We have all stopped to listed to the beautiful sound of the birds in the trees or flying overhead. The majority of bird sounds are so unique to modern archosaurs that 9 times out of ten we can tell that its a bird making these beautiful calls. Archosaurs are a group of diapsids whose living representatives consist of birds and crocodilians. This group also includes all extinct non-avian dinosaurs, extinct crocodilian relatives, and pterosaurs.
One fact that has be recently guided into the spotlight is the about of birds that make certain humming or cooing sounds with their mouths closed. This is called "closed-mouth vocalization. Basically the animal is able to hum and sing using air pockets and pathways in their beaks, heads and necks. Loud enough for us to hear these sounds down hear on the ground. Birds are not the only animals to use closed-mouth vocalization. Other archosaurs like Crocodiles and Alligators also emit low vibrating sounds that can quietly echo amongst to rivers and swamps.
Colors in the chart below show the probability of each branch being an open-mouth vocalizer (blue) or a closed-mouth vocalizer (red). Pies show the probabilities that the ancestors of birds and crocodiles, palaeognath birds, and neognath birds used closed-mouth vocalization.
Here is the new theory!!! Scientists now believe that dinosaurs very well might have emitted closed-mouth vocalizations as well. Though they might not have been as high pitched as some of modern day dinosaurs (birds are dinosaurs).
Again, Closed-mouth vocalizations are sounds that are emitted through the skin in the neck area while the beak is kept closed. To make them, birds typically push air that drives sound production into an esophageal pouch rather than exhale through the open beak. The coos of doves are an example of this behavior. Compared with sounds emitted through an open beak, closed-mouth vocalizations are often much quieter and lower in pitch. Birds making closed-mouth vocalizations usually do so only to attract mates or defend their territory. At other times, they emit sounds through an open mouth.