Birds have a highly developed sense of hearing that allows them to navigate, communicate, avoid predators, and find food effectively. Unlike mammals, birds do not have external ears. Instead, they have developed specialized structures and adaptations that allow them to detect sound waves and interpret noises in their environment.
Bird Ears vs Mammal Ears
Mammals like humans have external ears known as pinnae that help collect and funnel sound waves into the ear canal. The waves then hit the eardrum causing it to vibrate. These vibrations are transmitted to the inner ear where they are converted into nerve signals that are sent to the brain for interpretation.
Birds do not have pinnae or an external ear canal. Instead, they have a single orifice on each side of their head called an ear opening. This leads to the middle ear cavity and the inner ear. While the outer part of a bird’s ear is simple, the middle and inner ear structures are highly adapted and complex.
The Avian Middle Ear
In birds, sound waves travel through the ear opening and hit the tympanic membrane, or eardrum. This membrane vibrates in response to the sound. On the other side of the eardrum is the columella, a bone homologous to the stapes or stirrup bone in mammals. The columella transmits and amplifies the vibrations from the eardrum deep into the inner ear.
A key difference between bird and mammal middle ears is the presence of a syrinx or sound box in birds. This box surrounds the columella and eardrum and helps amplify and modulate the vibrations before they reach the inner ear. The syrinx plays a crucial role in birdsong and communication.
Two muscles, the stapedius and quadrates, also attach to the columella in birds. Contractions of these muscles dampen vibrations and protect the inner ear from excessively loud noises. This is similar to the stapedius reflex in humans which activates in response to loud sounds.
The Avian Inner Ear
In the mammalian inner ear, sound waves are converted into nerve impulses using the organ of Corti which sits on the basilar membrane inside the cochlea. Birds lack an organ of Corti but their inner ear does contain a cochlea and basilar membrane.
Instead of hair cells on the organ of Corti, birds have hair cells located on multiple papillae in their cochlea. There are generally three papillae: upper, middle, and lower. The basilar membrane can be divided into regions that respond maximally to different frequencies based on their stiffness and mass. Different hair cells are located along the best responding regions of the basilar membrane. This tonotopic organization is similar to mammals and allows birds to discern different sound frequencies.
While mammals typically have one row of inner hair cells for transmitting signals to the brain and three rows of amplifying outer hair cells, birds can have up to five times as many hair cells in their papillae. This may contribute to their excellent hearing range and ability to perceive both small and rapid frequency changes.
How Do Birds Detect Sound Without Ears?
The lack of external ears means that birds cannot rely on pinnae to funnel sound into their ear openings. Instead, there are a few key adaptations that allow them to detect sound effectively:
- Owl ears are placed asymmetrically on their skull – one ear is higher than the other. This helps localize sounds in multiple dimensions.
- Some owl species, like barn owls, have concave facial discs that help collect and direct sound toward their ear openings.
- The ear openings are surrounded by ruffs of specialized feathers that may help refract sound into the ear.
- Having eyes on the sides of their head gives birds a wider range of peripheral hearing.
- Birds can turn their head 270 degrees or more to precisely pinpoint the direction of noises.
Overall, birds compensate for not having external ears through enhanced middle and inner ear mechanics. Their ears are extremely sensitive and adapted to detect faint noises, rapid modulations, and high frequencies beyond human hearing range. Well-developed neural circuits also allow birds to quickly process auditory information.
Hearing Range in Birds
Most small birds can hear frequencies ranging from 1,000 to 8,000 Hz. Larger birds like owls and parrots may be able to hear infrasounds below 100 Hz thanks to their large ear openings and ear canal volume.
For comparison, human hearing range is usually 20 to 20,000 Hz. So birds can detect many high-pitched sounds that humans cannot. Some bird calls and songs contain notes between 4,000 to 8,000 Hz.
Birds also surpass most mammals at detecting very low, infrasonic frequencies below 20 Hz. These extremely low rumbles are produced by atmospheric phenomena like storms or earthquakes and help birds sense impending danger.
| Animal | Hearing Range (Hz) |
|---|---|
| Humans | 20 – 20,000 |
| Dogs | 40 – 60,000 |
| Cats | 45 – 64,000 |
| Birds | 1,000 – 8,000+ |
As shown in the table, dogs and cats also detect higher frequencies than humans up to 60-65 kHz. But birds with their specialized adaptations have the widest hearing range that extends into both the infrasonic and ultrasonic extremes.
Uses of Hearing in Birds
Birds rely heavily on hearing for various behaviors and needs:
Communication
Most birds use vocalizations for communicating with each other. Calls can signal alarm, identify family group members, attract mates, convey territorial warnings, provide flight instruction, and more. Many species also learn their melodious songs from an early age by listening and mimicking tutors. Good hearing maximizes the effectiveness of avian communication.
Navigation
Birds determine direction and position using landmarks as well as sound cues. For example, owls can locate prey up to 100 feet away based on faint rustling noises. Seabirds may orient themselves using the low frequencies of crashing ocean waves. Urban birds find their way using honks, squeaks and other city noises.
Predator Evasion
Detecting faint sounds helps birds become aware of danger as early as possible. Many species use alarm calls to alert others about nearby predators. Woodpeckers and parrots can hear the high-pitched chewing sounds of a predator attacking a tree cavity from afar and take evasive action.
Foraging and Hunting
Whether seeking seeds, fruits, nectar or prey, birds rely on hearing to help guide their search. Owls can detect prey under leaves or snow by the sounds made from tiny movements. Vultures can find animal carcasses by tuning in to the sounds of other scavengers. Hummingbirds zero in on the buzz of insect wings.
Unique Ears in Different Bird Groups
While all bird ears share common features, some unique specializations exist in different groups.
Owls
Owls have some of the most developed hearing among birds. The facial disc surrounding each ear opening acts like a radar dish to amplify faint sounds and pinpoint location. Lopsided ear placement improves vertical sound localization. Many owls have soft edged feathers to dampen self-noise while flying to hunt by ear.
Parrots
Parrots have relatively large ear openings and enhanced hearing sensitivity, especially at high frequencies used in communication calls. Some species have eardrums that vibrate asymmetrically to help discern directional information.
Pigeons
Pigeons have fleshy wattles around their ear openings that may collect sound. They can hear infrasonic noises from hundreds of miles away which may help with navigation.
Oilbirds and Swiftlets
These unusual birds use echolocation for orientation in dark caves. Their ear openings are large to capture the returning echoes of ultrasonic clicks made by the birds.
Songbirds
Songbirds have excellent hearing within the range of bird vocalizations allowing them to learn songs during development. Some species like starlings and thrushes can even recognize and respond to human-made sounds in urban areas.
Turkeys
Turkeys have small openings to the ear canal that are surrounded by bare, colorless skin. Hairs protrude from the skin around the ear opening and help channel sound.
Waterfowl
Ducks, geese and other waterfowl have ears adapted for hearing even when swimming with their heads submerged. Ear flaps cover the openings to keep water out.
Birds of Prey
Eagles, hawks, falcons, and other raptors are adept at detecting faint rustling noises from small animals moving in vegetation below. Their eyesight and hearing are equally crucial for locating prey.
Unique Anatomy Allows Excellent Bird Hearing
The avian auditory system is uniquely adapted for enhanced sound detection and discrimination. While birds lack external ears, their modified middle ear and cochlea allow great sensitivity over a wide range of frequencies. Specialized feathering, head mobility, and neural circuits also compensate for the lack of outer ears. Together, these adaptations equip birds with keen hearing that is superior to our own.
Conclusion
Birds have excellent hearing despite the absence of external ears. Instead of pinnae to collect sound, adaptations like asymmetrical ear placement, feather ruffs, and large ear canals allow birds to channel sound to their inner ears. The avian middle ear contains a sound box and muscles that fine tune vibrations. In the inner ear, birds have hair cell-based papillae that encode the pitch and intensity of sounds. Excellent frequency range, sensitivity to volume changes, and neural circuits specialized for auditory processing gives birds an advantage over most mammals in detecting and making sense of the noises around them. This keen sense of hearing is crucial for birds to navigate, communicate, avoid danger, and find food.
