As I said, there is an interesting wrinkle to the subject of flightless birds.
There are recognized categories of Cretaceous creatures (145.5 to 65.5 million years ago) that were very much like modern flightless birds including Ornithomimosauria and Oviraptorosauria.
"The Ornithomimosauria, ornithomimosaurs ("bird-mimic lizards") or ostrich dinosaurs were theropod dinosaurs which bore a [more than] superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia (now Asia, Europe and North America). The group first appeared in the Early Cretaceous and persisted until the Late Cretaceous."
"Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head."