SCIENTISTS DISCOVER HOW BIRDS
LOCALIZE SOUND SOURCE
Unlike mammals, birds
have no 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.
Now a research team from Technische Universitat Munchen
(TUM) has discovered how birds are able to localize these sounds, namely by
utilizing their entire head. Their findings were published recently in the PLOS
ONE journal.
It is springtime,
and two blackbirds are having a sing-off. They are both competing for the
attentions of a female. But to pick a successful suitor, the female must first
be able to find him.
“Because birds have no external ears, it has long been
believed that they are unable to differentiate between sounds coming from
different elevations,” explains Hans A. Schnyder from 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 identify sound sources in the vertical plane
using their external ears, which absorb, reflect or diffract the sound waves
because of their special structure. Their sense of hearing uses this
information to determine the elevation of the sound source. But how do birds
perceive these differences?
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,” relates Schnyder. All sounds
originating from the same side as the ear were similarly loud, regardless of
their elevation. The ear of 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,” continues Schnyder. “The system is highly accurate: at the highest
level, birds can identify lateral sounds at an angle of elevation from – 300
to +300.”
Why have birds developed sound localization on the
vertical plane? Most birds have eyes on the sides of their heads, giving them
an almost 3600 field of vision. Since they have also developed the
special ability to process lateral sounds coming from different elevation, 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
totally 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. The owl hears sounds coming from in front of it
better than the other bird species studied by Schnyder.
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