All birds are technically avian dinosaurs, but there’s still controversy over exactly how and when the first actual birds emerged.
UC Berkeley’s J. Lee Kavanau, the author of the paper, argues that there is ample evidence supporting that troodontids and oviraptorids were secondary flightless birds. These birds, like today’s ostriches and emus, lost their ability to fly, but retained their feathers.
“This evidence ranges from bird-like bodies and bone designs, adapted for climbing, perching, gliding, and ultimately flight, to relatively large, highly developed brains, poor sense of smell, and their feeding habits,” Kavanau writes. “Because ratites also are secondarily flightless and tinamous are reluctant, clumsy fliers, the new evidence strengthens the view that troodontids and oviraptorids were secondarily flightless.”
He also points out that “secondary flightlessness apparently favors paternal care of clutches of large, abundant eggs.” That’s been observed for both the “non-avian theropods” and today’s flightless birds. A single ostrich egg, for example, can weigh around 3 pounds and is equivalent to about 24 chicken eggs.