How birds localize sound sources sans external ears

A new research has discovered how birds localize sound sources without external ears.

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.

A research team from Technische Universitat Munchen (TUM) has discovered how birds are able to localize these sounds, namely by utilizing their entire head.

Researcher Hans A. Schnyder explained that because birds have no external ears, it has long been believed that they are unable to differentiate between sounds coming from different elevations, but a female blackbird should be able to locate her chosen mate even if the source of the serenade is above her.

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.

Schnyder added that they measured the volume of sounds coming from different angles of elevation at the birds' eardrums. All sounds originating from the same side as the ear were similarly loud, regardless of their elevation and 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. The system is highly accurate: at the highest level, birds can identify lateral sounds at an angle of elevation from -30 degrees to +30 degrees.

Schnyder concluded that their latest findings are pointing in the same direction, which seems that the combination of sight and hearing is an important principle in the evolution of animals.

The findings were published recently in the PLOS ONE journal.