Unlike mammals, birds have no external ears and their head does the work of external ears.

They can also perceive whether a particular sound's source is above them, below them, or at their level, the study adds.

"Because birds have no external ears, it has long been believed that they are unable to differentiate between sounds coming from different elevations," said the study's lead author Hans A Schnyder from Technische University Munchen (TUM), Germany.

"But a female blackbird should be able to locate her chosen mate even if the source of the serenade is above her," Schnyder said, adding that he could identify how birds learn about the elevation angles by studying crows, ducks and chickens.

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," Schnyder said.

The head screens the sound coming from different angles. Other sound waves pass through the head and trigger a response in the opposite ear.

The avians' brain determines whether a sound is coming from above or below from the different volumes of sound in both ears. This is how birds identify where exactly a lateral sound is coming from, for example at the height of the eyes.

"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," Schnyder said.

The findings were published in the journal PLOS ONE.

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