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The Western Roof-Owl: Bird of Mystery

April 1, 2010

First in a series of biographies of our local avifauna.

Western Roof Owl in typical roosting posture (A.Non 2/10)

The Western Roof-Owl, Bubo pneumatikus (WRO), is in many regards unique among the owls of the world. Most owls are nocturnal predators which roost during the day in hard-to-see locations, deep in foliage, high in trees or cliff holes or on rafters in dark barns, in order to avoid detection by their justifiably annoyed prey. At night such owls are often heard calling and occasionally seen pursuing their prey: small mammals, especially rodents, and small songbirds.

In contradistinction, the WRO eschews dark and hidden perches and does its daytime roosting right out in the open, usually on peaks or edges of roofs, its preferred perch, occasionally also on large antennas and fence posts. This atypical behavior causes it to be perhaps the most commonly seen owl in Western North America, although it is not the most abundant.

This peculiar roosting behavior permits the easy observation of its most recognizable and remarkable behavioral characteristic – complete immobility. Once it has chosen its diurnal roosting spot, it never moves: neither broiling desert summer sun nor freezing winter mountain storm can cause this bird to do so much as blink an eye. Many species of small birds – potential nocturnal prey for the WFO, one presumes – notice this lack of movement and actually seek out its company. European Starlings and Rock Pigeons are often observed to perch right on the WRO’s head, sometimes for hours. It is conjectured that such birds are attempting to demonstrate friendship with this large predator, perhaps in order to impress their friends or frighten away potential rivals, but no one knows for sure. [This possibility provides an intriguing subject for a Ph.D. thesis in Avian Ethology.]

The Western Roof Owl – or at least the most intensively studied local subspecies B.p.immobilus –  is about the size, shape and coloration of its more infrequently seen congener, the Great Horned Owl Bubo virginianus. In fact, the best way to separate these two species is by roosting location and behavior. If you see it roosting immobile on a roof, it is most likely the WRO. If you can’t see it, it’s the Great Horned Owl.

Right-looking behavioral morph (function of the tiny feather-curl at nape of neck is unknown)

So secretive and little-known is the nesting and feeding behavior of the WRO that there is not a single recorded observation of the bird leaving or arriving at its roosting site. One millisecond they are not there, the next millisecond they are, never to move until they again vanish, unseen.

Researchers conjecture that they hunt only on moonless cloud-covered nights. In utter pitch-black skies, no living creature ever sees them in flight, and their prey die never knowing what hit them. If true, this would go far to explain the lack of fear they elicit from potential prey species at their roosting sites.

If these conjectures are correct, such behavior necessitates certain physiological characteristics. They must have exceptional hearing as does the Barn Owl which can locate a vole rustling in the grass at 100 meters in complete darkness. [Bizarrely, no researcher has ever been able to detect any external auditory canal on the WRO. However, unless the owl can detect body heat in the infra-red, they must have excellent hearing. This is another excellent subject for an enterprising Ph.D. candidate.]  They must be able to catch great numbers of prey during their infrequent hunting expeditions, as it may be a long wait until the next suitably pitch-black night occurs. This explains why they are not found north of the Arctic Circle where the sun may not set for months. Their digestive systems must be extremely efficient in order to extract every calorie of energy from each morsel of whatever it is that they eat. This would explain the complete lack of regurgitated pellets around their roost sites: there are no pellets as they digest everything – fur, feather, bone, gristle, shell, skin. It would also explain why they never move: they are conserving energy in order to insure survival through what may be a very long fast. It should be noted that their apparent sleep must actually an exceptionally deep form of torpor, a form of near-suspended animation also used by hummingbirds at night and by the Common Poorwill during the winter. The WRO’s torpor is so deep that no medical equipment can detect any heartbeat, breathing or thermoregulatory activity.

Needless to say, nothing is known of their breeding biology. No nest has ever been found, no downy or juvenile bird has ever been seen. They simply appear, full size and in adult plumage. Neither has any sign of molting ever been detected.

The Western Roof Owl has yet one more unique feature: it is the only avian species known to have behavioral morphs. Many species have color morphs: dark-phase and light-phase Red-tailed Hawks for instance. Such color morphs do not indicate subspecies status,  they are simply a coloration variation that the individual possesses throughout its life. As far as researchers can tell, the straight-ahead and the right-looking forms  are lifelong and invariant behavioral morphs.

All-in-all, the Western Roof Owl is one of our most interesting local species. Its easy visibility when roosting recommends it to any diligent observer of birds. The difficulties one encounters in actually witnessing it doing anything only make the eventual documentation of its mysterious behavior that much more rewarding a pursuit.

If you found this article plausible, you may be interested in the other monographs in our annual series:
2011:   New Hummingbird Species Discovered in Los Angeles County
2012:   Canyonlands Roadrunner Captured on Film
2013:   Birders Take Their Lumps with their Splits

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