Birds have incredible sight and hearing abilities. Being around them, it does not take long to realize they are much more visually and auditorily acute than we humans. This post will discuss hearing in birds and try to explain some of the reasons birds are so 'blessed'.
Birds depend on sight and hearing to locate food, to navigate, to protect themselves and their offspring from harm and predators, and to locate mates and other members of their species. The Trichomonas victims (House Finches) we are seeing this winter around some feeders are a good example of how handicapped a bird is without good sight or hearing.
Hearing is the second most important sense possessed by a bird, second to sight. Birds can hear sounds located at a much greater distance and much lower in frequency than we can hear. We have had a flock of Wild Turkeys visiting our yard lately and the slightest sound in the house causes them to raise their heads and look at the house. The same is true of the many migrants arriving daily to feed.
Let's begin by describing how bird anatomy assists their hearing. Not all birds are the same but many species have developed anatomical features that enhance their hearing abilities. These features involve the head mainly. Owls, for example, need acute hearing since they hunt mainly at night. They often feed on rodents and they need to be able to hear a vole, for example, as it tunnels through heavy grass. Owls have developed flat faces and facial discs which collect sound like a parabola. Note the facial discs in the Great Gray Owl and the Barn Owl shown above. The sound is collected by these discs and is funnelled to the ear openings. Also note that the beaks of owls are positioned flat against the face (pointing downward) so that they do not distort the sound. Head shape is also significant. Studies have shown that the oval head shape of most birds helps with the processing of sound waves similar to the external ears of humans. The feather ruffs on the face and around the facial discs work similarly. Also do you know that they can change the shape of the facial disc as they wish in order to focus sound better?
Great Horned Owls and some other species (Long-eared Owl, Short-eared Owl, Eastern Screech-Owl) show ear tufts on their heads. Let's make it clear. These have nothing to do with hearing but are for display. They are made of tufts of feathers and have no connection to the auditory system. They do, however, sometimes help birders with finding and identifying the species. I recently found a female Great Horned Owl hunkered down deeply in her nest and those ear tufts helped in finding her.
Birds do not have external ears (pinnae) like humans do. They are descended from reptiles and external ears are not a reptilian trait. They have, however, adapted wonderfully! Their external ear canal openings (apertures) are on the sides of their heads. If they have facial discs they are placed on the edges of the discs so that the sound is funnelled to them. Some apertures are just covered with feathers for protection and to stop the muffled sound of air rushing past. Some species have a cover over the aperture. In some owls this covering (operculum) is round and acts like a valve. In other species it is just a fold of skin.
Many (most?) bird species have their apertures (ear openings) symmetrically placed on the sides of their heads. Each side of the head matches the other. Some species of owls, however, have asymmetrically set apertures. The opening is set higher on one side of the head than on the other. This is an amazing anatomical adaptation to enhance owl hearing. Some other species of owls (Ural, Great Gray, Boreal, Saw-whet Owls) have another amazing adaptation. They have asymmetry but it is in the temporal area. There is an enlarged area on one side of the head in the area of the eye and where the jaw moves under the eye. This slightly lop-sided anatomy is used to help improve sound acuity for these species.
So how do birds locate the sound source accurately? When a bird hears a sound it is able to pinpoint the direction it is coming from because the sound hits each ear at a different time. This is a minute difference but an owl can detect a difference of 30 millionths of a second! As the owl hears the sound it can tell which ear heard it first and it turns its head toward that direction. When the sound hits both ears simultaneously, it knows it is directly facing the source of the sound. In owls with auditory openings unevenly placed on the head, they can tell better if the sound is coming from higher or lower areas. So these owls can locate a sound in 4 ways; left, right, up or down. These stimuli are computed instantaneously in the brain and the owl then knows exactly where the prey is located spatially.
Owls have another adaptation to make them even more adept at prey location. The area of the brain where sound is processed is called the medulla. Studies have shown that owls have more complex medullae than other bird species. A Barn Owl medulla is estimated to have 95,000 neurons which is 3 times more than crows! (Campbell, Wayne. 1994. Know Your Owls. Axia Wildlife). Once the owl has located the prey it flies directly to it, keeping its head zeroed in on it. If the prey moves, the owl does another quick calculation and adjusts its flight path.
Note the apertures (auditory openings) shown in the above photos. I have selected featherless heads to make them more visible except for the cardinal. Some birds have round apertures and some have slits. Some have developed muscles which pull folds of skin over the aperture for protection.
The story of bird hearing is not complete without a word about what happens once the sound waves enter the ear canal. Birds have a smaller hearing range to humans but they have a more acute sound recognition. They are very perceptive to pitch, tone, and rhythm. This translates into different sounds, calls and songs which they use for territory defence, sourcing food, protection against predators, etc. Some birds are capable of using echolocation (like bats). These are cave-dwelling species and they use rapid chirps and clicks to navigate in dark caves. Swifts are one species that uses echolocation.
As in humans, as the sound enters the auditory canal it meets the tympanic membrane. Here the similarity ends. The tympanic membrane in birds is double. The outer membrane protects the inner, more sensitive membrane. The sound causes the tympanic membrane to vibrate and this vibration is transferred to the ossicular chain which transmits the vibration to the cochlea. The ossicular chain is in 3 parts in humans but in only one part in birds. This is called the columella. The cochlea in birds is not spiral-shaped as in humans but is straight or banana-shaped. The cochlea contains vibration-sensitive hair-like structures which transmit the sound stimuli to the medulla in the brain where it is interpreted for the bird.
Birds hear over a wide frequency range. They usually have a high degree of sensitivity in the low and middle range and it decreases rapidly as the frequency gets higher. One study shows Bullfinches hear in the 100 to 12,800 Hertz range. Pigeons can hear up to the 10,000 to 11,500 Hertz range. The hearing range of most birds is from 1000 to 4000 Hz but it differs some among species (e.g. Long-eared Owl, 100 to 18,000 Hz). Birds have another amazing hearing trait. As we know, human hearing gets worse with age and with exposure to loud noises because of damage to the sensitive hair cells in the cochlea. Not so with birds! Avian hearing maintains itself over the life of the bird. They do that by regenerating the sensitive hair cells in the cochlea as they wear out.
And that is not all! Bird hearing has even another amazing trait. Birds living in temperate zones have hearing ability that fluctuates throughout the year. When do they need better hearing? During breeding season, of course. They need to locate their mates, find their offspring, protect them, etc. They need good hearing to do this. Studies have shown that the auditory regions of their brains enlarge during breeding season. Later in the season when song is not so important, that part of their brains then shrink.
All this information puts 'birding by ear' in a new light. We use bird sound to identify birds often in our study of birds. It is good to think about how much more important hearing is to them. It also makes us wonder about what noise pollution is doing to them. How much damage is it causing to them physically and how does it disrupt their breeding and navigation behaviours? And, what about airports where they are using noise to remove birds? What is that doing to their hearing? And, considering birds' ability to regenerate hair cells in the cochlea, would further study provide information which could help with neurodegenerative diseases in humans? Without a doubt, birds are fascinating!!
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