Brian Switek has a great article on bird evolution on the blog Phenomena at National Geographic.  

The spark for this evolutionary explosion isn’t yet known. Flight could be a major factor, allowing birds to be adapted in starkly different ways from their earthbound ancestors. (Although, of course, nonavian dinosaurs like Microraptor found their own way into the air.) Paleontology thrives on such mysteries. For now, though, the new study underscores the fact that the change from dinosaur to bird is one of the most surprising and best-documented evolutionary transformations of all time. There is no sharp dividing line between dinosaur and bird. “Birds”, Brusatte and coauthors conclude, “are a contiuum of millions of years of theropod evolution.”

 

 

 

http://phenomena.nationalgeographic.com/2014/09/26/how-dinosaurs-set-up-an-avian-explosion/