Although birds don’t have external ears as mammals do, scientists have discovered how they use their entire heads to perceive and localize sound. For humans and other mammals, the outer ears have an important function: they help us identify sounds coming from different directions or elevations. According to an article published online in the November 12, 2014 issue of PLOS ONE, birds are also able to perceive whether the source of a sound is above them, below them, or at the same level. The paper came from a research team at the Technische Universität München (TUM) in Germany led by Dr Hans Schnyder.
“Because birds have no external ears, it has long been believed that they are unable to differentiate between sounds coming from different elevations,” said Hans A. Schnyder, PhD, the TUM Chair of Zoology. “But a female blackbird should be able to locate her chosen mate even if the source of the serenade is above her.”
Mammals reportedly identify sound sources in the vertical plane using their external ears, which absorb, reflect, or diffract the sound waves because of their special structure. The research team aimed to find out how birds perceive different sound sources without external ears. By studying three avian species—crow, duck and chicken—Schnyder and colleagues discovered that birds are also able to identify sounds from different elevation angles. Apparently, a bird’s slightly oval-shaped head transforms sound waves in a similar way to external ears.
“We measured the volume of sounds coming from different angles of elevation at the birds’ eardrums,” said Schnyder. “All sounds originating from the same side as the ear were similarly loud, regardless of their elevation. The ear on the opposite side of the head registered different elevations much more accurately—in the form of different volume levels.”
How Birds Localize SoundThe shape of a bird’s head plays an important role in directional hearing, according to Schnyder. Depending on where the sound waves hit the head, they are reflected, absorbed, or diffracted. The scientists reportedly discovered that a bird’s head completely screens the sound coming from certain directions, while other sound waves pass through the head and trigger a response in the opposite ear.
The avian brain determines whether a sound is coming from above or below from the different sound volumes in both ears. Schnyder says that this is how birds identify exactly where a lateral sound is coming from.
Birds Use Both Hearing and Sight for Sound Orientation
Most birds have eyes on the sides of their heads, giving them an almost 360° field of vision. Since they have also developed the special ability to process lateral sounds coming from different elevations, they combine information from their senses of hearing and vision to useful effect when it comes to evading predators.
A few birds of prey, like the barn owl, have developed a different strategy. This species hunts at night and, like humans, its eyes are front-facing. The feather ruff on their face modifies sounds in a similar way to external ears.
According to Schnyder, the owl hears sounds coming from in front of it better than the other bird species that were studied. So there is a perfect interaction between the information they hear and the information they see, as earlier studies were able to demonstrate. Schnyder believes that the combination of sight and hearing is an important principle in the evolution of animals.
Source: Technische Universität München (TUM)