Friday, February 3, 2017

Great Horned Owls

It's February, and Great Horned Owls (Bubo virginianus) are already nesting in California; in fact, some owls in this area begin breeding as early as January.  These large, nocturnal predators are particularly active at this time of year, and I've seen quite a few Great Horned Owls while I've been out and about this winter, even during the day.  Look for these magnificent birds roosting during daylight hours in thick stands of trees, often near the trunk.   

As the afternoon begins to fade to dusk, listen for the distinct and familiar series of mellow hoots that indicate a Great Horned Owl is near.  During the late fall and winter, you will likely hear a pair of owls calling to each other as courtship begins.  The pair may sing (or "hoot") simultaneously in a duet, or alternately, and often part of their songs will overlap.  The male's voice can be distinguished from the female's by its deeper tone, although he is the smaller partner in size.  In late December and January, I heard owls calling to each other as early as 3:00 p.m. on overcast days.  Most commonly, these owls are most vocally active during the first hour after sunset and just before dawn.

Great Horned Owls mate for life, though they roost separately apart from the nesting season.  They are quite territorial, with both members of a pair usually remaining on their territory throughout the year.  One pair requires around 2.5 acres of suitable habitat for their territory, which they will defend from neighboring owls through hooting contests.

The Great Horned Owl is North America's most widespread owl.  This species has successfully adapted to a wide variety of habitats, including virtually all California habitats except for the crest of the Sierra Nevada range.  Provided there is ample prey and adequate nesting sites, Great Horned Owls are equally likely to be found in oak woodlands, low-elevation conifer forests, chaparral, desert and riparian habitats; they are less common in high-elevation conifer forests, though not absent.  It is not unusual to find that these powerful predators have even adapted to suburban life; Eric and I have seen Great Horned Owls on several college campuses in the Great Central Valley, where there are stands of mature trees and open fields with plenty of gophers!

Owls in general are not known for their nest-building prowess; most species simply "renovate" an existing nest or cavity that has been abandoned by its previous tenants.  Great Horned Owls will use the past-season nest of other large birds such as crows or ravens, herons and hawks.  They make themselves at home with relative ease: stick nests in trees or on platforms will do, and so will old tree snags and cavities or niches in cliffs.  By utilizing natural cavities or niches in rocky cliffs, Great Horned Owls have been able to establish themselves successfully in desert regions.  The same nest may be used by a pair of owls for multiple years.

Great Horned Owls are formidable hunters, taking whatever prey happens to be available.  Adapting to eat nearly anything of manageable size, from large geese to small Jerusalem Crickets (Stenopelmatus fuscus), has allowed these owls to reproduce successfully and establish themselves in almost every habitat across North America.  They prey on other birds, including other owls, raptors and waterfowl; snakes, frogs and large insects round out the menu.  But by far, the most significant component of the Great Horned Owl's diet is small mammal species, such as rabbits, gophers, woodrats and mice.  Owls have a poor sense of smell, and are known for preying on the occasional skunk.  Swooping down silently on its prey, the Great Horned Owl snatches its victim with long talons, gripping with almost 30 pounds of pressure.  Prey is killed with a quick bite to the back of the neck from the owl's powerful beak. 

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