There were man sized flightless dinosaurs cloaked in feathers that were cousins of early birds in the mesozoic. But they did not survive.
Modern flightless birds are all descended from birds that could fly. Ostriches and Moas did not evolve directly from dinosaurs-but from small modern looking flying birds that got big and secondarily lost flight ( they retain many physical adaptations to flight that they no longer need).
But all birds today, flying and nonflying, seemed to have evolved from small minaturized versions of the hunting dinosaurs like raptors and T-Rex. Some say that they not only evolved from dinosaurs-that modern birds all ARE dinosaurs.
Since bird species outnumber mammal species that would mean we are still living in "the age of dinosaurs".
But there is atleast one bird scientists (the dissenting voice on pbs shows) who still resists this new fangled notion "pushed by dinosaur scientists and latched onto by the public" that "birds are descended from dinosaurs- or even ARE dinosaurs". He still pushes a non dinosaur model for bird origins.
But even he classes birds with the thecodonts- the group of reptiles that includes crocodiles and alligators that also gave rise to the dinosaurs. So even if birds are not themselves dinosaurs, they are atleast cousins.
Source: http://www.wrongplanet.net/postt220249.html
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