of food in the vicinity of a likely nest; the female either accepts or rejects according to parameters that ultimately determine the mixture of evolutionary traits. An unusual result of this selection is sexual dimorphism; male owls in general are smaller than females. One theory is that smaller males are selected because they are better hunters, their diminutive size allowing for greater speed and agility in a woodland hunt venue. This would be consistent with the observation that the male does almost all of the hunting, delivering food to the nest as often as ten times a day; the female broods. Barred owls nurture their brood of two to four chicks for four months, a long term commitment of some distinction requiring tenacity and superior hunting skills; the match must be good.

Owls do not build nests; they expropriate abandoned hawk, crow and sometimes squirrel nests becoming squatters as well as brooders. This relatively unusual avian strategy is not without consequences; it is considered probable among ornithologists that owl populations are limited by adequate nesting sites and not by the availability of food. Consequently, most owls are fiercely defensive of their territorial boundaries and will attack any intruder with vicious vigor. This includes other owls and humans, whose errant and mostly innocent incursions into the wrong neck of the woods can and has resulted in severe injury. Nest building is one of the more notable avian traits; its absence in owls is noteworthy and is likely be an unintended consequence of natural selection for some other key attribute. Owls do not maintain or improve the nests they occupy; only the accumulated fur and feathers of prey brought to the nest for dismemberment and evisceration and the occasional owl pellet cushion the bottom for the delicate eggs and hatched fledglings. Owl pellets consist of animal victim parts that were neither removed prior to ingestion nor digested for the extraction of nutrients.

Owls produce pellets because they have no crop; everything they eat passes directly to the digestive system., The crop, also known as a craw or croup, is a food storage repository in the gullet of meat eating raptors and most other birds (excepting also geese) from which inedible constituents can be regurgitated. Owls are not finicky eaters, and, with the exception of food brought back to the nest for nestlings, prey is largely eaten whole or in large chunks. Owls have the two stomach arrangement of other birds consisting of an enzyme and acid producing proventriculus and a muscular ventriculus or gizzard. The former produces gastric digestion factors and the latter grinds meaty body parts for nutrient extraction in the intestines and retains or filters the bones, teeth and other detritus. As with all living things, all that is ingested must be digested for energy and growth or expelled as waste. The ground meat that made it past the gizzard is drained of its food value on the way to the cloaca, a holding area for bird “droppings.” The mixture of white and brown fecal material is all that remains, as most birds have no bladder; the cloaca vents to expel its contents not infrequently over parked cars. The indigestible fur, bones and teeth are retained in the gizzard as a hardened pellet that must also be expelled. The now manifest owl pellet is passed back up the alimentary canal to the proventriculus where it blocks the digestive system until it can be regurgitated. Owl pellets are prized by biologists and are frequently used in science classes as a somewhat grisly exercise in literally coming face to face with the realities of the food chain.

Owls in lore and legend range from evil harbingers of ill will to beneficent founts of wisdom; the Harry Potter assignation as messenger between magical and muggle is only the most recent. Their peculiar behaviors appear preternatural, the agent of an unseen entity whose will they manifest. According to ancient Greek tradition, owls symbolized sagacity. The tetradrachm coin of Athens portrays an owl as the symbol of its patron Athena, the goddess of wisdom in the Greek pantheon. While there is no mythological explanation for the association, it is theorized that the ability of the owl to see in the dark led to the attribution of clairvoyant powers. However, it may also be due to the plethora of owls on the Attic peninsula. The goddess Minerva is the Roman equivalent to Athena with the same wisdom and penchant for owls; the association accordingly spread to inculcate the wise old owl as a foundational aspect of western civilization. As Pax Romana succumbed to the gradual onslaught of the Germanic tribes, the owl was supplanted from its position as a deity’s consort to something more nefarious. It is plausible that the owl as agent of the devil was a natural segue in the medieval era with its dichotomy of holy light and Stygian dark. The nocturnal nature of owls was a clear sign of being in league with Satan; the piercing gaze and rotating head clear evidence of possession by demons.

Owls as omens of death and misfortune are the predominant historical leitmotif of cultural associations ranging from India and China to Native Americans both north and south. Shakespeare regularly used the owl as an omen of malevolence; Lady Macbeth responds to the return of her murderous husband with an acknowledgment “I heard the owl scream.” The range of owl associations is highly regional indicating individual local traditions. In Wales, an owl’s cry meant an unmarried girl lost her virginity; In Spain, owls cry ‘cruz-cruz’ (cross, cross) because they witnessed the crucifixion of Jesus; In Germany, if an owl hoots when a child is born, it will have an unhappy life. In direct contradiction to the benevolent Athenian owl, the Aztec god of death Mictlantecuhtli is usually depicted with owls. The predominant owl association of the other North American Indians would equate to the European “bogeyman” lurking in the night shadows to abduct recalcitrant children. And for all of this, owls live and thrive, and really couldn’t give a hoot.