Birds belong to the group archosaurs, which also includes crocodiles and dinosaurs. The team found that closed-mouth vocalization has evolved at least 16 times in the group. And, only birds the size of doves or larger, they noted, "spoke" with closed beaks (see the video below for an example of one of the biggest birds we have, an ostrich, making closed-mouth vocalizations).
Why only larger birds? The researchers say for larger bodies the wall of the esophageal pouch is less tense than in smaller bodies and thus easier to inflate.
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With the last living archosaurs -- birds and crocodiles -- communicating in this way, and because so many dinosaurs had large bodies, the researchers suggest closed-mouth vocalization was likely to be in the vocal repertoire of some dinos.
"A cool thing about this work is the demonstration that closed-mouth behavior evolved many times," said lead author Tobias Riede, of Midwestern University, in a statement. "That suggests it can emerge fairly easily and be incorporated into mating displays."
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As of now, the team hastens to add, no one has yet proven what dinosaurs really sounded like. But, they say, the key to finding out may lie with the birds we see just outside our windows.
"To make any kind of sense of what non-avian dinosaurs sounded like, we need to understand how living birds vocalize," said study co-author Julia Clarke, of the University of Texas at Austin.
Of course, she added, "this makes for a very different Jurassic world. Not only were dinosaurs feathered, but they may have had bulging necks and made booming, closed-mouth sounds."
WATCH VIDEO: Ostrich Mating Call