Scientists have found that birds join together in song to threaten incomers away from their territory.
The biologists at the University of St Andrews found birds who formed defensive duos and enacted a battle of singing skills by singing louder and better than potential intruders, as a way to intimidate them. The threatening behaviour demonstrated in wrens shows that duetting in birds is used to signal aggression, and for the first time, this co-operative trait has been shown to conceal more selfish behaviour.
Dr Lorraine Marshall-Ball and Professor Peter Slater studied wrens in Costa Rica, who are well known for their co-operative duetting behaviour. It is common for males and females of this species to duet with each other, often in complex songs and to signal devotion within pairs. They looked for clues which might demonstrate that the act of duetting is used for any other purpose. By simulating territorial intruders using playback devices they found that subsequent duets were used as a definite sign of intimidation.
When new ‘birds’ arrive on the scene, the resident wrens duet in pairs, matching the song types of the incoming birds. However, a bit like the song, ‘anything you can do, I can do better’, the resident birds forge a co- operative combat by singing faster and more frequently. In some cases, they go further by approaching and attacking the incomers whilst singing loudly. The incoming birds usually flee when approached in this aggressive manner.
Dr Marshall-Ball said: “The two pairs respond to one anothers duets in a ‘listen then reply’ pattern known as countersinging. This countersinging allows both pairs to assess the others duetting ability, and their aggression level, by listening to how often they sing, how close they are, and whether or not they are singing the same song types (matching the song type being sung by an intruder is an aggressive signal). Countersinging is not a welcoming behaviour, it forms the basis of territory defence in most birds.”
Both males and females successfully defended their territory by appearing to work together. Interestingly, however the aggressive behaviour only manifested itself with birds of the same sex, ie females only aimed their aggression at incoming females and the same with males. While the use of bird song is a well-known part of the mating ritual, this is a demonstration of both sexes using co-operative song as a way of getting rid of potential mating threats.
The researcher continued: “When a pair duet, this joint song seems to work in two ways, both as a signal to other pairs that they are ready to defend their territory, and as a signal to one another of their ‘devotion’ to each other. It does seem to form part of the process of mating, and of maintaining a pairs commitment to each other, but it probably evolved mainly to increase the threat of the territory defence by saying that there are two birds ready to defend the territory instead of just one.”
Previous studies have shown that individual singing (and song- matching) by birds is used as a territorial signal, but no-one has previously looked at duetting birds for this aggressive behaviour. The research paper ‘Duet singing and repertoire use in threat signalling of individuals and pairs’ is published in The Royal Society’s ‘Biology Letters’.
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