Carl Zimmer in his excellent blog, The Loom:
Birds are like nothing else on Earth–in the sense that they have lots of traits in common that are not found in quite the same package in other animals. All birds have feathers, for example, and they can either fly or have evolved birds that did fly. You don’t see some feathered reptile running around on all fours. But that distinctness is merely an artefact of extinctions. The closest living relatives of birds today are alligators and crocs, and they share a common ancestor that lived some 250 million years ago. All of the species on the lineage that led from that ancestor to birds are extinct. The ground-running dinosaurs disappeared. There are fossils of birds that could fly 120 million years ago, but they became extinct too. To see how birds became so unique, scientists have to burrow down the evolutionary tree, and burrow into the ground.
I’ve written here about some of the things they’ve discovered in the process, such as flightless dinosaurs that probably used feathers to show off to the opposite sex. Feathered dinosaurs also laid eggs in nests and incubated them much as birds do today. And today (as in, this particulary day of the week), David Varricchio of Montana State and his colleagues are reporting that it was the father dinosaurs who were sitting on the eggs.