In a recent publication of a landmark study where 40 species of birds DNA were sequenced and cross compared some really illuminating findings about probable evolutionary reasons for specific traits and relationships were revealed. One of the most interesting things is what was not realized about the origins of the birds beak.
"Although birds are descended from dinosaurs, they have no teeth. Most previous studies have suggested the common ancestor of modern birds lacked teeth, but other work has disagreed. The new findings add support to the idea of a toothless common ancestor, because all birds sampled share some mutations that turn off five genes for building teeth.