Montana Best Times: Big Sky Birding Column

Great Horned Owls -- Stealth Bombers of the Night (March 2006)

Ernest T. Seton (1890), one of the foremost outdoor writers of his time, summed the Great Horned Owl best when he wrote : "their magnificent bearing; their objection to carrion, and strictly carnivorous tastes--would make me rank these winged tigers among the most pronounced and savage of the birds of prey". There is no question, if the Golden Eagle (Aquila chrysaetos) is to many like myself the king of birds during the day, then the Great Horned Owl is the stealth bomber of the night. When one realizes the variety of prey both of these species are able to take, coupled with their ferocity, aggressiveness and morphological adaptations, it is no wonder they are rated way above the rest when it comes to avian predators.

The Great Horned Owl (Bubo virginianus) is one of the most common and consequently easily recognizable of fourteen nesting species (and fifteen total species) of owls found in Montana. The Latin term Bubo literally means "eagle-owl", and virginianus refers to its first discovery and description coming from the state of Virginia in 1788.

What makes Great Horned Owls so unique?
So what is it about the Great Horned Owl, otherwise known as a GHO, that gives it such an edge above other nocturnal owls? One of the answers lies in its size. It weighs on average about 3.3 lbs, with females weighing slightly more and males weighing slightly less. In North America, in size and in weight it ranks second only to the Montana winter migrant known as the Snowy Owl. In total length it ranks third, with Great Gray Owls and Snowy Owls being on average slightly longer and taller. For wingspan, the Great Horned Owl ranks second only to the Snowy Owl. Great Horned Owls have an average wingspan (i.e wing to wing) of approximately 62 inches. As far as tail length is concerned, the Great Horned Owl ranks fifth, with Great Gray Owls having the longest tail followed by Snowy Owl, Barred Owl, and Spotted Owl (a species not found in Montana). And lastly, Great Horned Owls have the second longest beak length of all North American owls, second only to the Great Gray Owl.

What other morphological or physical features gives Great Horned Owls and edge over other avian nocturnal predators? Great Horned Owls are the only large owl with distinctive feathered "ear tufts". They are not true ears, but simple plumage ornaments that serves numerous functions from attracting a mate to signaling behavioral changes.

These ear tufts can be elevated at times and can be totally hidden, depending on the circumstances. When the ear tufts are not visible, it is often a sign of the bird trying to hide or blend in with the environment to avoid detection. The actual ear canals that are instrumental in hearing are found in back of the eyes on the skull, and on the edge of the large feathered facial disk. One of the two ear canals is very slightly lower than the other, allowing nocturnal birds such as Great Horned Owls and opportunity to locate prey through the triangulation of sound. The "ear conch" is a kidney-shaped opening or flap of feathered skin that directs sound to the ear canal. Its hearing is superbly acute, assisted by a large round "facial disc"-feathers that direct sound waves to its ears. Its body feathers are exceptionally soft thus providing superb insulation, and its flight feathers possess leading edge fringes, serrations or flutings (functioning like a silencer on a gun) allowing for silent stealthy flight.

The eyes of the GHO are round, large, and fixed in orbits in the skull. Inside each eye is a sclerotic ring that resembles bone and surrounds the yellow iris, sort of like an old large, wide, white wall tire. This sclerotic ring gives strength and protection to the eye, especially when they come in contact with prey or obstacles such as branches or bushes. Their large eyes are equipped with many rods ideal for night vision, and pupils that open widely in the dark. Although their eyes are set in sockets and do not move, flexibility in the atlanto-occipital joint (i.e. where the skull attaches to the vertebrae) enables this owl to swivel its head more than 180° from either direction. The eyes can be converted from wide open to narrow slits depending on whether the bird is hunting or trying to hide. Lastly the feet and the talons of the Great Horned Owl are extremely large and powerful for their size. There is feathering to the toes on the feet, with the feathering on the toes more obvious in the winter than in the summer. This degree of feathering on the feet is connected to insulation and seasonality. Owls also possess a reversible outer toe, allowing each foot to perch with two toes pointed forward and two toes pointed backward.

Hunting and Nesting
Great Horned Owls are quintessential avian nocturnal predators, but also can hunt the crepuscular hours of the day (dawn and dusk) and very uncommonly during the day. GHO's, like most owl species do not build nests, but rather take over a wide variety of sites for nesting ranging from: raptor, heron, magpie, eagle, osprey nests; mistletoe; alcoves in cliffs; cavities in trees; buildings; mine shafts; to even geothermal features. They can nest in Montana anywhere from 1,850 feet in elevation up 8,500 feet. Most GHO's are found on their territories year-round. GHO's are very vocal during the months of December through February, when they make their characteristic "whooooo-who-who--------whoo,whoo" calls. Breeding displays involve actively bowing, hooting, tail flexing, and also inflating and fluttering of the white throat feathers. Egg laying starts February through April depending on the elevation, with incubation at approximately 33 days. Egg clutches range from one to four, with two being an average number of eggs. The eggs are white and round. Young are born usually days apart, hence the owlets are different sizes due to asynchronous hatching. This is theorized to be a mechanism to counteract food shortages should they occur. Larger owlets are more likely to survive these food shortages due to competetion for food. Fledging is different in owls in that they don't truly fledge, but prematurely leave the nest usually by hopping, thus the term "branching or branchers". Branchers can first leave the nest at 6 weeks of age, but are quite reliably found around the area of the nest site for at least 9-10 weeks.

So when you are driving down a road or walking down a trail in the evening or at night, and you see a large brown-gray owl, bigger than a large frisbee. Check it out! And take notice as to whether it has a direct flight trajectory or is very loud and is hooting up a storm, and nine times out of ten you will find it is a Great Horned Owl-the stealth bomber of the night.

Now that the reader has a better understanding of Great Horned Owls-Stealth Bombers of the Night, the next topic scheduled for the May issue of Montana Best Times will be something different. It will a series of short stories about my personal experiences with birds taken from a new book about to be published entitled Lucky Feathers-Adventures and Experiences of a Yellowstone Ornithologist.

In the meantime enjoy! And the Best of Big Sky Birding to you!