Photo: Great Horned Owl, Michael Rosenbaum/Audubon Photography Awards
Deidre Bryant
Autumn is in full swing. November and December are the best times to enjoy the beautiful sound of hooting, especially from Great Horned Owls (Bubo virginianus). Although they call a variety of wilder environments home, they can live among us on farms and in small woodlots and city and suburban parks. We can hear them most effectively now because the end of our calendar year is courting and territory setting time for Great Horned Owls, and late fall weather creates conditions that allow us to hear the birds’ calls better.
Great Horned Owl, Michael Rosenbaum/Audubon Photography Awards
The Great Horned Owl is probably one of the most recognizable birds in the Americas, with its large fluffy feathers, pointed tufts on its head, and unmistakable low, but soft call. A common hooting pattern is a longer “hooooooot,” followed by three or four shorter hoots. During a duet, females will reply to a male with a raspy hoot in a higher pitch. Great Horned Owls have a range of other vocalizations, too, some of which sound like a screeching cat. Despite their notable appearance and recognizable calls, they are not always easy to locate because of their nocturnal habits and preference for places with little to no light pollution.
Nevertheless, at the right location, and especially at this time of the year, you can hear a chorus of hoots piercing the night and providing an enchanting soundtrack from dusk till dawn. That’s because in November, male Great Horned Owls begin scouting and claiming vast territories (an average of about 1.5 square miles) around their nests. Come January, mating will already be underway and a pair will begin settling in their home. They do not build nests, instead typically taking over nests previously used by squirrels, red-tailed hawks, or other arboreal animals.
When I was much younger, I used to think owls only called during the cooler months and at night. Owls will actually hoot throughout the year and quite often, even when the sun is up. But, in many ways, it’s much easier to hear them on crisp, cold nights. If you’ve ever been to a night football game and listened to a marching band during halftime or when they’re in the stands, you’ll know exactly what I’m talking about. Sound doesn’t get louder as the air gets colder, but it travels farther.
Sound is carried by the collision of vibrating molecules in a longitudinal wave, and sound waves travel faster in warm air because warm air molecules are more energetic than cold air molecules. Faster moving molecules mean sound waves will, in fact, travel faster. Relative humidity also affects sound. Because increasing humidity lowers the density of air and its ability to absorb sound, sound waves move faster in high humidity than in low humidity. On the other hand, fall and winter conditions carry sound farther, even though it may travel more slowly. A number of fall conditions can cause this effect.
Temperature inversion. Sound moves in all directions from its source, including up. During cooler months, a layer of cold air sits under a layer of warm air, and the boundary between them refracts or bends the sound waves, redirecting them toward the ground. (Think of how a prism affects light.) When the sound waves oscillate, or move back and forth, between the cold and warm air layers, they are amplified and can be heard over a greater distance.
Leaf fall. The absence of leaves from deciduous trees and shrubs prevents sound from being dampened, allowing sound to travel farther.
Windy conditions. The wind on breezy days and nights also can refract or bend sound waves. Wind moves faster at greater heights because it encounters fewer obstacles. The difference in speed of the wind causes sound waves to bend towards the ground and travel farther when they travel with the wind (but upwards if they’re travelling against the wind). When an owl is hooting on a windy day, its hoots travel farther downwind than on a still day.
The fall acoustics help owls claim and hold nesting territory. The distance sound travels in some fall conditions allows hooting to be more effective in alerting intruders who enter a claimed territory or in signaling the presence of a threat to other owls. Better acoustics also facilitate courtship displays over long distances.
Of course, even without the assistance of fall conditions, territorial hooting, because it is generally very low-pitched, can travel farther without significant reduction. So why does low-pitched or deep sound travel farther? Pitch refers to frequency, and low frequency sound is from a sound wave that oscillates fewer times per second. Low frequency waves travel farther than high frequency waves because there is less energy from oscillation transferred to the medium (in this case, it’s air) allowing them to have relatively more energy left over to travel longer distances. Think of a fog horn, which produces a low-pitched sound.
If you’re interested in more than territorial hooting, here’s a link to an interesting article on the variety of owl vocalizations.
As with many birds, great acoustics are paramount in acquiring a mate and claiming territory. However, it’s quite fascinating how Great Horned Owls are able to take advantage of their cold, dark environment while other birds, who breed in the spring and summer, have migrated elsewhere. And, anthropomorphizing just a little, isn’t their calling out to a long-term (maybe life) mate over the miles just a little romantic?
Happy hooting season!