owls can hunt by sound --4/23/25

23 Apr 2025

owls can hunt by sound --4/23/25

0 COMMENTS

Today's encore selection -- from What an Owl Knows by Jennifer Ackerman. Owls can hunt in complete darkness by using their extraordinary hearing:

“Roger Payne was the first to show that barn owls can hunt their prey in complete darkness using only sound. Payne is better known for his discovery of songs in Humpback Whales. But before he went to whales, he conducted a set of brilliant studies on barn owls, exploring the precision of their strikes and the exact sensory cues they use to locate their prey. In one experiment, Payne blocked all light from a room so it was pitch black, and placed an owl on a perch in a corner. He covered the floor with leaves and then dragged a mouse-size wad of crumpled paper through the leaves. The owl tried to strike the rustling paper. The bird wasn't using sight or smell or body heat in targeting its prey. ‘The paper wad and the leaves through which it was dragged were at the same temperature,’ wrote Payne. ‘Therefore, the owl could not have located it by any infra-red contrast with its surroundings. The paper wad put out no mouse-like odour, so homing on scent would be of no value. Because the lights were out, the owl could not see the wad .... The only possibility left, as I see it, is that the owl was orienting acoustically to the sounds.’

“Just to be sure, Payne tried blocking the owl's ear with a small cotton plug, first one ear, then the other. He let a mouse loose to scurry in the leaves. ‘In both cases the owl flew, in darkness, directly towards the mouse but landed about eighteen inches short of it,’ wrote Payne. ‘After each trial the cotton was removed, and the owl was allowed to try in total darkness to catch the same mouse it had just missed. In both cases the owl then struck successfully.’

“Payne also filmed owls targeting prey on the wing in complete darkness. The results were mind-boggling. When a mouse shifted direction, the owl would twirl its head toward the creature and adjust its attack midflight.

“How in the world does a bird do this?

“With a head designed for listening, like Percy's. The flat, gray facial disk of a Great Gray Owl is like one huge external ear, a feathered satellite dish for collecting sound. Not all owls have the big, pronounced facial disks of Great Grays, Boreal Owls, and barn owls. It's smaller in owls that rely less on sound for hunting--Great Horned Owls, Little Owls, pygmy owls. And in some species, like fish owls, it's dramatically reduced. This makes sense. Streams are noisy, water is noisy, and sound reflects off the air-water surface. So, presumably, a hunting owl can't hear a fish. But fish owl expert Jonathan Slaght thinks the birds may be using sound more than we think. He showed me a photo of a Blakiston's Fish Owl on a riverbank in which the bird ‘really looks like it's using its facial disk,’ he says. ‘So I think these “owly” features are diminished but not gone.’

Adult male great grey owl (Canada)

“The facial disk in owls that hunt primarily by sound is outlined with a ruff, or ring of stiff interlocking feathers that capture sound waves and channel them toward the ears, like people cupping their hands around their ears. Feathers in the back of the disk direct high-pitched sounds toward the ears, so the owl hears less noise from its surroundings and can focus on prey cues. ‘The diversity of feathers on a Great Gray Owl's facial disk is just phenomenal,’ says Jim Duncan, an expert on the species. ‘Seven or eight different kinds. The ones you see are very loose and filamentous, and sound travels through them quite easily. And then there are these curved solid feathers that form the back of the facial disk and act like the parabolic reflector part of the disk. The curve probably reflects the optimal angle for sounds hitting the disk to be directed into the ear cavities.’ Percy can even change the shape of the disk by using muscles at the base of the feathers, shifting from a resting state to the alertness of an active hunt. It's remarkable to watch an owl do this, adjust its facial disk when it hears something interesting. It's like the disk itself is a kind of aperture, an ‘eye,’ that opens wide to let in more sound and bounce it toward the ears.

“The use of the term eared in the common names of some owls is confusing. Long-eared and Short-eared Owls have tufts of feathers on the tops of their heads, called ‘plumicorns’ (from the Latin for ‘feathered horn’), which look a lot like mammal ears. But these tufts have nothing to do with hearing and everything to do with camouflage and, sometimes, display.

“An owl's actual ears are just openings in each side of the head, well covered with specialized feathers that allow sound to pass through. Their size varies from species to species, depending not just on whether they hunt in daylight or darkness, but on the general invisibility of their prey. The night-hunting Long-eared Owl, which feeds heavily on small rodents, does in fact have long ears in addition to long tufts, with ear slits that run from the top of its head all the way down to its jaw. Barred Owls and Boreal Owls, which are strictly nocturnal, also have large ear openings. But so do pygmy owls, which often hunt by day, because the little rodents they prey on are often hidden in dense grassland and must be sussed out by sound.

“And Percy's prey? In the wild anyway, it's often buried deep in snow, which not only obscures everything visually but also creates what's called an ‘acoustic mirage,’ distorting the location of sounds beneath it and making it harder for a bird to pinpoint its prey. As we'll see, Great Grays have evolved some truly spectacular strategies to deal with the challenge.

“In any animal ear, a little sliver of tissue called the ‘cochlea’ collaborates with the brain in the hard work of hearing. The cochlea contains hair cells sensitive to the vibrations of sound, and its length in an animal is a pretty good measure of hearing ability. In most owls, the cochlea is enormous relative to body size and contains huge numbers of hair cells compared with other birds. The cochlea of a barn owl, for instance, is massive. ‘It's the race car equivalent of the bird inner ear,’ says Christine Koppl, who studies barn owls at the University of Oldenburg in Germany. In her talks, Koppl shows a slide comparing the cochlea of a barn owl with a range of other bird species--blackbirds, jays, buzzards, and hawks. The owl's cochlea is easily three or four times as long as those of other birds, giving it extraordinarily acute hearing.

“An owl's auditory system shares with other birds another superpower we mammals don't possess: it doesn't age. To see whether the hearing of barn owls changes over time, Koppl collaborated with two colleagues, Ulrike Langemann and Georg Klump. The scientists trained seven owls varying in age to fly from one perch to another to receive a treat in response to an auditory signal. Then they separated the birds into ‘young’ and ‘old’ age groups and tested their hearing by changing the tones, moving them up and down the frequency scale. The team found no age-related hearing loss at all between the young barn owls and the older ones. In fact, the star of the study, a twenty-three year-old Methuselah of a bird named Weiss, could hear the full range of tones just as well as the two-year-old birds in the study. This suggests that owls, like other birds, have the capacity to regenerate their hair cells, keeping their hearing keen throughout life.

“We mammals are not so fortunate. Growing old as a human or mouse or chinchilla brings with it age-related hearing loss, especially in the range of high-frequency sounds. In our ears, damaged hair cells are not replaced as they are in birds, and we can only envy the regenerative powers of owl ears.”