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Songbirds and shrill shouters

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Dr K-lynn Smith, behavioural biologist from Macquarie University, looks at how birds make sounds and why this is important.    
Birds produce a wide range of sounds, from the grunts, grumbles and hoots of turkeys to songbirds which can make two sounds simultaneously and even the infra sounds that female emus produce low to the ground and which can travel great distances.
Birds have tracheas similar to humans but their larynxes are only used to open and close, preventing food and water entering the tracheas. The song production part of birds is found in either branching parts of the tracheas or bronchi and one of their air sacs. This air sac from the lungs presses on a membrane, thus producing sound. Tracheas develop over the life of birds and the thickness levels are related to testosterone levels in males, which change over time, accompanied by a related change in sounds made. Geese and swans have long, looped-back tracheas which allow them to create higher pitched sounds.
Baby songbirds have to learn the sounds and they learn them in two distinct periods. The first is in the nest where, much like human babies learning to talk, there is a lot of song babbling early on, until the exact sounds are mastered. The next song learning period is when the fledglings, which have left their original territory, either return or take up a new territory meaning they have to learn the songs established in that area.
Female songbirds prefer males with a wide repertoire of songs, reflecting a higher cognitive ability to successfully reproduce multiple songs. Males also carry out aggressive song duets, with one bird copying another. Females monitor this interaction and are much more likely to mate with the winner, whether that is her current mate or not. Australian songbirds have evolved very differently to those in the northern hemisphere where there were many terrestrial predators, causing songbirds to evolve as small and cryptic sounding creatures. Australian songbirds, beset more by aerial predators, are larger and could be less cryptic, which may well explain why Australian bird calls sound so different to their northern counterparts.

Dr K-lynn Smith was interviewed by Ruby Vincent for A Question of Balance. Summary text prepared by Victor Barry, June 2011; revisited August 2016

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