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Barn Owl

Unlike mammals, birds have no external ears. The outer ears of mammals means that they can help identify sounds coming from different elevations and it has long been believed that birds do not have that ability due to their lack of external ears.

But a new study from a research team from Technische Universität München (TUM) has found that birds can also perceive whether the source of a sound is above, them below them or at the same level and discovered how birds are able to localize those sounds.

Mammals identify sound sources in the vertical plane using their external ears, which absorb, reflect or diffract the sound waves because of their structure. Their sense of hearing uses this information to determine the elevation of the sound source. But how are birds able to perceive these differences?

For example during breeding season two blackbirds are competing for the attentions of a female and she needs to be able to locate her chosen mate even if the source of the serenade is above her.

Dr. Hans A. Schnyder from the TUM Chair of Zoology, part of the team who worked on the research which was published in PLOS ONE journal, studied three species of birds; crows, ducks and chickens. He discovered that birds are able to identify sounds from different elevation angles and they do so by using their heads to do the work of external ears. 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 Dr. 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.”

Depending on where the sound waves hit the head, they are reflected, absorbed or diffracted. The scientists discovered 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 bird’s 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," added Dr. Schnyder. "The system is highly accurate: at the highest level, birds can identify lateral sounds at an angle of elevation from -30° to +30°."

The team also questioned why birds have developed sound localization on the vertical plane. Most birds have eyes on the sides of their heads, giving them an almost 360° field of vision.

With the ability to process lateral sounds coming from different elevations, they can combine this information from their senses of hearing and vision to useful effect when it comes to evading predators.

A few birds of prey such as the barn owl have developed a totally different strategy. Barn owls hunt at night and like humans its eyes face forward. The feather ruff on their face modifies sounds in a similar way to external ears and hears sounds coming from in front of it better than other species of birds. Owls also use their eye sockets to help them hear and as it hunts for prey it tucks it beak down to help with the process of collecting sounds.

There is a perfect interaction between the information they hear and the information they see as earlier studies had demonstrated.

"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," concludes Schnyder.

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