In reading Jennifer Ackerman’s Book, What an Owl Knows, many interesting scientific papers are mentioned, detailing some of the journey to our current level of owl understanding. Many of these are available online, but they are not always the most approachable things to read. I’ve bookmarked a few to explorer and try to share with you in a more friendly and condensed manor.
Today’s is a study about how we learned about owl’s amazing hearing abilities and how we began to understand their 3-dimensional ability to understand sound. It’s a hugely useful ability in how they get their food, and to stay one step ahead of their own predators. Knowing more about this will allow you to better understand these amazing animals and to get a deeper feel for how they are able to do some of the things they do.
ACOUSTIC LOCATION OF PREY BY BARN OWLS {TYTO ALBA) BY ROGER S. PAYNE Rockefeller University and New York Zoological Society Received 20 January 1970
Studies go back to the 1930s and 1940s to determine how owls locate their prey in the dark. After checking the minimum distance in low light for owls to both locate prey and navigate to it while avoiding obstacles on moonless nights under tree cover making light even less available it started to become evident the owls had some other means of navigating a lightless world.
One early theory proposed owls were able to see infrared. This was quickly rules out by seeing the owls eye did not have the proper anatomy to do so, and exposing owls to incredibly high levels of infrared light produced no response in the irises. This ability would also not explain how owls could see prey obscured by grass or snow tunnels.
After that, a theory was circulated that they used a heightened sense of smell. This never seemed to make it far into experimentation. Birds in general other than vultures are not known to have well developed sense of smell, and the distances owls would need to smell prey seemed extreme. Also in many of the experiments involving darkness, other observers had noted owls would many times bump into or step on the dead rodent specimens before noticing them.
Echolocation was also ruled out in the 1950s after another series of low light experiments determined at a certain darkness, owls lost the ability to avoid obstacles. If owls could echolocate, they would be able to avoid the obstacles in any light level.
Prior to Payne’s research, it was known many owls had asymmetrical ears that were thought to contribute to the owls’ ability to locate prey. What was not likely understood was just how much the structures contributed to those abilities. It was very difficult to examine the structure of the ear flaps, as most attempts to remove the flap feathers to see the actual flaps would destroy the flaps themselves. With Payne’s successful attempts, it was able to be seen the drastic increase in hearing ability the owl gained from being able to contort the angle of the flaps to create near parabolic structures to magnify available sound.
The ear flap feathers are unusually thick and are the densest packed feathers on the owl’s body, positioned about as close as physically possible, in a hexagonal structure. The skin surrounding these feathers is also exceptionally thick. All this adds up to a structure that can collect and reflect sound to an extraordinary degree. The heart shape of the facial disc splits incoming sound equally between sides of the face, directing it straight to the ears.
The feather structures of the face and ears themselves do not have a reducing effect on incoming sound either. A microphone placed in an owl’s ear only measured a <1 decibel reduction in sound across a wide spectrum.
Even the downward facing beak and tilting down posture of the head play a role in sound delivery. Most birds have horizontal beaks, while owls’ beaks point down to the chest. This removes it as an obstacle to incoming sound. By having a downward curving beak and holding the head so the beak is tucked in even more removes the largest blockage to the parabolic shape of the face.
There are numerous inner ear structures of the owl which are different to most other birds. They have larger cochlea and ear drums than average, and the ear drums extends into the cochlear fluid to better transfer vibrations and to amplify low energy sounds. Inner ear structures are symmetrical. The asymmetrical external structures seem to be the only parts shaped differently on opposing sides. There are also 3 large air chambers in the owl skull, making up to 25% of the volume of the head. These openings are connected to the middle ear structures, possibly allowing sounds from one side to be heard by both ears.
Payne participated in prey location experiments using owls in a darked room. The windows were sealed with Masonite and electrical tape over the windows. After ensuring no light appeared to be entering the room, the confirmed it by leaving camera film exposed on the floor and then overdeveloped it to confirm no light was entering the room. They also only conducted the experiments at night on top of everything else.
The room was equipped with 2 inches (5cm) of leaf cover on the floor, and a 7 foot (2.13 m) perch was placed for the owl to hunt from. A hand raised barn owl was given 5 weeks to adjust to living in the room in increasing amounts of darkness.
For the experiment, a live deer mouse was released in the room. During the first 3 nights, the owl did not attempt to catch the mouse. Starting the fourth night, the owl did go after the mouse. Over the next few days, there were 16 attempted strikes on the mouse at a distance of >12 feet (3.66 m) with only 4 misses, and no miss was by greater than 2 inches (5 cm).
The ability to use infrared or smell was ruled out by dragging a mouse sized ball of paper at the same temperature as the surrounding leaves across the room and it was successfully captured by the owl. Being the same temperature would leave no infrared difference from the surroundings, and the paper did not smell like a mouse or anything else the owl would eat. This seemed to leave passive acoustic location as the means the owls hunted. The next series of tests involved blocking one ear with cotton and releasing the mouse. With either ear closed off, the owl was able to determine the correct direction of the mouse, but the strikes came up 18 inches (46 cm) short. After the failed attempt, the cotton was removed and all subsequent strikes caught the mouse.
These experiments were successfully duplicated at a similar test room at Cornell. Further experiments there were done replacing the leaf layer with sand, sand with small piles of leaves, or a leaf tied to the mouse’s tail. Tests were also done in the light and different levels of darkness to see if the light level affected the owl’s striking position or the way it hunted. Hundreds of strikes were observed or recorded, and after the owl got accustomed to hunting in the light, there were no significant differences noted. Strikes in total darkness were viewed with infrared light.
Observations were able to be made of the owls changing direction or following the mouse with their face if the mouse moved while the owl was coming at it. The mouse holding still is the only thing that kept the owl from locating it in the darkness. As soon as it moved a significant amount, the owls were able to locate it no matter how dark the room was.
Some interesting notes were discovered in observing strikes in the light. The owls did not normally have their talons open until they were about 6 inches (15 cm) of the mouse, and also in 22 out of 24 strikes, the owls would give a final wing flap within striking distance to make a last minute acceleration at the mouse just as the feet came forward. No strike itself killed the mouse. The owls would apply a crushing grip to prevent the mouse escaping or biting them.
Owls did not seem to do much of their unusual head movements in total darkness. This leaves it open for further testing to see if the head bobbing and turning upside down is to aid in hearing or in vision. Other differences in hunting in darkness included the owls would fly at about half speed, and would also swing their legs like a pendulum during flight. It was thought this was to reduce any injury due to collisions with unseen objects.
The way an owl aligns with its target is also quite interesting. When the owl dives, it lowers its head to near where the feet are positioned. It then goes face first at the prey, until the last moment where it flips the feet to be where the ears were, negating the need to adjust the calculation of the distance from the ears to the target to that of the feet. The talons are opened near equilaterally and evenly spaced to create the largest grasping area possible and conforming roughly to the shape of the mouse’s body.
Another experiment was done to see if owls could determine the direction the prey was facing. A dead mouse with a leaf attached was dragged across the floor 12 times. In all 12 strikes, the owl oriented itself to have the talons come in parallel to the mouse, maximizing grasping area. The thought was a miss in potentially better than a strike landing only 1 talon, as then they could not get a secure grip on the mouse, which could lead to the owl being bitten by the unrestrained mouse. The owl would always adjust its flight path, even in total darkness, to come in with the talons lined up with the length of the mouse’s body.
Some further tests were done about frequency owls can hear and some calculations to determine how far away an owl will strike at a sound. Payne’s calculations based one the mouse’s size relative to the claw spread of the test owl determined a maximum distance of 20 feet (6 m), which correlated to observed strike distance of a maximum of 23 feet (7 m), making the calculated distance fairly accurate.
Many of the total darkness tests were attempted with other owls with different ear structures. Testing with 2 Screech Owls and 3 Great Horned Owls were unsuccessful on all attempts. Those species both have symmetrical ears. Experiments with Saw Whet Owls and Long Eared Owls were successful, even though both of those species use different physiological methods to achieve the asymmetry. Saw Whets have asymmetrical skulls, Long Eared Owls have asymmetrical membranes at the entrances to the ear canals, and Barn Owls have an operculum, a valve that changes the shape of the ear openings. Payne also mentions Barred Owls have yet another type of asymmetry, though it isn’t mentioned what that is. He states this implies evidence of the owls’ ability to locate prey acoustically has developed numerous times.
The results of all this testing show these owls have the ability to “see” sound in 3 dimensions, as they are able to determine frequency (tone/pitch), intensity (energy of the soundwave), phase (timing relationship between 2 soundwaves), and time-of-arrival (difference of time between both ears hearing a sound) of any observable sounds. They are able to internally calculate these values to track their prey. The time from observation to an attempted strike is lower the more head on the sound is coming to the owl, as it reduces the time of these calculations.
There were other experiments done that I don’t know how to interpret of Payne firing different sounds at a suspended dead owl with microphones placed in the ears and he created plots of how the sound is received and how the owl can interpret it, but that’s all beyond my level of understanding. Also some other ones where a dead mouse is moved while suspended on silk strands at different heights off the floor. The owls missed most of those strikes, and the thought seemed to be since the owls knew what the environment actually looked like in the light, they targeted past the actual mouse to the spot on the floor past the mouse if you would draw a line directly from the owl’s eyes through the actual location of the mouse, to the floor beyond.
Some earlier theories on how owls hear were ruled out, as those had stated the owl could visualize the sound regardless of the location of the source, but in all Payne’s experimentation, the owl always turned to face the sound directly before taking flight. The prior theory’s principles implied they could receive accurate location data no matter their facial orientation to the source. In his results, the owls required 2 sounds, and initial sound to alert them something was there and to get their head oriented to that sound, and then a follow up sound to focus in to determine the precise location.
There are more results concerning frequencies they are able to hear, and how they use them to determine sound location that are beyond my understanding and some explanations on how the head and ear flaps move to cycle through frequency ranges to maximize specific parts of sound they hear, so if you can understand any of that, the final summary of the paper will probably hold some more great insights for you. This is the best I can make of it though.
There’s a lot more to this paper than I presented here of course, it’s 36 pages of actual tests and results, and I’ve gotten this down to about 4. I know I’ve learned a good bit by reading it, and I hope you enjoyed this summary.
