Nightingales quarrel at eye level

Male nightingales that sing to defend their territory quarrel more at eye level, say scientists. During night-time singing bouts, males are known to interrupt each others calls and rapidly change their songs. Researchers wanted to investigate whether the birds tried to gain a height advantage by moving higher up in their chosen trees. They found that the birds were actually more aggressive when singing from branches that were at the same height. The findings, published in journal PLoS One, surprised the researchers. "We expected nightingales singing from higher song posts to appear more threatening to their rivals," said Dr Valentin Amrhein from the University of Basel, who was the senior author of the study. Alongside their colleagues from the Netherlands Institute of Ecology, the scientists theorised that the birds' positions could have a significant effect on how "threatening" their calls were. Dr Amrhein explained that higher branches were thought only to be accessible to the fittest and most dominant birds, because taking up a loftier post increased threat of predators and reduced protection from the elements. The team believed, therefore, that males singing from higher positions would represent more of a threat to their rivals, and elicit more of a defensive response from other males.

Singing competition

The team tested their idea by playing audio recordings of male nightingales at varying heights and monitoring how a real bird responded. In the experiment, males responded more aggressively to calls made at the same height as them by increasing their "song rate". This meant that the birds took fewer pauses and changed their tune more often. According to Dr Amrhein the results suggest that the birds have an advanced awareness of both their position and that of their rivals, and that they factor this information into their vocal performances. "In future studies on vocal communication in birds, signalling height should be considered, because it apparently determines how birds perceive their rivals," he said. Nightingales are found in tropical Africa and migrate to Europe, north west Africa, the Middle East and Central Asia to breed. Males are famed for their nocturnal musical performances, which are made up of an impressive range of sounds including trills and whistles. Females are thought to choose their mates based on the quality of these songs.