How birds listen without external ear decoded

Birds use their head to differentiate between sounds coming from different elevations as they do not have external ears like mammals, a new study has found.
The outer ears of mammals play an important function in that they help the animal identify sounds coming from different elevations.
But birds are also able to perceive whether the source of a sound is above them, below them, or at the same level.
Now a research team from Technische Universitat Munchen (TUM) in Germany has discovered how birds are able to localise these sounds, namely by utilising their entire head.
"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 from the TUM Chair of Zoology.

By studying three avian species - crow, duck and chicken - Schnyder discovered that birds are also able to identify sounds from different elevation angles.

It seems that their 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.

It all comes down to the shape of the avian head. Depending on where the sound waves hit the head, they are reflected, absorbed or diffracted.
What the scientists discovered was that the head completely screens the sound coming from certain directions. 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.
"This is how birds identify where exactly a lateral sound is coming from - for example at eye height," said Schnyder.
Most birds have eyes on the sides of their heads, giving them an almost 360 degree field of vision.

The finding was published in the PLOS ONE journal.