Why do scientists think that birds evolved from dinosaurs?

Many dinosaurs have birdlike traits. The basic body plan of theropods (the two legged carnivores) is somewhat birdlike. However some of these are especially birdlike. Among these are Archeopteryx, which is widely attributed to be the earliest known bird and clearly has traits of maniraptoran theropods. A number of maniraptorans such as Velociraptor are now known to have had large, well-developed feathers. One relative, Microraptor, is believed to have been able to glide between trees.