High-speed video shows how crows 'caw'
By Catherine Brahic Video: Inside a crow’s voicebox High-speed video shot from inside a crow’s voice box has revealed striking similarities between the bird’s signature caw caw call and a low, raspy sound produced by humans. “We used an angioscope, which are small optic fibre devices that doctors use to investigate blood vessels in humans,” says Franz Goller at the University of Utah in the US, whose team shot the video. Goller first used the technique in the mid-1990s. At the time, they allowed him to solve the mystery of how the vocal organ of songbirds works. The syrinx is the bird equivalent to a human’s larynx and is remarkable in that it is essentially a double voice box. Whereas in humans air passes through a single “valve”, in songbirds each lung is hooked up to its own valve. These earlier videos revealed that sound was produced when muscles pull two heavy folds of tissue on either side of the valve apart. Outrushing air sets the folds of tissue vibrating, just as happens with human vocal cords. But this all happens very quickly, so in the early videos the researchers could not discern the details of how this was happening. In the new experiment, Goller used newer angioscopes that are able to take better-quality high-speed video images. He and his colleagues filmed the inside of a hooded crow‘s syrinx. To activate the syrinx, they pressed on the anaesthetised crow’s chest, pushing air out of its lungs, through the double voice box and into the trachea. This produced the crow’s signature caw. The video showed that the cawing was produced when the valves opened briefly then remained shut before briefly opening again, and at a speed which is impossible to see without high-speed cameras. The mechanism is very similar to a hoarse, raspy sound produced by humans called “vocal fry” – a very low frequency crackling, sounding a bit like static radio noise. “Vocal fry is produced when the human larynx opens very briefly for a pulse then has long closed phase,” Goller told New Scientist. “It looks like birds do the same, but with two openings.” Goller does not know the function of these “pulsed” vocalisations in birds. They appear frequently in starling songs and one of Goller’s students is looking at whether females react differently to a male who makes these sounds more than others. The researchers are also preparing to take high-speed images in the syrinx of songbirds that are trained to sing with an angioscope in place. Journal reference: Proceedings of the Royal Society B (DOI: