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How birds get by without external ears

A new study looks at how birds localize sounds. It seems that their slightly oval-shaped head transforms sound waves in a similar way to external ears.

Photo credit: Mr. TinDC/Flickr

Photo credit: Mr. TinDC/Flickr

Unlike mammals, birds have no external ears. The outer ears of mammals help the animal identify sounds coming from different elevations. Mammals’ ears absorb, reflect or diffract the sound waves because of their special structure. But birds are also able to perceive whether the source of a sound is above them, below them, or at the same level. How do they do it?

Namely by utilizing their entire head, according to research from Technische Universität München (TUM) published in PLOS ONE in November.

The researchers studied three avian species – crow, duck and chicken. The study suggests birds’ slightly oval-shaped head transforms sound waves in a similar way to external ears.

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.

Hans A. Schnyder is the TUM Chair of Zoology. He said:

This is how birds identify where exactly a lateral sound is coming from – for example at eye height. The system is highly accurate: at the highest level, birds can identify lateral sounds at an angle of elevation from -30° to +30°.

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 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.

So there is a perfect interaction between the information they hear and the information they see, say the researchers. Schnyder said:

Our latest findings are pointing in the same direction: it seems that the combination of sight and hearing is an important principle in the evolution of animals.

Bottom line: Research from Technische Universität München (TUM) published in PLOS ONE in November, 2014, suggests that birds’ slightly oval-shaped head transforms sound waves in a similar way to external ears.

Read more from Technische Universität München

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