Woodlot in Columbus, Ohio. Photo by M., Wikimedia Commons
(Click on the following link to listen to an audio version of this blog … Silent flight
When I was a graduate student at Ohio State in the late 1970’s, I took a course entitled ”Forest Biometry.” The course was mix of extremely useful topics for anyone who wanted to do field research in forested ecosystems, or who was working toward a position in which they would need to be able to make sense of field research reports. We covered simple, field mapping techniques (compass and pacing), the design of sample plots and sampling/cruising patterns, how to measure trees (heights and dbh’s)), and spent a lot of time on very practical statistics and data interpretation. It was a good course.
I was the only ecology student in the class. There were forest product majors, natural resource management majors, general agriculture majors and so on. There were 15 or 20 of us in the class, and we were divided up into working groups of four for our semester projects and reports. My group mates were taking the class as a degree requirement and were amazed that I was taking it as an open elective. There was a lot of math in this class, something most of my fellow students tried to avoid!
One of the things that the long history of Forest Biometry students had to do was map a two acre woodlot over on West Campus. I don’t know if this small piece of mixed hardwood forest still exits or not (Ohio State has done a lot of growing and changing in the past 40+ years), but even in the 1970’s these twoacres were, possibly the most measured and described bit of woods in the entire state of Ohio. Over the years, not only had the boundaries of the woodlot been mapped over and over and over, but, quite possibly, every tree in the woodlot had been identified, measured and located on map after map.
It was a very well-known space!
Great horned owl. Photo by C. Hume, Wiimedia Commons
One afternoon, I was out in the woodlot doing some tree measurements when I either saw something out of the corner of my eye or felt something move in close to me. I turned to my right and there, on a low branch of the red maple tree I was measuring, was a great horned owl!
The owl fixed both of his front-facing eyes on me and silently watched me for several minutes. He (based on his fairly moderate size, I assume that he was a “he”) then lifted up off of his branch and silently flew away. I never heard a whisper from his incoming flight or from his departing flight. It was like he was in a soundproof bubble, moving about separately from everything else around him! This encounter was like a dream.
I have had other owl encounters that have emphasized the quietness in flight. The great horned owl up in my front yard spruce tree back at my home in Pennsylvania. The great horned owl settling down like a ghost out in the front yard here in Greeley when I was out last year looking at the lunar eclipse. Only owls fly with such stealth. Even little song birds raise a rattling ruckus when they lift off from the ground or a perch, and large birds pulse great waves of sound out from their liftoffs and landings.
Why and how are owls so silent?
Spotted owl with mouse. Photo by E. Brauar. Flickr
The why seems obvious. Owls must be quiet so that their prey doesn’t hear them coming! This is called the “mouse ear” hypothesis. It is very straightforward and very logical and probably not correct. There is a range of quietness that different owl species exhibit and those owl species on the noisier side of owl-normal don’t seem to have any difficulty catching prey. Certain owl species, though, are extremely quiet. If it’s not to conceal themselves from their prey, then why?
Great gray owl. Photo by USFWS, Public Domain
Another explanation of why owls fly so silently is called the ”owl’s ear” hypothesis. All owls rely on hearing to find prey but certain owl species preferentially utilize their sense of hearing to locate their food. These owls must be extremely quiet or else they couldn’t hear the tiny rustlings in the leaf litter or under the snow that mark the location of a potential meal! Owls with broad facial discs (like great gray owls, boreal owls and barn owls) use this forward facing feature like a big, sound gathering disc. They pull in even very faint sounds and transfer them to their asymmetrically located “true” ears on the side of their heads. Their brains are wired to pinpoint the position of the sound source, and their strikes are aimed at those positions with amazing accuracy.
So the owls are silent especially so that they don’t mask the faint sounds of their prey!
How, then, can owls fly so quietly?
Noise comes from turbulence and friction. Air passing over a wing gets organized into turbulent flow which raises quite a racket. I remember a sharp-shinned hawk at a demonstration at the National Aviary in Pittsburgh being turned loose to fly over the heads of an audience. The clatter rough whirr of her wings flying a few inches over our heads were deafening! Lots of turbulence and lots of feathers- rubbing-on-feathers friction.
Owl wings. Photo by T. Higett. Wikimedia Commons
Owls lessen turbulence via two adaptations: 1. The front edge of their wing has an extending row of fine bristles (called “the comb”) that claw into the air rising over the wing and break it up into smaller and smaller volumes. This dispersion breaks up the turbulence of the air and dramatically reduces noise. 2. On the back edge of the wing there is a line of ragged feather edges that break up the air flowing over the wing and prevent it from gathering into a turbulent mass.
So the wing design of an owl keeps air from forming large, noisy, turbulent air masses.
Barn owl feather. Photo by K.C.Schneider. Flickr
Also the feathers of the owl are designed to minimize sound. The wing feathers have soft, elastic tips that dampen noises. If you have petted an owl, something you should do carefully and only with permission, you know how soft and velvety their feathers are. Their feathers have a surface feature called a pennula. The pennula is a fine layer of fluffy fibers that separate each individual feather. Air flows through the pennula layer and the feathers pass over each other with a minimum of friction and a minimum of noise.
So some advance aerodynamics in wing construction and advanced material science in feather composition all work together to make the flight of an owl extremely quiet!
There is a new book by Jennifer Ackerman entitled What an Owl Knows. It is an impressive blend of science and colorful annecdotes about owls. Such wonderful and mysterious birds! I recommend the book very highly!