There is a lot of evidence that birds are dinosaurs. For example some of their traits like clawed toes and scales covering their feet. One of the most famous bird fossils from the Jurassic period, the archaeopteryx bird had these traits. Although the archaeopteryx bird could not fly because they don't have feathers and had a flat sternum. Regular birds have a keeled sternum. Thomas Huxley was one of the first people to make a connection between dinosaurs and birds. John Ostrom found 22 features in theropods and birds that could not be found in any other animal groups. The archaeopteryx had a long bony tail as well and had claws on its wings believed to be used to catch prey or climb on trees. Most common of the species of dinobirds are Enantiornithes a fairly diverse group of birds, mostly flying forms, Hesperornithiformes toothed birds which were mostly flightless swimmers, and Ichthyornithiformes toothed flying birds that probably fed on fish.
The dinosaur birds did not have hollow bones like regular birds have today, if they did have hollow bones they probably wouldn't have gotten preserved over so many years. I think that birds are dinosaurs because there is a lot of evidence from fossils and other types of evidence that is found every year.