Speech almost defines the human race. But we are not the only animals with a versatile vocal repertoire; birds are also renowned for learning and producing intricate trains of sound. Verena Ohms and her colleagues from The Netherlands, Germany and USA explain that speech scientists are particularly interested in bird sound production because of the parallels between bird and human vocal learning. However, there are significant differences too. For example, humans use the tongue extensively to articulate sounds while most birds do not use the tongue; instead, they expand the oesophagus and adjust how wide they open their beaks. Yet, there are several groups of birds that may buck this trend. Ohms explains that speech-imitating parrots have a prominent tongue that they may use to modulate sounds produced by their simple syrinxes. But, there was no direct evidence that parrots use their tongues while calling, so Ohms and her colleagues, Gabriel Beckers, Carel ten Cate and Roderick Suthers, decided to make X-ray movies of four squawking monk parakeets to find out whether or not they articulate sounds with their tongues (p. 85).
Placing minute metal markers in the birds' tongues and at two locations on the tracheae near the top of the throat, the team filmed the parakeets with 10 ms bursts of X-rays as they chattered and made contact and greeting calls. Analysing the markers' movements, the team saw that the birds use their tongues extensively as they squawk and chatter. Having lowered the tongue and opened their beaks before initiating a call, the parakeets then further lowered their tongues by as much as 6.7 mm as they began calling. They also varied the strength of their calls using two strategies: opening the beak wider and varying the height of the tongue to alter the distance between it and the beak's upper mandible.
Analysing the birds' throat, beak and tongue movements during contact and greeting calls, the team found that the parrots prepared for a call by lowering the larynx, opening the beak and lowering the tongue before uttering a sound. They also shortened the distance between the two markers on the tracheae by as much as 44%. Then, as the birds began squawking, they continued depressing their tongues and pulling their larynxes down. Comparing the greeting and contact calls, Ohms and her colleagues found that the throat structure movements were slower in the longer greeting calls. However, when the birds were chattering – rattling out a train of alternating notes – the team noticed that the birds produced two distinct tones by changing the relative timing of their beak, tongue and tracheal movements.
So monk parakeets modulate their calls by adjusting their tongue height, beak opening and tracheal length and the team points out that this is the first observation that a bird contracts the trachea to articulate distinct sounds. However, the parakeets did not perform one tongue movement that is particularly important in human speech. Explaining that humans extend the tongue back and forth, the team noticed that the parakeets appeared not to move the tongue horizontally. However, knowing that the parakeets produce a repertoire of nine calls when communicating naturally, they say, ‘It cannot be ruled out that in some of the other call types these birds use the front–back dimension more heavily.’ They are also keen to analyse the beak, tongue and throat movements of birds mimicking human speech to discover more about the mechanisms of speech production.